Recent developments in the synthesis of regioregular thiophene-based conjugated polymers for electronic and optoelectronic applications using nickel and palladium-based catalytic systems

Thiophene-based conjugated polymers hold an irreplaceable position among the continuously growing plethora of conjugated polymers due to their exceptional optical and conductive properties, which has made them a centre of attention for the past few decades and many researchers have contributed tremendously by designing novel strategies to reach more efficient materials for electronic applications. This review aims to highlight the recent (2012–2019) findings in design and synthesis of novel thiophene-based conjugated polymers for optical and electronic devices using organometallic polycondensation strategies. Nickel- and palladium-based protocols are the main focus of this account. Among them nickel-catalyzed Kumada catalyst-transfer polycondensation, nickel-catalyzed deprotonative cross-coupling polycondensation, palladium-catalyzed Suzuki–Miyaura and Migita–Kosugi–Stille couplings are the most popular strategies known so far for the synthesis of functionalized regioregular polythiophenes exhibiting fascinating properties such as electronic, optoelectronic, chemosensitivity, liquid crystallinity and high conductivity. This account also presents a brief overview of direct arylation polymerization (DArP) protocol that has shown a great potential to lessen the drawbacks of conventional polymerization techniques. DArP is a cost-effective and green method as it circumvents the need for the synthesis of arylene diboronic acid/diboronic ester and distannyl arylenes using toxic precursors. DArP also puts off the need to preactivate the C–H bonds, hence, presenting a facile route to synthesize polymers with controlled molecular weight, low polydispersity index, high regioregularity and tunable optoelectronic properties using palladium-based catalytic systems.


Signicance of the substitution on thiophene monomer
Unsubstituted polythiophenes were synthesized via chemical polymerization at the initial stages of the history of polythiophenes. [85][86][87][88][89] These unsubstituted polythiophenes were thermally stable and highly conductive but were insoluble. 90,91 In order to prepare soluble polythiophenes, alkyl substituents were introduced at position-3 of the thiophene unit which was then polymerized using the protocol previously used for the synthesis of unsubstituted polythiophenes in the late 1980s. [92][93][94][95][96] However, these chemical and electrochemical polymerization techniques resulted in the random couplings in the poly(3alkylthiophenes) (P3AT) yielding only 50-80% head to tail couplings due to the multiple head-head and tail-tail couplings. Head-to-tail regioregular P3ATs were synthesized for the very rst time using McCullough method in 1992. 97 While it enhanced the electrical conductivity of the polymer due to the formation of well-organized three-dimensional polycrystalline structure, this method also paved the way for the synthesis of many other functionalized polythiophenes.

Effects of various functional groups on the properties of polythiophenes
To date many functionalize polythiophenes have been synthesized consisting of thiophene monomers with various substituents at 3-position. While different alkyl substituents are the most commonly used, the use of esters (-COOR), 98,99 acetyl (-COR), 100 amide (-CONHR), 101 alkoxy (-OR), 102-105 alkylthio (-SR), 106-108 sulfonyl (-SO 3 R), 109 alkylamino (-NHR and NRR 0 ) 110 and uoroalkyl [111][112][113][114][115][116][117][118][119] groups have also been reported. It has now been established that functionalization of polythiophenes not only enhanced their solubility and processability, it also altered their optical and electronic properties. In addition to this, the side chains of the polymers were reported to be helpful in the chemical sensing as molecular recognition units. 120,121 1.3.1. Substituents with electron withdrawing inductive effect. Introduction of ester group at position-3 of thiophene monomer resulted in a blue shi in l max compare with that of P3ATs, which might be due to the wider band gap resulting from the electron-withdrawing nature of the carbonyl moiety. 98 Polythiophenes substituted with partially uorinated alkyl chains exhibited unique properties including chemical and oxidative resistance, and hydrophobicity. Such polymers formed highly ordered solid-state structure and were also reported to show liquid crystalline behavior. 122,123 Regioregular polythiophenes (rrPT) with ether substituents, where oxygen is not directly connected to the ring, exhibited ion-binding properties toward Pb 2+ , Hg 2+ and Li + . These polymers showed very high conductivities aer iodine doping. 124 1.3.2. Substituents with electron donating inductive effect. Introduction of electron-donating groups presented several advantages over alkyl substituted ones. For instance, alkoxy group, with a heteroatom directly attached to the thiophene ring resulted in a decreased band gap by raising their HOMO level, which resulted in low oxidation potential, and the conducting state in the polythiophene was stabilized. 124 rrPT substituted with alkylthio groups showed low solubility in common organic solvents such as THF, xylene and chloroform, while they were fairly soluble in carbon disulde. 55 These polymers exhibited high conductivity of 100 S cm À1 aer iodine-doping. 125 Relatively fewer polythiophenes with 3,4alkylthio substituents have been studied so far. One of the most interesting properties of these polythiophenes that distinguished them from others is the complexing ability of sulfur atoms of thioether groups towards various "so" metallic ions. This capability of alkylthio substituted polythiophenes introduced the possibility of using the sensitivity of optoelectronic properties of these polymers to develop sensor systems to detect the presence of species that may alter these properties. Such compounds may therefore be used as catalysts for the hydroformylation reaction or as active site of the chemosensors. [126][127][128] 1.4. Polythiophenes substituted with chiral side chains p-Conjugated polymers substituted with chiral side chains represent a special class of optically active polymers that has attracted much attention in view of their possible applications as enatio-selective membranes and electrodes, suitable for chiral sensing as well as electrochemical asymmetric synthesis. [129][130][131] Side chain chirality imparts helical order in solid state and in aggregates. Moreover, chiral side chains have great ability to self-assemble into novel nanostructures. 132,133 In general, these chiral polymers display strong chiroptical properties only when their chains aggregate to form chiral superstructures. Signicant optical activity could be induced into polythiophenes provided the substituent at position-3 of the thiophene monomers are placed in a regioregular head to tail fashion. This induced optical activity is either the result of an intermolecular chiral orientation of the predominantly coplanar chains of the polymer with a kind of super-coiling in crystalline hexagonal phase or result of an intramolecular helical conformation of the polymer backbone. 134 Chiral aggregations in optically active polythiophenes are reported to be inuenced with slight differences in the structure such as regioregularity and substituents as well as processing conditions, e.g. solution temperature, 135,136 solvent 137,138 and solvent additives. [139][140][141] The polymer solution consisting of chiral regioregular polythiophenes showed strong circular dichroism (CD) signal at its p-p* transition upon addition of poor solvent or at low temperature suggesting the formation of helical backbone in the aggregate, 142,143 although few conjugated polymers have also been reported to self-assemble into aggregates even in chloroform and chlorobenzene that are considered as "good" solvents. 144 produced structurally homogenous and defect-free polymers that have greatly improved photonic and electronic properties as compared to regioirregular ones. 146,147 Regioregular polythiophenes have led to a large number of novel and important micro-and nano-scale electronic materials and devices. 6,[148][149][150][151][152][153] The random sequence of monomer units in polythiophenes hinders the close packing of the polymer chains and reduces the electrical conductivity due to the twisting of thiophene rings out of the conjugation planarity resulting from the steric repulsion among the substituted alkyl chains. Self-assembled polythiophenes exhibit better performance in electrical and optical devices as compared to regiorandom polythiophenes. 144,154 Sterically twisted structure is produced in the polythiophene backbone due to the multiple head-head (HH) and tail-tail (TT) couplings, resulting in the loss of extended p-conjugation 2. This sterically twisted back bone leads to the reduction of high conductivity and other desirable properties in polythiophenes.
Regioregular polythiophenes possess excellent electrical properties due to their planar backbone and solid-state self-assembly that form well-organized three-dimensional polycrystalline structure, making them highly conductive due their efficient interachain and interchain charge carrier pathways. 90,154 For instance, the mobilities of regioregular poly(3-alkylthiophenes) are higher than those of regioirregular poly (3-alkylthiophenes). 6,155,156 The solution of regioregular polythiophenes, containing chiral side chains, show a strong circular dichroism signal due to the formation of helical backbone in aggregates, whereas regioirregular polythiophenes show only a weak CD signal. Head to tail (HT) polymers are superior compare with their head to head (HH) and regiorandom isomers and possess higher eld-effect mobility and light-emitting ability. 157,158 2. Transition metal catalyzed synthesis of thiophene-based polymers Perfect control over the incorporation of each thiophene unit in a consecutive head to tail manner is important for the synthesis of regioregular polythiophenes, and employment of transition metal catalysts has effectively served this purpose since the rst synthesis of regioregular head-to-tail coupled poly(3alkylthiophene) (P3ATs) by McCullough early in 1992 using nickel catalyst. 97
Formation of conjugated polymers essentially lies in the efficient carbon-carbon bond formation between unsaturated carbons in aromatic monomer units. In addition to the oxidative and electrochemical polymerization, 159-161 transition metal catalyzed cross-coupling reactions provide a particularly powerful tool for the synthesis of conjugated polymers. 162 The reaction in general, involves oxidative addition of transition metal catalyst across C-X bond of an electrophile, which leads toward transmetallation with the main group metal of organometallic nucleophile. Finally, the C-C bond is formed via reductive elimination with a concomitant regeneration of active catalyst.
Most commonly used transition metal catalysts for polycondensation reactions are palladium and nickel-based catalysts, although some other metals have also been used. The organometallic nucleophiles employed in transition metal catalysts include stannyl reagents (Stille coupling), 163 boron reagents (Suzuki-Miyaura coupling), 164 copper (Sonagashira coupling) 165 and Grignard reagents (Kumada-Corriu coupling). 166 Thus, consecutive transformations in the catalytic cycle can be made in order to extend the conjugation lengths. Regioregularity of the polymer can be achieved easily when nucleophilic and electrophilic centers of monomeric units are readily accessible. The enhancement of regioregularity through these advanced metal-catalyzed methodologies leads to various benecial outcomes such as an intensied extinction coefficient, an increase in the mobility of the charge carriers and a red shi in absorption in solid state.
