Synthesis of highly syndiotactic polymers by discrete catalysts or initiators

Abstract

Progress in the synthesis of highly stereoregular polymers with syndiotacticity ≥90% from an array of monomer substrates is surveyed, with a focus being placed on the use of syndiospecific discrete catalyst or initiator systems also exhibiting high activity and efficiency in the polymerization reactions. The monomer scope encompasses nonpolar α-olefins (propylene, styrene, and higher α-olefins), conjugated diolefins, bicyclic olefins, polar conjugated olefins (acrylic monomers such as methacrylates and (meth)acrylamides), and cyclic esters (lactides and β-lactones), while the polymerization catalysts or initiators enabling such syndiospecific polymerizations cover those discrete molecular complexes of main-group, early transition, and lanthanide metals. Several free-radical polymerization systems capable of producing highly syndiotactic polymers are also highlighted.

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Garret M. Miyake

Garret Miyake received his B.S. from Pacific University in 2005 and Ph.D. from Colorado State University in early 2011 under the direction of Professor Eugene Chen. He was an NSF-EAPSI fellow, carrying out his summer research in 2009 in the laboratory of Professor Eiji Yashima at Nagoya University, Japan. Since March 2011, he has been a postdoctoral fellow working with Professor Bob Grubbs at Caltech. His research interest is in the area of polymerization catalysis for the precision synthesis of chiral and sustainable polymers by transition-metal, main-group, and organic catalysts.

Eugene Y.-X. Chen

Eugene Chen is a Professor of Chemistry and Engineering (adjunct) at Colorado State University. His current research interests include: precision synthesis of stereoregular polymers through development of new stereoselective catalysts or polymerizations; renewable energy & sustainability through catalytic conversion of nonfood biomass into fuels or chemicals and polymerization of renewable feedstocks into environmentally sustainable polymers; and nanostructured materials through in situ synthesis and synergistic assembly of organic and inorganic nanocomposites or hybrid polymers.

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Introduction

Polymeric materials are the most important synthetic materials to modern society, exhibiting vast ranges of materials properties that enable their application in countless venues. An impressive repertoire of polymerization techniques have been developed for the synthesis of well-defined polymers through various ionic (anionic and cationic),^1–4 radical,^5–9 and coordination^10–13polymerization mechanisms. Such advancements have allowed for the polymerization of diverse classes of monomers to polymers with specific target applications, accessible by the vast range in polymer properties, governed not only by the chemical composition of the monomer building blocks but equally so in how these building blocks are assembled together. This assembly gives rise to the potential to produce stereoregular, crystalline polymers exhibiting enhanced properties, as compared to their non-stereoregular, amorphous counterparts. Thus, the development of rapid and efficient syntheses of target-oriented polymers still represents a major goal in polymer science.

This review focuses on the polymer tacticity control by discrete catalyst or initiator systems, a topic of ever growing interest.¹⁴ The tacticity of a polymer, or the relative stereoregularity of stereocenters within a polymer's main-chain, determines its overall crystallinity. A tactic polymer folds and packs its chains in an ordered fashion to render crystallinity, while an atactic polymer lacks any long range order and is amorphous. Hence, tacticity is directly related to the polymer's physical and mechanical properties such as melting transition temperature (T_m), glass-transition temperature (T_g), resistance to solvent and fatigue, as well as modulus and impact strength. In the case of water-soluble, stereoregular polyacrylamides, the polymer tacticity also influences the polymer's effectiveness to act as kinetic hydrate inhibitors in oil field applications.¹⁵

More specifically, this review concentrates on the synthesis of highly syndiotactic polymers by discrete catalyst or initiator systems. Chart 1 depicts mono- and disubstituted tactic vinyl and vinylidene polymers showing the stereogenic center sequence distributions, where mmmmmmmm, rrrrrrrr, mrmrmrmr, and mmmmrrrr correspond to isotactic (it), syndiotactic (st), heterotactic (ht), and it-block(b)-st stereoblock (sb) stereomicrostructures, respectively. Exemplifying the effects that tacticity plays on the properties of a polymer, poly(methyl methacrylate), PMMA, exhibits a wide range of T_g's,¹⁶ which are determined by its stereomicrostructure: ca. 55,¹⁷ 87,¹⁷ 110,¹⁷ 130,¹⁸ and 140 °C¹⁹ for highly isotactic (96% mm), isotactic-b-syndiotactic (46% mm, 46% rr), syndio-biased atactic (60% rr), syndiotactic (81% rr), and highly syndiotactic (95% rr) PMMA, respectively.

Chart 1 Four stereomicrostructures of tactic 1- or 1,1-disubstituted vinyl or vinylidene polymers.