Since the advent of the concept of regioregular polythiophenes and an understanding of their useful properties and their utilization in various potential applications, number of research groups put their efforts to synthesize novel thiophene-based monomers, and their subsequent polymerization to afford homopolymers as well as copolymers with some acceptor monomeric units.
A brief survey has been provided for the synthesis of differently substituted and fused polythiophenes, including recent contributions made by different research groups, regarding the synthesis and characterization of novel thiophene basedmonomers and their subsequent polymerization using nickel and palladium based catalytic systems with a special emphasis on the recent advancement in Ni-catalyzed C-H functionalization polymerization, Ni-catalyzed Kumada catalyst-transfer polycondensation and Pd-catalyzed direct arylation polymerization approachs. The survey includes how they are helpful in addressing the drawbacks of widely used conventional crosscoupling polymerization protocols.
3. Nickel catalyzed synthesis of thiophene-based polymers been developed and are being utilized succesfully in the synthesis of novel conjugated polymers. Ni-catalyzed C-H functionalization polymerization and Kumada catalyst-transfer polymerization (KCTP) are the most popular protocols being used for the synthesis of conjugated polymers nowadays. Some researchers have utilized Ni-catalyzed Suzuki, Negishi and Murahashi coupling reactions to polymerize thiophene-based monomers. Some of the very recent work reported by exploiting Ni-catalysis is briey discussed in this review.

Ni-catalyzed C-H functionalization polymerization
Ni-catalyzed dehydrobrominative polycondensation is conducted by deprotonation at C-H bond of thiophene monomer using bulky magnesium amide, TMPMgCl$LiCl (chloromagnesium 2,2,6,6-tetramethylpiperidide lithium chloride) also known as Knochel-Hauser base. 167,168 This method helps polymerization to take place at room temperature within shorter time period. [169][170][171] Knochel-Hauser base is more effective for regioselective deprotonation of arenes as compared to the strong bases such as lithium amides (R 2 NLi) and alkyl lithium reagents (RLi), traditionally used for deprotonation. These strong bases lead to undesirable reactions due to their strong nucleophilicity (e.g. Chichibabin addition) and high reactivity. Low stability of lithium amides in THF solutions at room temperature is another serious limitation, which requires in situ generation of these reagents. Moreover, the requirement of low temperatures (À78 to À90) for the protonation of arenes further complicates the scale-up of these reactions. 170,[172][173][174] Employment of 2-halo-3-substituted thiophenes instead of 2,5-dihalothiophenes along with the Knochel-Hauser 167,175 base have been the key for the successful synthesis of thiophenebased polymers. 170,[176][177][178][179][180][181][182] Magnesium amide does not interfere with the propagation of the polymer during the course of the reaction. 164 Dehydrobrominative polycondensation proceeds with the higher atom efficiency compared to the dehalogenative polycondensation employing Grignard reagent. The loss of two halogen atoms from the thiophene monomer could be problematic as it results in a greater mass loss in the polymerization reaction while dehydrobrominative polycondensation results in the synthesis of highly regioregular head-to-tail polythiophenes with the improved atom efficiency. 170 This protocol has been successfully employed by different research groups to synthesize highly regioregular thiophene-based polymers, from which some of the recent works have been highlighted here.
Shunsuke Tamba and co-workers, in 2014, polymerized 3hexylthiophene to afford head to tail type regioregular poly(3hexylthiophene) by using nickel(II) catalyst for the deprotonative C-H functionalization polycondensation of 2-(phenylsulfonyl)-3-hexylthiophene. 184 2-(Phenylsulfonyl)-3hexylthiophene 13, was used as a monomer precursor, which was subjected to deprotonation with Knochel-Hauser base at room temperature. Ni(dppe)Cl 2 was added as a catalyst to the reaction mixture, which yielded polythiophene with number average molecular weight of (M n ) 9300 g mol À1 . 1 H NMR analysis conrmed 99% head-to-tail (HT) regioregularity of the desired polymer.
It is worth noting that carbon-carbon bond formation using transition metal catalysis occurred through C-S bond cleavage, which is a new class of cross-coupling polycondensation reactions. Polymerization of 3-hexylthiophen-2-yl phenyl sulde monomer 14 was performed under similar conditions to obtain P3HT 16 with much lower yield and molecular weight (M n ¼ 1610 g mol À1 ). 2-Phenylsulnyl-3-hexylthiophene 15, a sulfoxide monomer, was also polymerized under similar conditions to afford the corresponding polymer with 74% yield and molecular weight of 3840 g mol À1 (M n ). These results showed that phenyl sulfone served as the most effective leaving group in the polymerization reactions of thiophene (Scheme 7).  Concerning the polymerization mechanism of halothiophenes, the initiation reaction in polymerization is considered to be reductive tail to tail homocoupling of metalated sulfonyl thiophene and oxidative addition of Ni(0) species into the C-S bond. Homocoupling of metalated monomer 17 takes place initially to give 18, propagation reaction occurs by incorporation of the monomer 17 at the terminal C-S bond. An end group of 19 is the terminal thiophene having SO 2 Ph group (Scheme 8).
In 2013, Shunsuke Tamba and co-workers reported the use of [CpNiCl(SIPr)] catalyst for the synthesis of extremely high molecular weight head-to-tail type regioregular poly(3-alkylthiophene) via dehydrobrominative polycondensation. 179 Polymerization of 2-bromo-3-hexylthiophene proceeded with a catalytic amount of [CpNiCl(SIPr)] and an equivalent amount of Knochal-Hauser base to yield regioregular poly(3hexylthiophene) 16 up to M w ¼ 815 000. A self-standing lm of polythiophene was obtained with M w ¼ 414 000, while the attempted formation of corresponding lms with lower molecular weight (M w ¼ 38 000) was not successful. Mechanical characteristics of this self-standing lm are of great interest in terms of the relationship of its physical properties with its mechanical treatment as a high performance material.
Polymerization of 2-bromo-3-hexylthiophene 20 was conducted using 2 mol% of nickel catalyst and 1.2 equivalents of TMPMgCl$LiCl at 25 C for 24 hours yielding the corresponding polymer 16 in 90% yield with head-to-tail regioregularity of 98%. The obtained high molecular weight polymer had reasonable solubility in 1,2-dicholorobenzene and chloroform.
[CpNiCl(SIPr)]-catalyzed polymerization was performed under several different conditions and the results are enlisted in Table  1, which showed that optimum conditions for obtaining high molecular weight poly-3-hexylthiophene are room temperature and 0.5-2 mol% catalyst loading (Scheme 9).
The optimized polymerization conditions of 2-bromo-3hexylthiophene were also applied to the synthesis of other 3substituted bromothiophenes in the presence of [CpNiCl(SIPr)] catalyst. Polymerization of these monomers proceeded efficiently to afford high molecular weight polythiophenes ( Table  2).
[CpNiCl(SIPr)] catalyst was also found to be effective in GRIM polymerization of 2,5-dibromo-3-hexylthiophene. Treatment of 21 with i PrMgCl$LiCl in THF at room temperature and subsequent addition of Ni-catalyst initiated the polymerization reaction. Further stirring at room temperature yielded poly(3hexylthiophene) 16 with M n ¼ 107 000 (Scheme 10).
Keisuke Fujita and co-workers synthesized polythiophenes substituted with siloxane moiety at 3-position in 2016. 185 Synthesis of siloxane bearing monomer started with radical bromination of 2-bromo-3-methylthiophene 22 with NBS in the presence of azoisobutyronitrile (AIBN) to afford bromomethylated intermediate 23, which was subjected to allylation by treatment with allyl magnesium bromide to give u-olenic product 24. Silylated product 25 was obtained in a quantitative yield by hydrosilylation of 24 using platinum catalyst (Scheme 11).
Nickel-catalyzed deprotonative polycondensation of 2bromo-3-substituted thiophene 23 was performed using TMPMgCl$LiCl. Deprotonation step was conducted at 60 C for 1 hour to yield the corresponding thiophene magnesium species, which was then treated with 0.1-5 mol% Ni(PPh 3 )Cl 2 to afford the desired regioregular head to tail polythiophene 24. The obtained polythiophene bearing siloxane as a substituent was found to be dissolved in a variety of organic solvents especially in hexanes, allowing formation of thin lms (Scheme 12).
Chia-Hua Tsai and co-workers in 2016 demonstrated the synthesis of periodic p-conjugated polymers of group 16 heterocycles (thiophene, furan and selenophene) with relatively low dispersities and controlled chain length using catalysttransfer polycondensation. 186 In order to ensure the well-dened sequence, the copolymers were synthesized by linking short oligomers through catalyst-transfer polycondensation (CTP). The redox potentials and optical band gaps were reported to vary with the composition of the copolymers in a predictable manner. Moreover, the periodic sequences exhibited well-dened morphologies, and the packing patterns mimic those of regioregular P3HT.