This example of greatly enhanced thermal properties of syndiotactic PMMA, as compared to other tactic PMMA, highlights the importance of exploring all stereo-forms of stereoregular polymers, particularly syndiotactic nonpolar and polar polyolefins. The synthesis of highly syndiotactic, technologically important nonpolar polyolefins,²⁰ especially, st-polypropylene (st-PP)^21–23 and st-polystyrene (st-PS)^24–27 has been thoroughly investigated and reviewed. In 1955, Natta discovered that classical Ziegler–Natta catalysts produced PP containing fractions of highly syndiotactic PP that can be isolated.²⁸ However, highly syndiotactic PS was not achievable by such catalysts, and its first synthesis was realized only 30 years later by Ishihara and co-workers, utilizing half-sandwich organotitanium complexes activated with organoaluminum compounds such as methylaluminoxane (MAO).^29–31 Since these initial reports of the synthesis of st-PP and st-PS, numerous successful approaches or catalyst systems have been developed towards their synthesis under industrial conditions and several of them have been commercialized, which have been recently and thoroughly reviewed prior (vide supra). Hence, a comprehensive review on the synthesis of st-PP and st-PS is beyond the scope of this survey, although a brief history and significant recent advancements will be highlighted. In contrast, the synthesis of highly syndiotactic polar conjugated olefin (acrylic) polymers, such as st-PMMA,⁴ has been limited to low temperature (typically at −78 °C or lower) anionic polymerization, and its synthesis by chiral catalyst-site-controlled coordination polymerization at industrially desirable conditions (i.e. ambient or above temperatures) has been achieved only very recently.¹⁰ The ability to synthesize highly syndiotactic polar polyolefins under industrial settings will lead to an expansion of the applications of such polymers.

Accordingly, the goal of this article is to survey reports in the synthesis of highly syndiotactic polymers, where the term “highly syndiotactic” is arbitrarily defined by the authors as a polymer exhibiting racemic triads of rr ≥ 90%. This review is organized by polymer type, including polymers from nonpolar α-olefins (propylene, styrene, and higher α-olefins), conjugated diolefins, bicyclic olefins, polar conjugated olefins (acrylic monomers such as methacrylates and (meth)acrylamides), and cyclic esters (lactides and β-lactones), which is further subdivided, when accomplished, by polymerization mechanism. It should be pointed out that, although most of the catalyst species described herein are only active for polymerization in their cationic or other active forms derived from the activation of the precatalyst with an appropriate cocatalyst (activator), the term catalyst will be used to describe both neutral and cationic or other active forms. In addition, where the term “catalyst” is used for living/controlled coordination polymerization of polar conjugated olefins, it emphasizes the catalyzed monomer enchainment. In this context, it is a catalyst when emphasizing the fundamental catalytic event of monomer enchainment (i.e., the propagation “catalysis” cycle), but it is not a “true” catalyst if the catalytic production of polymer chains is concerned. Furthermore, several other characteristics of the polymerization and polymers will be described, including: polymerization medium (solvent), temperature (T_p), time (t), turnover frequency [TOF, defined as mole of monomer (M) consumed (or polymer formed) per mole catalyst (or initiator, I) per h], and catalyst or initiator efficiency [I^*, defined by I^* = M_n(calcd)/M_n(exptl), where M_n(calcd) = molecular weight (MW) of M × [M]₀/[I]₀ × conversion % + MW of end groups and M_n (M_w) = number (weight)-average MW], as well as polymer MW (M_n and M_w) and MW distribution (MWD or polydispersity index, PDI = M_w/M_n). To provide a means of directly comparing different polymerization systems to each other, polymerizationactivity in this review has been converted to TOF values with the same unit of h⁻¹. Lastly, a well-definedpolymer is characterized by a high degree control on its chain structure (regiochemistry and MW characteristics such as M_n, M_w, and MWD), topology (linear, branched, dendritic, etc.), or stereomicrostructure (tacticity or helicity), all of which can have drastic impact on the materials properties of polymer.

1. Polypropylene

Syndiotactic PP is a crystalline thermoplastic material that exhibits lower T_m (150–155 °C) and crystallization rates, but greater impact strength, higher flexibility, and better optical clarity, than its isotactic counterpart, governed not only by the stereoregularity but also through the error sequences which in tandem dictate the crystallization kinetics and morphology.³² In 1988, Ewen and co-workers discovered that C_s-symmetric metallocene dichlorides (1, Chart 2) bearing an isopropylidene (Me₂C<) bridged Cp-Flu (Cp = cyclopentadienyl, Flu = fluorenyl) ligand set, upon activation with excess MAO, are highly active catalysts for the syndiospecific polymerization of propylene at ambient or higher temperatures leading to highly syndiotactic, fully regioregular PP.^33,34 The polymerization of liquid propylene at 25 °C catalyzed by 1 is highly active (TOF = 4.76 × 10⁵ h⁻¹) and produces st-PP (86% rrrr) with a M_W of 133 kDa and a PDI of 1.9. Increasing the polymerization temperature to 50 °C resulted in a dramatic increase in activity by ∼15-fold (TOF = 7.12 × 10⁶ h⁻¹), which was accompanied by a decrease in polymer MW (M_w = 69 kDa) and syndiotacticity (81% rrrr). The activity of the Hf analog at 50 °C is lower than the Zr catalyst, but the resulting polymer MW is more than 11 times higher (M_w = 777 kDa) and the syndiotacticity is noticeably lower (74% rrrr).³³ Keeping the same C_s-ligand framework, Razavi revealed that diphenylmethylidene (Ph₂C<) bridged catalyst 2 produces st-PP with slightly higher syndiotacticity, but much higher MW (4-fold enhancement), albeit with lower polymerization activity, than the PP produced by 1.³⁵ Bercaw and co-workers examined the propylene polymerization behavior in more detail by this Ph₂C< bridged catalyst and found that, at 20 °C, catalyst 2 produced highly syndiotactic PP (97.5% rrrr) with high MW (M_w = 840 kDa, PDI = 2.3) and T_m (147 °C); the activity was also impressive, with a TOF of 1.72 × 10⁶ h⁻¹.³⁶ Increasing the polymerization temperature to 40 °C and 60 °C resulted in variations in polymerization activity (Table 1), but a sharp drop in polymer syndiotacticity to 83.6% rrrr and 81.1% rrrr, and polymer T_m to 137 °C and 132 °C, respectively.

Chart 2 Representative discrete catalyst systems that produce highly syndiotactic PP.

Table 1 Characteristics of propylene polymerization by highly syndiospecific catalysts^a

catalyst	T p (°C)	TOF (h^−1)	M w (kg mol^−1)	PDI (Mw/Mn)	rrrr (%)	T m (°C)	ref.
a MAO as cocatalyst for all runs; n.r. = not reported.
1	25	476,000	133	1.9	86	n.r.	33
1	50	7,121,000	69	1.8	81	138	33,35
2	20	1,723,000	840	2.3	97.5	147	36
2	40	265,000	380	2.0	83.6	137	36
2	60	1,192,000	270	2.0	81.1	132	36
3	40	n.r.	766	n.r.	91.0	150	37
3	60	n.r.	509	n.r.	88.5	143	37
4	0	105,000	961	2.1	>99 (r)	153	38
4	20	259,000	843	1.8	>98 (r)	148	38
5	0	13,000	n.r.	n.r.	>98 (r)	140	38
6	–15	31,000	58.6	2.25	>99	165	39
6	25	921,000	35.7	2.17	96	157	39
7	20	4,039,000	980	2.0	97.5	151	36
7	50	17,671,000	290	2.0	81.0	123	36
8	0	72	26.7	1.08	94 (rr)	156	43
8	25	139	50.8	1.08	93 (rr)	152	43
8	50	113	43.2	1.23	90 (rr)	149	43
9	0	245	107	1.11	96	148	45

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Much effort has been directed towards introducing substituents into the Flu ring for further improving the syndiospecicifity of Ph₂C< bridged C_s-symmetric metallocene catalysts. In this context, Razavi and co-workers found that catalyst 3 with 3,6-di-tert-butyl substitutions on the Flu ring is indeed more syndiospecific than the parent catalyst 2.³⁷ Thus, the propylene polymerization by 3 at 40 °C yielded st-PP with high MW (M_w = 766 kDa), syndiotacticity (91% rrrr) and T_m (150 °C). Impressively, even at 60 °C, the resulting polymer still had a high syndiotacticity of 88.5% rrrr. Miller and Bercaw developed a series of sterically expanded Flu derivatives of Zr and Hf complexes incorporating an octamethyloctahydrodibenzofluorenyl (Oct) (4) or a tetramethyltetrahydrobenzofluorenyl (5) ligand.³⁸ In toluene at 0 °C, complex 4, when activated with 2000 equiv of MAO, exhibited a good activity of TOF = 1.05 × 10⁵ h⁻¹, producing st-PP with high MW (M_w = 961 kDa, PDI = 2.1), syndiotacticity (r > 99%), and T_m (153 °C). Increasing the polymerization temperature to 20 °C decreased MW (M_w = 843 kDa, PDI = 1.8) and slightly reduced the syndiotacticity and T_m (r > 98%, T_m = 148 °C), but more than doubled the polymerization activity (TOF = 2.59 × 10⁵ h⁻¹). The hafnium analog produced PP with noticeably lower syndiotacticity and with lower activity. The unsymmetrically substituted Flu derivative 5 also produced highly syndiotactic PP at 0 °C (r > 98%) but with lower T_m (140 °C) and activity (TOF = 1.30 × 10⁴ h⁻¹). The Me₂C< bridged Oct derivative is less active and syndioselective than the Ph₂C< bridged catalyst 4.