All the monomers were synthesized by the cross-coupling reactions of individual heterocyclic ring using Pd 2 dba 3 in 1,4-dioxane at 100 C, and then NBS was used to introduce
The term "Kumada catalyst-transfer polycondensation" was created by Yokozawa and co-workers, which reects an essential step (intramolecular catalyst transfer process) during the catalytic cycle of polymerization, while the term Grignard metathesis (GRIM) refers to the stage of monomer synthesis rather than to the chain-growth process itself. 187 Chain growth mechanism offers more control over molecular weight distribution and end group functionalization compared to step-growth polymerization mechanisms (mostly Suzuki and Stille couplings) and is more effective in achieving reproducible material properties and subsequently, reproducible device parameters. 187,204 Low degree of control over growing polymer chain oen leads to batch-to batch differences and altered material properties which is undesirable to obtain reproducible results. Discovery of chain growth mechanism has extended the scope of KCTP and is widely used to synthesize novel thiophenebased polymers with tailored architecture. Some of the very recent work in this regard is reviewed below.
Colin R. Bridges and co-workers in 2014 reported the rst example of the synthesis of electron-rich/electron-decient, all conjugated diblock copolymers using Ni(II) diimine catalyst, dibromonickel (MesAn), with an electron donating ligand. 205 This catalyst formed strong association with both electron-rich and electron-decient monomers due to which it could be very effective in performing their controlled polymerization.
Poly(3-hexylthiophene) (P3HT) 16 and polybenzotriazole (PBTz) 33 were chosen as the electron-rich and electron Scheme 19 One-pot synthesis of P3HT triblock copolymers 45-47. decient-blocks, respectively. MesAn catalyst had been used previously for the synthesis of polyolens but was never tested for the synthesis of conjugated diblock copolymers via Kumada catalyst transfer polymerization (KCTP). MesAn association complex with P3HT and PBTz monomers showed stabilization of 148.3 and 143.8 kJ mol À1 , respectively. These complexes exhibited greater stability than other Ni(II)-diimine catalystmonomer systems, 206 suggesting a good control of MesAn over both benzotriazole and thiophene polycondensation. Chain transfer or chain termination reactions are prevented by strong catalyst affinity to the monomer, thus, allowing more control over polymerization. Controlled polymerization is evident by the narrow dispersities of the polymers with the molecular weights that could be controlled by the catalyst to monomer ratios. To test this, homopolymers 16 and 33 were synthesized from their respective monomers 2,5-dibromo-3-hexylthiophene  (Table 3). These newly synthesized donor-block-acceptor copolymers exhibited interesting electrochemical and phase separation properties (Scheme 15).
In 2015, Zhuping Fei and co-workers reported two strategies for the synthesis of regioregular 3-alkyl-4-uorothiophenes, F-P3HT 37, F-P3OT 38 and F-P3EHT 39, containing straight (hexyl and octyl) and branched (2-ethylhexyl)alkyl groups, respectively. 207 Comparison of the properties of the uorinated polymers with their non-uorinated analogues revealed that backbone uorination results in an increase in the polymer ionization potential without causing a signicant change in the optical band gap, indicating that uorination leads to lowering of both the HOMO and LUMO energy levels. Average charge carrier mobilities for the uorinated polymers are found to be increased up to a factor of 5 in the eld-effect transistors. Fluorination also enhances the tendency to aggregate in the solution.
For the synthesis of uorinated polymers 37, 38 and 39, Grignard metathesis (GRIM) route was used due to its wellknown robustness and good control over the synthesis of P3HT. Thiophenes were uorinated in the 3 and 4 positions by electrophilic uorination of lithiated thiophenes, although the uorination of electron rich aromatic thiophenes at these positions is non-trivial. 2 and 5 positions of the thiophenes were protected via TMSCl in order to prevent the rearrangement of 3 or 4 lithiated thiophenes to thermodynamically more stable 2 and 5 positions (Scheme 16).
Early-stage introduction of the alkyl side chain proved to be problematic, which means that all the four steps needed to be repeated in order to change the alkyl side chain. Keeping in view the tedious nature of separation of uorinated monomer from non-uorinated byproduct, an alternate synthetic route was designed in which the side chain was introduced aer the uorination step. Fluorinated intermediate 40 was prepared from commercially available 2,3,4,5-tetrabromothiophene in two steps and a reverse-phase chromatography could be used to separate uorinated product from the non-uorinated byproduct. Surprisingly, 40 was found to be unreactive to the standard Kumada coupling conditions used for 3-bromothiophene, thus, Negishi cross-coupling with octyl or 2-ethylhexyl zinc bromide was used in order to incorporate alkyl side chains to the synthesis of monomers 41 and 42, employing Pd(dppf)Cl 2 as a catalyst (Scheme 17).
Silvia Destri and co-workers in 2015 reported the synthesis of a novel poly(3-alkyloxythiophene), bearing a chiral centre with particular reference to the evolution of the optical activity in Table 4 Results for one-pot synthesis of triblock copolymers through sequential living block copolymerization using the Ni(II) complex as a single catalyst passing from true solution to the solid state (ordered powders and lm). 208 Analysis of this compound supplied a strong indication to be possibly used as an inverse chiral probe as it completely lost chiroptical signal aer the crystallization. Kumada catalyst-transfer polycondensation was used to synthesize regioregular poly{3-[(S)-(2-methylbutyloxy)methyl] thiophene} 46, starting from 2-bromo-5-iodo-3-[(S)-(2-methylbutyloxy)methylbutyl]thiophene 45, in which one equivalent of isopropyl-magnesium chloride was added prior to the addition of catalyst Ni(dppp)Cl 2 . A low molecular weight polymer (M n ¼ 3.4 kg mol À1 , nearly 20 repeating units) was obtained aer 24 hours of reaction at room temperature with greater than 90% regioregularity. In order to increase the molecular weight of the polymer, lithium chloride was used along with the Ni(dppp)Cl 2 catalyst, following the procedure reported by Ueda and co-workers for the monomers containing oxyethylene side chains. Use of lithium chloride resulted in number average molecular weight of 10.6 kg mol À1 and polydispersity index of 1.39 (Scheme 18).
This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 4322-4396 | 4339 equivalent of le-and right-handedness. The helicity of these assemblies could be easily tuned by introducing chiral cholesteryl pendants on the polyallene segments. Moreover, same synthetic strategy was applied to synthesize water soluble amphiphilic triblock copolymer PTA-b-P3HT-b-PTA 46, containing hydrophobic P3HT and hydrophilic poly(triethyleneglycolallene) (PTA), which was found to exhibit multiresponsiveness including pH, temperature and solvent (Scheme 19). Results for one-pot synthesis of triblock copolymers through sequential living block copolymerization are summarized in the Table 4.
Chunhui Zhao and co-workers in 2017 utilized the cyclopolymerization technique for the synthesis of polythiophenes for the rst time. 210 Cyclopolymerization is useful for the monomers containing two polymerizable moieties and results in the formation of insoluble cross-linked polymer networks. In cyclomerization, alternating intramolecular-intermolecular chain propagation produces a series of cyclic molecules along the polymer chain and, to achieve this, the monomers are generally designed such that thermodynamically favored ve or six-membered rings are formed. However, with an elaborate monomer design, large macro cycles can be produced as well. Monomer for cyclopolymerization, indicated as GMm (GM ¼ Gemini monomer and m ¼ strap length), was synthesized in seven steps, in which 2-bromo-5-iodo-thiophenes were tethered by alkylene straps and they were able to be processed by catalyst-transfer polymerization (CTP).
Zhi-Peng Yu and co-workers in 2017 reported one pot synthesis of triblock copolymers consisting of poly(3hexylthiophene) (P3HT), poly(phenylisocyanide) (PPI) and poly(hexadecyloxylallene) (PHA) blocks, through three sequential living polymerizations of the corresponding monomers using Ni(II) complex as a single catalyst. 211 Ni(II)-terminated P3HT 49 was rst prepared through the polymerization of 2-bromo-3-hexyl-5-chloromagnesiothiophene 48 with Ni (dppp)Cl 2 [dppp ¼ 1,3-bis(diphenylphosphanyl) propane] as a catalyst in THF at room temperature, following Scheme 23 Synthetic route for head to tail regioregular P3ATTs and P3ASTs. the Kumada catalyst-transfer polymerization (KCTP) mechanism. Aer the completion of the polymerization, the polymer solution was treated with hexadecyloxyallene 50 in THF at room temperature, which was considered on the basis of its good solubility. Aer the formation of Ni(II)-terminated block copolymer 51, third monomer, tert-butyl-4-isocyanobenzoate 52 was added to the copolymer solution under dry nitrogen atmosphere and stirred for 2 hours. Triblock copolymer 53 was afforded aer the subsequent workup of the crude polymer mixture. The properties of the copolymer could be tuned by changing the sequence of the monomers in the polymer chain by changing the order of their addition (Scheme 21). This synthetic protocol also proved advantageous for the synthesis of amphiphilic triblock copolymers. Monomers 48, 54 and 55 were incorporated to prepare triblock copolymer 54 composed of hydrophobic P3HT, hydrophilic poly (triethylene glycol allene) and hydrophilic PPI bearing triethylene glycol monomethyl ether. Interestingly, these amphiphilic triblock copolymers exhibited tunable light emissions with response to various environmental stimuli such as pH, temperature and solvent. Remarkably, white-light emission can be readily achieved in solution, gel, and also solid state (Scheme 22).
Pan Wang and co-workers in 2018, developed sulfonecontaining chiral helical polymers poly-3-(alkylsulfone) thiophene (P3AST) 63 and 64, which conrmed a new design for the creation of large Faraday effect. 212 These polymers exhibited tunable and large Faraday rotations with absolute verdet constants up to (7.63 AE 0.78) Â 10 4 deg per T per m at 532 nm. It was determined that the magnitude and sign of the verdet constant are related to the helicity of the polymer at the measured wavelength. These magneto-optic MO values rival the present record material and also demonstrated that verdet constants can be tuned that would be challenging to obtain using present inorganic materials.
P3ATTs were oxidized with m-CPBA to sulfone-containing polymers SO2-P1 63 and (S)-SO2-P2 64 in >99% yield with an excellent solubility in most of the organic solvents. The respective molecular weights and polydispersity indices of all Scheme 24 Synthesis of end-functionalized regioregular P3HTs, 68a-d via KCTP. the polymers are given in Table 5. The effective molecular weight of sulfone-based polymers was smaller compared to their sulde counterparts, which might be attributed to the conformational differences or undetected degradation from oxidation via m-CPBA.
In 2019, Koomkoom Khawas and co-workers reported the catalytic-initiated Kumada catalyst transfer polymerization (KCTP) protocol for the synthesis of aromatic endfunctionalized, defect-free poly(3-hexylthiophene), P3HTs, with controlled molecular weight. 216 Oxidative addition of aromatic bromide to in situ formed Ni(0) complex of diphenylphosphino propane (dppp) 2 , generated Ni(II) catalytic initiators 66, which were used to synthesize a series of end-functionalized P3HTs having different aromatic end groups (Scheme 21). For the synthesis of Ar-terminated P3HTs (Ar-P3HT), four aromatic bromides namely (4-bromophenoxy)(tert-butyl)dimethylsilane 65a, 4-bromobenzyl bromide 65b, 4-bromotoluene 65c and 4bromoanisole 65d were employed. The reason of considering silyl protected 4-bromophenol as one of the Ar-Br is that phenolic OH end-functionalized P3HT could be obtained by simply deprotection of silyl ether aer the completion of the polymerization.
In order to synthesize nickel catalytic initiator, reduction of anhydrous NiCl 2 was performed using zinc dust in dry DMF, which was followed by addition of diphenylphosphinopropane (dppp) under argon atmosphere to generate Ni(0) complex, Ni(dppe) in situ. Then the solution of aryl bromides 65a-d were added dropwise to the solution of Ni(0) complex at room temperature to synthesize Ni(II) catalytic initiators 66. 2-Bromo-5-chloromagnesio-3-hexylthiophene 67 was synthesized from 2bromo-3-hexyl-5-iodothiophene by metal exchange reaction using isopropylmagnesium chloride. For the polymerization, Ni(II) complex solution was added to the Grignard reagent solution of the monomers to obtain the corresponding polymers 68a-d. Corresponding to 65a, 65b, 65c and 65d respectively (Scheme 24). An overview of the properties of the polymers has been provided Table 6.
The versatile nature of the catalytic initiators consisting of different aromatic groups was exhibited by the initiation of polymerization and these initiators are believed to be useful for the synthesis of different P3HT based architectures and polymer brushes. This protocol is considered to be highly useful for large scale synthesis of conjugated block copolymers, endfunctionalized P3HTs and complex architectures of P3HT, as all the steps are in situ, well connected to each other and do not require any separation and purication of the intermediate compounds. Molecular weight of the polymer could easily be controlled by varying the amount of NiCl 2 with respect to the feeding monomer.
In 2019, Christoph Horn and co-workers reported the synthesis of novel side-chain semi uorinated thiophene monomer 69, synthesized by reacting 3-thiophene ethanol with 4,4,5,5,6,6,7,7,8,8,9,9,9-tridecauorononan-1-ol (Scheme 22). 217 Kumada catalyst transfer polycondensation protocol was employed to synthesize regioregular side-chain semiuorinated thiophene polymer P3sfT 75 with narrow polydispersity index of 1.11, number average molecular weight (M n ) of 25 900 g mol À1 and high regioregularity (>98%). P3sfT exhibited high self assembly and crystallinity in the solid state. The order is even more pronounced compared to the P3HT because of the extension of the side chains by uorinated methylene groups. It was concluded that the order of the backbone polymer was enhanced due to the stronger self-organization of side chains, which resulted in the strong formation of layered structure as well as p-p stacking. P3sfT 75 showed high potential as a semiconductor in organic electronics due to its high selfassembly.

Ni-catalyzed Suzuki, Murahashi and Negishi coupling polymerization
In 2013, Kanta Fuji and co-workers explored Murahashi coupling as a versatile preparative tool for polycondensation of hetero arylenes having extended p-conjugation. 218 Nickel catalyst bearing an N-heterocyclic carbene (NHC) ligand was shown to be highly effective in contrast with the previous reports, in which palladium was an effective catalyst for Murahashi coupling whereas nickel was considered to be less effective transition metal catalyst. Three classes of lithiations including lithium-bromine exchange, deprotonation and halogen dance were exhibited to form lithiated monomers, which were subsequently polymerized using Ni-NHC complex to obtain the corresponding polymers. Only chlorothiophene 76 underwent direct lithiation whilst the polymerization of bromothiophene did not give desired results. Ni-catalyst, NiCl 2 (PPh 3 ) proved highly effective in polymerizing the lithiated chlorothiophene Improved polydispersity index was achieved by using cyclopentyl methyl ether (CPME) as a solvent under similar conditions (PDI ¼ 1.35, M n ¼ 12 500 g mol À1 ) (Scheme 26).
Block copolymer of lithiated monomer could be obtained by an end functionalization of 78. Corresponding block copolymer 79 was obtained by addition of lithiated chlorothiophene to the reaction mixture containing polyarylene 78. In copolymer, monomer ratio (polyarylene/polythiophene) was 1 : 0.90, which was conrmed by 1 H NMR analysis (Scheme 27).
Organolithium species, generated by halogen dance rearrangement, was also found to undergo Murahashi coupling polymerization to yield a new class of polythiophene. Treatment of 2,5-dibromo-3-hexylthiophene 21 with lithium diisopropylamide ( i Pr 2 NLi) in THF induced halogen dance rearrangement at À78 to 0 C leading to the formation of 4-lithiated intermediate 80, which, upon addition of NiCl 2 (PPh 3 ) i Pr, afforded the corresponding polythiophene 81 in 77% yield bearing a bromine substituent at position 4 of the thiophene. It could lead to further transformations of the C-Br bond (Scheme 28).
Negishi-type catalyst-transfer polycondensation (NCTP) protocol was introduced by Goto and co-workers in 2014 for the synthesis of P3HT-b-poly(3-octadecylthiophene), using zincate complex, Bu 4 ZnLi 2 . 219 Furthermore, two-stage polymerization of poly(3-hexylthiophene) was also achieved by NCTP using zincate complex. Polymerization results of NCTP using Ni catalysts with varied phosphine ligands were found to be strongly inuenced by electron donating ability of ligands as well as steric hindrance based on the factors of bite angle and cone angle of Ni-catalysts.
Living nature of NCTP method was conrmed by adding monomer 82 to the remaining solution of the rst P3HT block. Increase in the number average molecular weight from M n ¼ 5060 g mol À1 to M n ¼ 16 100 g mol À1 conrmed efficient chain extension. The same two stage-polymerization protocol was applied to synthesize block copolymer of 3-hexylthiophene with 3-octadecylthiophene using 85 as a second monomer. P3HT-b-poly(3-octadecylthiophene) 86 with M n of 15 000 and dispersity index of 1.08 was afforded with 3HT/3ODT molar ratio of 42/58.
In 2016, Yunyan Qiu and co-workers for the very rst time, reported chain growth Suzuki cross coupling for catalysttransfer polycondensation of ester-functionalized thiophene, employing commercially available nickel precatalysts (Scheme 30). 220 This protocol was exploited for the controlled synthesis of poly(hexylthiophene-3-carboxylate) P3HET (88) and poly(3hexylthiophene) P3HT (90). Borylation of the thiophene ring of the monomers with pinacolborane was achieved using an iridium-catalyzed C-H borylation reaction. 221 Versatility of this method was also illustrated by synthesizing alternating copolymer 92 from borylated thiophene monomer and 3-hexylthiophene. It was revealed that water was necessary for promoting the controlled polymerization of all three monomers. Water from K 3 PO 4 $H 2 O was found to be sufficient for promoting the controlled reaction of monomer 87, while additional water resulted in an excellent control over dispersity and molecular weight of the polymers, produced from monomers 89 and 91.

Pd-catalyzed synthesis of thiophene-based polymers
Palladium-based catalysts are one of the most efficient catalysts for cross-coupling reactions known so far. An avalanche of data is available regarding Pd-catalyzed reactions and progress is still continued. Palladium catalysis has played a vital role in the synthesis of conjugated polymers and various Pd-catalyzed cross-coupling reactions, especially Suzuki and Stille couplings, are widely used in the synthesis of monomers as well as polymers. Some of the recent progress in synthesis of polythiophenes using different Pd-catalysts is reviewed below.

Pd-catalyzed Suzuki-coupling polymerization
Kazuyoshi Watanabe and co-workers in 2012 reported the synthesis of new derivatives of polythiophenes and their phenylene copolymers via introduction of chiral alkoxy substituents into their side chains. 222 These polymers exhibited uorescence ranging from blue to red in lms and from blue to orange in chloroform solutions. Enantiotropic main-chain liquid crystallinity was also shown by these polymers at elevated temperatures. For the polymers comprising upto three aromatic rings in repeating units, bisignate cotton effect was observed in p-p* transition region of CD spectra due to the formation of polymer assembly with an interchian helically p-stacked structures. NBS was used to brominate 2 and 5-position of 3-thiophenecarboxylic acid 97 to afford 2,5-dibromo-3thiophenecarboxylic acid 98, which underwent esterication with (R)-(À)- (2)  Josue Ayuso and co-workers in 2015 reported the synthesis of highly pure N-methyliminodiacetic acid (MIDA) boronate ester thienyl monomer 109 via direct electrophilic borylation of 2,5dibromo-3-hexylthiophene 21, and its subsequent use to obtain regioregular poly(3-hexylthiophene-2,5-diyl), rr-P3HT, by Suzuki-Miyaura polymerization. 223 This approach provides a simple route towards the synthesis of bifunctional monomers required for polymerization reaction, and also avoids the use of unstable boronic acid intermediates during their synthesis. The rigid tridentate MIDA group binds boron strongly to provide an exceptional stability to even electron rich heteroaryl boronate esters under acidic conditions. The hydrolysis of MIDA boronate esters proceeded slowly under mild basic conditions to gradually unmask the active boron transmetallating agent. In this way, the concentration of the sensitive boron species was minimized in the reaction mixture, which in turn, is helpful in In 2016, Hong-Hai Zhang and co-workers reported the rst synthesis of regioregular poly(5-alkyl-2,3-thiophene)s (P5HT) 111, which is an ortho-linked isomer of a well-known conjugated polymer, poly(3-alkyl-2,5-thiophene) (P3HT), via Suzuki-Miyaura cross-coupling polycondensation, using PEPPSI-IPr as a catalyst. 224 Strong repulsion originating from highly angled connections made this synthesis quite challenging, and very few examples of the synthesis of poly(o-arylene)s via direct polymerizations have been reported so far. [225][226][227][228][229] Commercially available thiophene was used to synthesize 2-bromo-5alkylthiophen-3-ylboronic acid pinacol ester 110 through three steps, consisting of alkylation using n-butyl lithium, bromination with NBS and then boronation in gram scale quantities. In situ quenching strategy was used to prevent halogen dance rearrangement during the borylation step (Scheme 35).
Commercially available PEPPSI-IPr was found to be the best overall catalyst, affording polymers 111a,b with narrow polydispersity index and tunable molecular weight ( Table 7). The comparison of UV-visible absorption of P5HT (l ¼ 345 nm) with that of P3HT (l ¼ 450 nm) showed low degree of conjugation in P5HT than in P3HT, which might be a result of helical geometry of the P5HT 111 compared to the P3HT's more planar geometry. Moreover, 111 was also reported to produce green uorescence under UV irradiation (l ¼ 360 nm).
Monomer 112, 2,5-bis(4,4,5,5-tetramethyl-1,3,2dioxoborolane)-3-hexylthiophene, was synthesized by slow addition of n-butyllithium to the solution of 2,5-dibromo-3hexylthiophene 21 in THF and stirring the resulting mixture at À78 C for 2 hours. Aerward, 2-isopropoxy-4,4,5,5tetramethyl-1,3,2-dioxoborolane was added into the reaction mixture and the reaction was stirred at À78 C for additional 1 hour, which was followed by an overnight stirring at room temperature. The reaction mixture was extracted with DCM and puried by column chromatography to obtain the monomer 112. In order to synthesize the polymer 114 via conventional Suzuki coupling protocol, Pd(PPh 3 ) 4 was added to the solution of 113 in toluene, and the reaction mixture was stirred for 72 hours at 90 C. The reaction mixture was extracted with ethylacetate aer cooling to the room temperature, followed by washing with acetone and methanol by Soxhlet extraction for a day to yield polymer 114 in 89%. Microwave-assisted polymerization of the monomers 112 and 113 was carried out by adding tetra-n-butylammonium bromide and Na 2 CO 3 to the solution of 112 and 113 in H 2 O in a microwave tube. Pd(dppf) Cl 2 was added aer bubbling nitrogen gas through the solution and the reaction mixture was heated at 80 C for 15 minutes. Polymer 114 was obtained in 52% yield aer washing with CH 3 OH and decanting.
In 2018, Kantaro Kosaka and co-workers investigated several palladium catalysts having bulky phosphine ligands other then tertiary tributylphosphine (t-Bu 3 P) for Suzuki catalyst transfer condensation polymerization. 231 Reaction of 2,5-dibromothiophene 116 with diphenylboronic acid ester 115 was chosen as a model reaction to investigate the variety of Pd precatalysts. It was found that diphenyl-substituted thiophene was formed exclusively when di-tert-butyl(4-dimethylaminophenyl) phosphine (AmPhos) was used. Disubstituted thiophene 118 was formed preferentially compared to the monosubstituted thiophene 117, because the intramolecular transfer of Pd catalyst on the thiophene occurred aer the rst substitution with 115, demonstrating high transfer ability of AmPhos Pd catalyst (Scheme 37). Polymerization of uorene and thiophene with 4-iodobenzonitrile 119 using an in situ generated 4-cyanophenyl Pd (AmPhos) initiator 121 from AmPhos Pd precatalyst 120, proceeded through CTCP mechanism, yielding polythiophene 122a and polyuorene 122b with controlled polymer ends and low dispersity (Scheme 38).
Block copolymerization of uorene and thiophene with 123 proceeded irrespective of the polymerization order. In block copolymerization, insensitivity of PdAmPhos to the polymerization order suggested possible synthesis of acceptor-donoracceptor and donor-acceptor-donor triblock copolymers via Suzuki-Miyaura CTCP (Scheme 39). Moreover, (tolyl)PdAmphos (Br) 123 was also synthesized, and it was found that the mixture of 123 and cesium uoride (CsF) yielded poly (3-hexylthiophene) with the dispersity index of 1.18 (Scheme 40). Table 8 summarizes yields and molecular weights of all the polymers obtained via polymerization of 122 with AmPhos Pd initiator 123.

Pd-catalyzed Stille-coupling polymerization
In 2012, Zhi-Guo Zang and co-workers reported the synthesis of two novel conjugated polythiophene derivatives PT4TV 133 (Scheme 39) containing thienylene-vinylene (TV) as a side chain and PT4TV-C 136 (Scheme 40) having thienylene-vinylene side chain attaching the carbonyl group via copolymerization of thiophene thienylene-vinylene side chain and unsubstituted terthiophene unit. 232 Side chain isolation approach was employed to preserve backbone planarity, which combined Raja Shahid Ashraf and co-workers in 2014 reported the synthesis of diketopyrrolopyrrole-based copolymers consisting of different chalcogenophenes such as thiophene, tellurophene and selenophene for organic photovoltaic devices and eld effect transistors. 240 The polymer band gaps were reported to be narrowed by increasing the size of the chalcogen atom due to the LUMO energy level stabilization. Moreover, the larger intermolecular heteroatom-heteroatom interactions were also increased by increasing heteroatomic size, which led to an enhanced eld effect mobilities of 1.6 cm 2 (V s) À1 due to the formation of polymer aggregates. All these polymers exhibited high photoresponse in near-infrared region with tremendous photocurrents, making these polymers promising candidates for tandem solar cells.
Pd-catalyzed Stille cross-coupling polymerization was used to copolymerize dibrominated C3-DPPT monomer with bis- In 2014, Iain Meager and co-workers reported the synthesis of a new thieno[3,2-b]thiophene isoindigo (iITT) based monomer unit 151, and its subsequent polymerization with thiophene, bithiophene and benzodithiazole to furnish three new copolymers with narrow band gap semiconducting properties for OFET applications. 242 It was found that extension of the fused ring system attached to the isoindigo core could serve to further enhance molecular orbital overlap along the polymer backbone and, hence, facilitate good charge transport characteristics. All the three newly synthesized polymers showed good ambipolar properties when used as a semiconducting channel in top-gate/bottom-contact OFET devices as well as good stability with high temperature annealing, exhibiting an increase in the crystallinity of the polymers which directly corresponds to the improvement in the charge carrier mobility.
Synthesis of the ilTT monomer is a rst example of conjugated six fused ring isoindigo system.    Both polymers showed chain aggregation even at dilute concentrations. PBTz-Th* 174 was also characterized via CD (circular dichroism) spectroscopy due to the presence of a chiral side chain, while PBTz-Th did not show any response to CD. CD spectra revealed that the chains of 174 are chiral in aggregate, and this chiral ordering was also found to translate from the aggregates in the solution to solid state upon deposition of the solution, due to the relatively small calculated barrier to rotation of the BTz-Th unit.
(S)-2-Ethylhexan-1-ol 169 was synthesized by reduction of a,b-unsaturated aldehyde 168 using Baker's yeast as a catalyst. rotational barrier compared to 180. This higher rotational barrier seemed to limit the ability of 183 to achieve chiral aggregates by adopting helical structure.
Baker's yeast was used as biocatalyst to reduce a,b-unsatu- Organic semiconducting materials based on 1,3-butadiyne unit were reported for the rst time in 2017 by Brian J. Eckstein and co-workers. 249 Alkyl-substituted 1,4-di(thiophen-2-yl)buta-1,3-diyne (R-DTB) donor building blocks were polymerized with thienyldiketopyrrolopyrrole (R 0 -TDPP) acceptor units to afford p-conjugated polymers TDPP-DTB, in which R and R 0 groups were varied to obtain four different polymers (199)(200)(201)(202). The solubility of the newly synthesized polymers was revealed to be strongly dependent upon the substitution pattern of the DTP superior electron transport properties. [254][255][256] Therefore, it was an interesting idea to incorporate p-conjugated PB units and semicrystalline DPP backbone into one material.
Near-infrared absorbing DPP polymer PDPP2TBTD, in which DPP is alternating with benzodithiophene (BD) units, was selected to synthesize a single-component polymer 208. 209 was constructed by tuning the distance between PBI and DPP. In 210 polymer, alkylthiothiophene side chain was introduced on the BDT unit. These structural modications helped to improve the nanophase separation of the polymers, which resulted in a PCE of 2.74% in polymer 210 based singlecomponent organic solar cells, compared to 0.5% in 208 polymer as a photoactive layer.
Starting from the DPP 203, precursors 204a and 204b were synthesized through alkylation and bromination reactions, where the four bromine atoms exhibited distinct reactivity enabling the introduction of perylene bisimide (PBI) side chain into diketopyrrolopyrrole (DPP) molecule with the help of monoalkylated PBI compound 205, affording DPP-PBI monomers 206a and 206b. Stille coupling polymerization was used to synthesize polymers 208, 209 and 210, using distannyl-BDT monomers 207a and 207b containing alkylthiophene or alkylthio-thiophene side chains. These polymers were readily soluble in chloroform. The molecular weight (M n ) determined by GPC in 1,2,4-trichlorobenzene at 160 C was 6.6, 23.5 and 20.7 kg mol À1 for the polymers 208, 209 and 210, respectively. All the polymers exhibited good thermal stability with 5% weight loss at temperature above 340 C (Scheme 62).
In 2018, Bogyu Lim and co-workers proposed a new strategy to synthesize well-dened regioregular alternative D-A polymers, using large molecular weight regioregular monomers (LRMs) 212. 257 A regioregular alternative D-A polymer, rr-PBTTh, was synthesized by a systematic introduction of various conjugated moieties in a single polymer backbone using thiophene as a donor and benzothiadiazole as an acceptor unit. This polymer exhibited a highly planar conjugated backbone because of regioregularity and S/F and S/O intramolecular conformational locks. rr-PBTTh 218a-c displayed well-balanced ambipolar transport characteristics largely due to the balanced D-A molecular structure and showed promising electrochromic performance with coloration efficiency upto 321.7 cm 2 C À1 and rapid response time below 0.5 s.
An intermediate compound 211 was prepared which provided various LRMs through simple coupling and bromination with various dibromo-Ar 1 compounds. A large variety of polymers can be produced by these LRMs with diverse diboronic-Ar 2 (for Suzuki reaction) or distannyl-Ar 2 (for Stille reaction) compound. Mono uorinated benzothiadiazole based intermediate 213 was coupled with 214 via Stille coupling reaction to afford LRM 216. An undesirable homo-coupled byproduct 215 was produced during the coupling reaction of compound 213 and 214. However, relatively high retention factor (R f ) and relatively good solubility of ditrimethylsilyl based by-product 215 in chloroform and DCM eluents for column chromatography explained that the by-product could be removed aer bromination. Finally, the regioregular polymers 218a-c were synthesized by a microwave-assisted Stille coupling polymerization of LRM 217 and distannyl-alkylthiophenealkoxybenzothiadiazole based monomer 217 (Schemes 63 and 64).
Yahui Liu and co-workers in 2018 developed a regioregular wide bandgap polymer as a donor material to enhance the performance of non-urrene organic solar cells. 258 In lms, regioregular polymer reg-PthE 225 (Scheme 66) showed closer packing of the polymer backbone and a large absorption coef-cient as compared to the corresponding random polymer ran-PthE 221 (Scheme 65). Devices based upon reg-PthE:FTIC realized a high power conversion efficiency (PCE) of 12.07%, while the devices based on ran-PthE:FTIC achieved PCE of 9.89%. With ITCC as an acceptor, PCEs of 11.21% and 8.38% were achieved for reg-PThE and ran-PthE, respectively. Semitransparent organic solar cells having reg-PthE:FTIC, as an active layer, exhibited a PCE of 8.69% and an average visible transmittance of $25%.  a All the active layers were spin-coated following the same method, resulting in a similar thickning of $100 nm. b The EQE-integrated J sc . c The statistic parameters were obtained from 10 individual devices.
This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 4322-4396 | 4369 compared to 230. These results suggested that the random polymer could be benecial for the improvement of photovoltaic properties of conjugated polymers for polymer solar cells.
The monomer 227, used as a starting material for coupling reactions, was synthesized by 1 eq. of ROBT 225 and 2.5 eq. of TT 226. In order to avoid over polymerization of 227, the   Table 9, whilest an overview of the PSCs properties of the polymer based on the polymers:PC 71 BM is given in Table 10.

Application of Pd-catalyzed direct arylation polymerization towards synthesis of thiophene-based polymers
Conventional carbon-carbon bond forming cross-coupling reactions such as Suzuki, Kumada, Negishi and Stille are of great relevance in organic and polymer chemistry. Hovewer, the use of various organometallic reagents is considered as a drawback of these coupling reactions because of the stoichiometric amount of byproducts formed during the coupling reactions. These reagents and the resulting byproducts are oen toxic and environmentally risky, especially stannyl derivatives. Synthesis of the organometallic monomers oen consists of multiple steps, purication of which is quite tedious. In order to address these shortcomings, a new synthetic approach, called direct arylation polymerization (DArP), came into the focus of attention of scientic community and has been developed to a great extent during the last few years. DArP offers a simple, relatively inexpensive and environment friendly protocol to construct C-C bond between an (hetero) aryl halide and a non-preactivated (hetero) aryl by directly activating one of its C-H bonds. [260][261][262] This synthetic strategy eliminates the need of functionalizing the monomers with expensive organotin and organoboron derivatives which are essential for Stille and Suzuki cross- coupling polymerizations. This development resulted in up to 35% reduction in the fabrication cost of organic electronics. 263 DArP is highly sustainable and attractive for the large-scale production of conjugated polymers due to high atom economy and few, if any, toxic byproducts. 264,265 Well-dened and high molecular weight conjugated polymers are now available with a courtesy of this approach and some of the very recent example are discussed herein.
Patrick D. Homyak and co-workers in 2013 reported the use of direct arylation polymerization to synthesize new low bandgap poly(thienothiophene-alt-dithienylbenzodithiophene) (PTB) polymers, 268 by reacting dithienylbenzodithiophene with thieno [3,4-b]thiophene acceptor blocks containing phenyl, octyl, peruorophenyl and peruorooctyl side groups. The strongly electron withdrawing peruorophenyl and per-uorooctyl were observed to signicantly lower the energy of both HOMO and LUMO levels. These materials showed favorably aligned energy levels compared to conventional fullerene type acceptors.
The monomers with non-uorinated monomeric units, 237 and 238, were synthesized by selective coupling of 3,4-dibromothiophene 233 with terminal alkyne 234 via Sonogashira coupling reaction yielding compounds 235 and 236 with 44% and 43% yields, respectively. Subsequent cyclization of 235 and 236 with sodium sulde in the presence of CuO at elevated temperature afforded 237 and 238 with 44% and 22% yields, respectively (Scheme 69).
In 2015, Eisuke Iizuka and co-workers reported a novel catalytic system for the synthesis of alternating copolymers of dithienosilole (DTS) 248 and thienopyrroledione (TPD) 249 via Pd-catalyzed direct arylation polymerization approach. 275 Combination of P(2-Me 2 NC 6 H 4 ) 3 (L2) and P(2-MeOC 6 H 4 ) 3 (L3) ligands was found to be very effective in preventing structural defects as well as formation of the side products. The polymer (DTS-alt-TPD) 250 was prepared by Migita-Stille cross-coupling polymerization and was reported for the rst time in 2011.
Attempts to make this polymer via direct arylation polymerization (DArP) resulted in the formation of insoluble byproducts, and the synthetic results were reported to be difficult to reproduce. The solution to this problem was found in the use of a mixture of two ligands, P(2-Me 2 NC 6 H 4 ) 3 (L2) and P(2-MeOC 6 H 4 ) 3 (L3), which proved very efficient and sufficiently reactive even in toluene as non-polar solvent and produced a copolymer with number average molecular weight of 15 000 g mol À1 (Scheme 73).
In 2015, Wei Lu and co-workers reported the use of direct arylation polycondensation for the synthesis of highly crystalline bithiazole-based donor-acceptor type copolymers 253a-c, where bithiazole 252 served as an acceptor unit while 3,4-ethylenedioxythiophene (EDOT) derivatives 251 were employed as donors. 276 Installment of long chain alkyls on the bithiazole monomers increased their solubility in the solvents used for polymerization reaction and, thus, proved helpful in obtaining high molecular weight polymers (Scheme 74).
Patrick D. Homyak and co-workers in 2016 presented a strategy for tuning the physical properties of P3HT-based copolymers by incorporation of folurinated thiophene repeating units. 277  Five copolymers 257a-d, were synthesized by changing the 253 : 254 feed ratio with the 0, 25, 50, 75 and 100% of uorinated monomeric unit corresponding to the P0, P25, P50, P75 and P100, respectively (Scheme 76). An overview of the composition, molecular weight and regioregularity of P(3HT-co-3H4FT)s is given in Table 11. Electronic properties of the polymers were strongly affected by increasing uorination as evidented by the decrease in the E HOMO level by 0.4 eV for P100 as In 2017, Amsalu Efrem and co-workers developed a direct arylation polymerization (DArP) protocol for the synthesis of high performance, narrow bandgap donor-acceptor conjugated polymer 263, composed of alternating alkylquaternarythiophene and 5,6-diuoro-2,1,3-benzothiadiazole units. 278 A series of DArP optimization led to the target molecule with a number average molecular weight (M n ) of 14.6 kDa without noticeable b-branching effects and homocoupling.
3-(2-Octyldodecyl)thiophene 258 was coupled with 4,7dibromo-5,6-diuorobenzothiadiazole 259 to obtain 4,7-bis(4-(2-octyldodecyl)thiophen-2-yl)-5,6-diuoro [2,1,3] benzothiadiazole 260, which was further brominated with NBS to generate monomer 259, which was polymerized with 2,2 0bithiophene (BT) 262 to afford polymer 263. DArP protocol produced maximum M n average when the reaction was conducted in the presence of Pd(OAc) 2 using (o-MeoPh) 3 P as a ligand, K 2 CO 3 as a base, PiVOH as an additive and o-xylene as a solvent at 100 C for 72 hours. Alternating polymer structure and C-H selectivity of polymer 263, synthesized via direct arylation polymerization, are comparable to those of the same type polymers, prepared using Stille coupling, and despite of their lower molecular weight as compared to the polymers synthesizes via Stille coupling polymerization, showed better performances in OPV and OFET devices tested in air without any device encapsulation (Scheme 77).
3-Dodecylthiophene 264 was brominated to afford 2-bromo-3-dodecylthiophene 265, which, upon reaction with 2-bromo-5-  Polymers synthesized using DHAP strategy showed essentially the same thermal and optical properties and are comparable to those observed with their analogues, obtained from Stille coupling and oxidative polymerization, while several synthetic steps were skipped compared to traditional Stille and Suzuki polymerization since the use of stannyl and boronate (toxic waste) comonomers are no longer required in this method.
Shotaro Hayashi and co-workers in 2017 reported direct arylation polycondensation of b-unprotected chalcogen heteroles including thiophene, furan and selenophene under phosphine-free conditions. 280 Phosphine-free polymerization is one of the low cost and green pathways towards the synthesis of p-conjugated polymers. Polycondensation of thiophenes and furans produced insoluble polymers, resulting from b-defects and network formation, when Pd(OAc) 2 was employed as a catalyst. Carbon-supported palladium catalyst, Pd/C, was found to be effective in performing polymerization of thiophenes and furans. The solid-supported palladium catalyst increased the regioselectivity of the reaction at the a-position of thiophene and prevented the formation of b-defects.
M. Wakioka and co-workers in 2017 reported the effect of mixed-ligand catalyst system towards Pd-catalyzed direct arylation polymerization. 281 The combination of P(2-MeOC 6 H 4 ) 3 (L4) and N,N-tetramethylethylenediamine as a ligand effectively prevented defect formation in the polymerization of diketopyrrolopyrrole (DPP) units to give donor-acceptor copolymers.
Homocoupling defects decreased to 1.6% when L4 was used in combination with TMEDA. Use of this ligand mixture also resulted in complete suppression of the formation of insoluble material, and the molecular weight of the polymer 283 also increased remarkably (M n ¼ 24 500 g mol À1 ). This mixedligand approach was also proved to be effective for the polymerization of 280 with thienopyrrolidione 279 to yield 281 (M n ¼ 36 800 g mol À1 ) and 3,4-propylenedioxythiophene 285 to obtain 286 (M n ¼ 19 000 g mol À1 ), donor-acceptor polymers (Scheme 81).
In 2017, Carl Roy and co-workers devised an efficient strategy for the synthesis and purication of novel mono-uorinated derivatives of thiophene 289 and 291 for synthesizing uorinated dithienobenzothiadiazole (DTBT) comonomers. 282 It was observed that the amount and the position of uorine atoms on the DTBT moiety could tune the regioselectivity and reactivity of direct (hetero) arylation polymerization (DHAP). Polymerization time for the uorinated DTBT monomers reportedly decreased to only few minutes from 66 hour, required for the polymerization of non-uorinated DTBT. Polymerization of the reaction mixtures containing 296 and 298 occurred rapidly (11 and 40 minutes, respectively) to give 300d and 300b, while 300a and 300c formed at slow rate (66 and 46 hours, respectively). 300d and 300b were found to be soluble in hot dichlorobenzene and 300a and 300c were soluble in DCM at room temperature. This difference in the reactivity of all the four monomers and their corresponding polymers is attributed to the difference in the amount and the position of uorine installed on the anking thiophene of the DTBT moiety (Schemes 82-84).
Colleen N. Scott and co-workers in 2017 reported the synthesis of three novel 2,5-dithienosilole with a series of diuorobenzodiimine-based acceptors, 5,6-diuoro[2,1,3]benzotriazole (DFBTA), 5,6-diuoro[2,1,3]benzothiadiazole (DFBT) and 5,6-diuorobenzoselenadiazole (DFBSe) using direct arylation polymerization reaction, which was considered as the rst known example of synthesis of 2,5-dithienosilole-based polymers. 283 Only oligomers with low molecular weight were yielded when Stille cross-coupling polycondensation reaction was performed to check the structural quality of the polymers, demonstrating the power of DArP protocol to synthesize polymers consisting of strongly electron-decient monomeric units. Newly synthesized polymers 306a-c had reduced band gaps (<2.0 eV), and the hole mobility values in the range of 10 À2 cm 2 (V s) À1 was provided by polymer 306a, which was superior compared to the previously reported values for the 2,3dithienosilyl-based polymers (10 À4 to 10 À6 cm 2 (V s) À1 ) and comparable to the dithienosilole-based polymers. Diol 301 was alkylated using 1-bromo-2-ethylhexane to yield compound 302, which was then capped with thiophene by Sonogoshira reaction in the presence of 2-bromothiophene to afford compound 303. Silol ring structure 304 was formed by a nickel mediated Kumada-type intramolecular cyclization, which was followed by bromination with NBS to yield monomer 305 (Scheme 85).
Polymers 306a-c, with moderate molecular weight, were obtained using Pd 2 (dba) 3 $CHCl 3 catalyst, phosphine ligand and K 2 CO 3 with pivalic acid. Relative molecular weights are in Table  12, which were measured by size exclusion chromatography using polystyrene as a standard. The much lower molecular weight of 306c could be the result of selenium (Se) atom inhibition of polymerization, presumably due to the coordination of Se atom with the Pd centre 284 (Scheme 86).
In 2018, Ludi Deng and co-workers synthesized a series of poly(3-alkylesterthiophenes) (P3ET) with high head-to-tail (HT) coupled regioregularity upto $92% via direct heteroarylation polymerization. 285 The role of the alkyl-ester side chains on the C-H coupling polymerization as well as their effects on the optical and electrochemical properties and crystallinity were Scheme 88 Synthesis of monomer 2-bromo-3-hexyl-thienyl-5-gold 310 by C-H activation.  investigated by changing the length and size of the alkyl groups. It was found that higher head-to-tail regioregularity was obtained with larger alkyl ester side chains on position 3 of the thiophene monomers. All P3ET products 309a-f were found to have low crystallinity and particularly lacked order in the pstacking direction in the solid state due to the steric hindrance of ester substituents. The lower ionization potential of the poly(3-alkylesterthiophenes) 309, compared to the corresponding poly(3-alkylthiophene), suggested that they could be potential candidates for p-type materials. Synthesis of the monomer was started by addition of suspension of thiophene-3carboxylic acid 307 in anhydrous alcohol, with a subsequent Scheme 90 Synthesis of monomer 314. addition of trimethylsilylchloride (TMSCl) via syringe. The reaction mixture was stirred at 80 C for 24 hours. For polymerization, potassium acetate (KOAc), silver carbonate (Ag 2 CO 3 ) and DMAc were added to the monomers 308a-f, and the reaction mixture was stirred at 110 C for 10 minutes, then degassed with argon, which was followed by addition of Pdcatalyst and stirring at 110 C for 48 hours to afford poly(3alkylesterthiophenes) 309a-f (Scheme 87). Various palladium catalysts were employed to examine their inuence on direct arylation polymerization reaction for the synthesis of P3ET derivatives. Effect of catalyst types on the yield, polydispersity index, regioregularity and molecular weight of the resultant poly(3-octylesterthiophene) 309b, are displayed in Table 13. Yield, regioregularity, molecular weight and dispersity for DCHCP on poly(3-alkylesterthiophenes) are given in Table 14.
In aforementioned examples of palladium-catalyzed synthesis of thiophene-based polymers, polymerization proceeded via step-growth mechanism. Developing a palladiumcatalyzed synthesis proceeding through chain-growth polymerization could be more effective in controlling molecular weights and dispersities of the polymers. Development of dual metal catalysis with orthogonal reactivity could be an effective pathway for this purpose. Luscombe's research group presented their ndings for living polymerization of poly(3hexylthiophene) in 2016. Sabin-Lucian Suraru and co-workers reported dual metal catalysis using gold and palladium. 286 Initially, gold was used for the C-H activation of the monomer, 2-bromo-3-hexylthienyl-5-gold 310, which then underwent palladium catalyzed cross coupling via chain growth polymerization to yield regioregular polythiophene without the use of sensitive organometallic reagents based on base metals such as Grignard reagent. Auration of 2-bromo-3-hexylthiophene 20 occurred by reacting it with chloro(tri-tert-butylphosphine)gold(I) using grounded NaOH in dioxane at 50 C for 48 hours to obtain the desired monomer 310 in 82% yield. Moreover, the same reaction was also performed in the presence of THF and NaOt-Bu in order to match the reaction conditions of C-H activation with that of chain growth polymerization of P3HT 16 more closely (Scheme 88).
Organo gold monomer 310 was polymerized at 60 C in THF to provide good solubility to the monomer and avoid precipitation of the polymeric product during the course of the reaction. The use of Pd-PEPPSI-iPr catalyst resulted in a very good yield, while Pd(PPh 3 ) 2 Cl 2 and combination of Pd 2 (dba) 3 with dppp or PtBu 3 did not show good activity and resulted only in the formation of short oligomers. This improved performance of PEPPSI catalyst compared to phosphine based ligands, was speculated to be the result of strong sigma donating character of N-heterocyclic carbenes (NHCs) as compared to phosphines (Scheme 89).
Polymerization step was performed by altering monomercatalyst ratio and four different reactions were conducted using 1, 2, 3 and 5 mol% of Pd-PEPPSI catalyst. Results are given in Table 15, indicating a remarkable agreement with the theoretical molecular weight M th n , expected for a living chain growth polymerization where the Pd catalyst does not leave the growing polymer chain.

Palladium-catalyzed Negishi coupling polymerization
In 2018, Marie-Paule Van Den Eede and co-workers investigated the chiral expression and supramolecular organization in tailormade conjugated polymers P3Ats, which were synthesized using Pd-RuPhos protocol. 287 Following two new parameters, effect of the chiral expression and supramolecular organization were determined.
(1) End-groups are required to break the symmetry in block copolymers comprising (R)-chiral and (S)-chiral blocks of equal lengths. Polymer chains did not aggregated without an endgroup.
(2) The supramolecular aggregation is completely disrupted if one single head-to-head (HH) coupling is present in regioregular poly(thiophene). This observation was in contrast to the presence of one tail-to-tail (TT) defect.
While the synthesis of the monomers 311-313 were available in the literature, 288,289 the monomer, 314 was synthesized for the rst time. For this synthesis, in situ prepared tetramethylpiperidinyllithium was employed to deprotonate 314a at position 5 and subsequent quenching with CBr 4 yielded 314b. PhI(OAc) 2 and I 2 were added to the solution of 314b in DCM to yield 314 (Scheme 90).
All the polymers were prepared via Negishi coupling polymerization. For the synthesis of polymers 316-321, initially, a polymerization method not incorporating an end group different from the rst bolck's monomeric unit was required. Since Kumada catalyst transfer polymerization of thiophene Scheme 92 Synthesis of 325 and 327. Table 16 An overview of M n and Đ of polymers obtained by GPC, the degree of polymerization DP calculated from the GPC data, and the DP m and n obtained by 1  block copolymers using a standard external o-tolyl initiator resulted in block copolymers with an o-tolyl end group, another external initiator was needed. [290][291][292][293][294][295] Thus, for the polymers 320 and 321 different reaction method was applied as an HH coupling needs to be formed in the beginning of the second block. Kumada catalyst transfer polymerization based on Ni(dppe) or Ni(dppf) catalyzed HH-coupling at much slower rate compared to HT-couplings, 295 which would result in the mixture of homopolymers of the rst block and diblock copolymers with a very long second block. However, Pd-RuPhos polymerization based on deactivation of monomer was developed. 296 It has also been demonstrated that HH-couplings could form easily and at a rate similar to the HT-couplings. The origin of the controlled nature of this polymerization was deactivation of the monomer, which could dissociate from the growing polymer, leaving a Brterminated chain. Reinsertion resulted in a further growth. For the polymerization; solution of monomer 311 underwent Grignard metathesis (GRIM) reaction with t BuMgCl for 60 minutes. Reaction of 311a with ZnBr 2 solution yielded organozinc monomer 311b, which was added to the solution of initiator 315 and polymerized for 50 min. Acidied THF was used to terminate this polymerization to obtain polymer 316. Predetermined quantities of precursor monomer 312 were transformed into the corresponding organozinc compound 312b to synthesize polymers 317-319. 312b was added to the rst (S)chiral block. These polymerizations were also terminated by acidied THF aer 50 min. For the synthesis of polymers 320 and 321, same procedure was followed as for 317-319, but with the precursor monomers 314 and 313 for the formation of the second block (Scheme 91). An overview of M n and Đ of the polymers 316-321 is given in the Table 16. There are few polymers that could be synthesized using three different protocols, namely Suzuki, Stille and direct arylation polymerization (DArP) reaction. In 2014, Ichiro Imae and coworkers reported the synthesis of poly(quinquethiophene), that is poly(3,3 000 -dihexyl-3,4 0 ,3 000 ,4 000 -diethylenedioxy-2,2 0 :5 0 ,2 00 :5 00 ,2 000 :5 000 ,2 0000 quinquethiophene),partially containing 3-hexylthiophene and 3,4-ethylenedioxythiophene (EDOT), via Suzuki, Stille and DArP protocol. 297 Among them, direct C-H coupling reaction produced the polymer with the highest molecular weight. The polymer was soluble in common organic solvents and a signicant red shi was observed with uorescence and absorption spectra compared to the corresponding monomeric unit due to enhancement of p-conjugation length. In order to synthesize 325, n-butyllithium was added to the solution of 3,4-ethylenedioxythiophene 322 in THF at À78 C to obtain lithiated 3,4-ethylenedioxythiophene 323, which was followed by addition of tri(n-butyl)tin chloride to afford 322. The product 322 was then added to a solution of 2,5-dibromothiophene and Pd(CH 3 ) 4 to yield 325. n-Butyllithium was added to a solution of 325 in THF at À78 C, followed by a subsequent addition of tri(n-butyl)tin chloride at À78 C to afford 326, which was added to a solution of 2-bromo-3-hexylthiophene 20 to afford 327 (Scheme 92).
Monomer 328 was synthesized by adding n-butyllithium to the solution of 327 and stirring the resulting mixture for 1 hour at À78 C, which was followed by a subsequent addition of 2isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and further stirring the reaction mixture at À78 C to afford 328 monomer in 16% yield. Same sequence of steps was applied for the synthesis of monomer 329 by the addition of tri(n-butyl)tin chloride and was used for the polymerization reaction without any purication due to the instability of 329. Monomer 330 was obtained in 47% yield by addition of CHCl 3 /CH 3 COOH solution of NBS to HE5T in CHCl 3 /CH 3 COOH and purifying the residual product via column chromatography (Scheme 93).
In order to investigate the protocol yielding 327 with higher molecular weight, three types of polymerization reactions were applied to synthesize polymer 331. Application of direct arylation polymerization reaction resulted in higher molecular weight of polyHE5T (M w ¼ 63 000), while Suzuki and Stille coupling polycondensation reactions resulted in yielding low molecular weight polymer 331 with M w ¼ 22 000 and 28 000, respectively, which was reported to be due to difficulty in the purication of 328 and 329 arising from their low stabilities (Scheme 94).
In 2017, Shu-Wei Chang and co-workers reported the synthesis of double acceptor copolymers 338, 338-DA, 339, 339-DA, 340-342, 342-DA, 343, 343-DA, 344, 344-DA, 345 and 345-DA, consisting of thieno [3,4-c]pyrrole-4,6-dione (TPD), benzothiadiazole (BT) and two cyclopentadithiophene (CPDT) units, and a series of copolymers consisting of CPDT-A-CPDT (A ¼ acceptor: BT or TPD) with different number of thiophene units, prepared by a combination of Stille and Suzuki coupling, oxidative and direct arylation polymerization. 298 Hybridized features were demonstrated by the double acceptor copolymer due to the presence of both TPD and BT. Band gap for both BT and TPD series was increased by increasing the number of electron donor units. Higher OPV performance was exhibited by these donor-accepter alternating copolymers, compared to other polymers with power conversion efficiency of 3-4%. Absorption of wide-range visible light by the active layer consisting of double acceptor polymers and fullerene derivatives exhibited subtractive color, found to be advantageous for the development of transparent building-integrated organic photovoltaics.
Monomers 334 and 335 consisting of two CPDT 332 and one TPD or BT units were synthesized by direct arylation of dibrominated BT 171 or TPD 333, using Pd(OAc) 2 , PivOH and K 2 CO 3 in DMF for 2 hours at 80 C. These monomers were further brominated with NBS to yield monomers 336 and 337 for subsequent use in Suzuki or Stille coupling polycondensation or direct arylation polymerization (Scheme 95). Results of the polymerization reactions are enlisted in Table 17. Direct arylation polymerization resulted in low molecular weight copolymers, which might be a result of low reactivity of 334 and 335 as compared to CPDT that resulted from the presence of electron-decient TPD or BT units. Use of harsh reaction conditions such as increased temperatures and longer reaction times could lead to the formation of insoluble branched polymeric product due to the presence of multiple reactive protons on CPDT 332. Moreover, CPDT-BT-CPDT 334 showed higher molecular weight than their CPDT-TPD-CPDT 335 counter parts. The monomers containing carbonyl moiety were reported to be effective coupling partners in Stille reaction, while they are less effective in direct arylation due the presence of base in the reaction mixture, which would affect carbonyl groups on TPD (Scheme 96).

Conclusion
Transition metal catalyzed polymerization has made tremendous contribution towards the synthesis of novel polymers of both isolated and fused ring thiophenes. Preferably, substituted thiophene monomers are used to enhance their solubility in organic solvents employed for their polymerization as well as characterization. Palladium and nickel-based catalysts have proved very helpful in the regioregular synthesis of homopolymers as well as block and copolymers of thiophene-based monomeric units with useful optical and conducting properties to be used in organic photovoltaics, eld effect transistors, light emitting diodes etc. Dehydrobrominative C-H functionalization polycondensation and Kumada catalyst-transfer polymerization (KCTP) are extremely fast growing synthetic strategies that have shown a great control over the polymerization of thiophene-based monomers but still more effort is needed to explore their full potential for the polymerization of more complex monomers. Direct arylation polymerization (DArP) is being developed to combat the shortcomings of conventional methods used for polymer synthesis. DArP protocol is economical to adopt because it consists of lesser number of steps as it excludes the synthesis of stannyl and boronate comonomers to afford desired polymeric products with the same optical and thermal properties as of the polymers synthesized by Suzuki and Stille coupling reactions. Hence, it also avoids the formation of toxic byproducts during the proceedings of the reaction and uncomplicate the purication process of the nal product. It has been more than twentyve years since the rst synthesis of regioregular polythiophene but this eld is still growing and is believed to have a bright future especially in the area of plastic electronics.

Conflicts of interest
There are no conicts to declare.