The above catalysts 1–5 are characterized as 1,1-disubstituted methylidene (R₂C<) bridged C_s-ansa-Cp-Flu metallocene catalysts. Exploring other bridging systems, Irwin and Miller synthesized a sterically expanded, “constrained geometry” type η¹-Flu-η¹-amidoansa-zirconium complex (6) and found that 6, upon activation with excess MAO, exhibits highly attractive characteristics in the syndiospecific polymerization of propylene.³⁹ At −15 °C, the polymerization of liquid propylene by 6 produced essentially quantitatively syndiotactic PP (rrrr > 99%), which displayed the highest T_m known for st-PP (165 °C for the unannealed polymer, 174 °C for the annealed polymer). The activity of the polymerization at −15 °C was modest (TOF = 3.10 × 10⁴ h⁻¹), producing st-PP also with a modest M_w of 58.6 kDa (PDI = 2.25). Increasing the polymerization temperature to 25 °C resulted in a large increase in polymerization activity by ∼30-fold (TOF = 9.21 × 10⁵ h⁻¹), accompanied by a small drop in the polymer syndiotacticity to rrrr = 96% and T_m to 157 °C and a considerable decrease in MW to M_w = 35.7 kDa. An analogous “constrained geometry” titanium catalyst supported by an η³-3,6-^tBu₂-Flu-η¹-amido ligand is less syndiospecific, affording st-PP with rr = 93% (86% rrrr) at T_p of 0 °C.⁴⁰ Doubly silylene-bridged C_s-ansa-metallocenes complexes (7), developed by Bercaw and co-workers, are also highly syndiospecific for propylene polymerization.^36,41 For example, complex 7 (R = CHMe₂), upon activation with 2000 equiv of MAO in toluene at 20 °C, was highly active and syndiospecific for propylene polymerization (TOF exceeding 4.0 × 10⁶ h⁻¹), producing st-PP with rrrr = 97.5% and T_m = 151 °C. Increasing the polymerization temperature to 50 °C led to a drastic increase in polymerization activity, with TOF exceeding 1.7 × 10⁷ h⁻¹, at the expense of the much reduced syndiotacticity (81% rrrr) and T_m (123 °C). Complexes with other substitutents (e.g., R = H, SiMe₃) also produced highly syndiotactic PP at 20 °C with rrrr ∼ 94% and T_m ∼ 151 °C.

Departing from the prototype C_s-symmetric ansa-metallocene catalyst system, where the propylene polymerization by Zr catalysts is much more effective and syndiospecific than analogous Ti and Hf catalysts and proceeds via 1,2-insertion regiochemistry and catalyst-site-control stereochemistry, Fujita and co-workers have developed a class of C₂-symmetric, octahedral non-metallocenetitanium catalysts supported by phenoxy-imine chelating ligands (FI catalysts) that polymerize propylene in a highly syndiospecific and living fashion via 2,1-insertion regiochemistry and chain-end-control stereochemistry.⁴² Thus, catalyst 8 with a bulky trimethylsilyl grouportho to the phenoxy-O and a C₆F₅ group on the imine-N produced highly syndiotactic PP with rr = 94%, 93%, 90% and T_m = 156 °C, 152 °C, 149 °C, for the polymerizations activated with MAO and carried out at 0 °C, 25 °C, and 50 °C, respectively.⁴³ However, the activity of <140 h⁻¹ is lower than the prototype zirconocene catalysts by several orders of magnitude (cf.Table 1). Replacing the orthoTMS group in 8 by the ^tBu group experienced considerable reductions in all aspects of the polymerization at 25 °C, including activity (TOF = 87 h⁻¹), syndiotacticity (rr = 87%), melting transition (T_m = 137 °C), and MW (M_w = 31.6 kDa). In fact, the syndiotacticity of the PP produced at 25 °C by the derivatives of catalyst 8 with different R subsitituents drops linearly on going from TMS (93% rr), ^tBu (87% rr), ⁱPr (75% rr), Me (50% rr), and H (43% rr). Coates and co-workers have developed analogous phenoxy-imine titanium catalysts for the synthesis of st-PP.⁴⁴ With an additional ^tBu group placed on the phenoxy-benzene ring, now titanium catalyst 9 (activated by MAO) with the ^tBu grouportho to the phenoxy-O can also produce highly syndiotactic PP at 0 °C, with rrrr = 96%, M_w = 107 kDa, PDI = 1.11, T_m = 148 °C, and enhanced activity (TOF = 245 h⁻¹).⁴⁵

Unlike the above C₂-symmetric bis(phenoxy-imine) titanium catalysts that polymerize propylene syndiospecifically via 2,1-insertion regiochemistry and chain-end-control stereochemistry, C_s-symmetric, R₂C< bridged ansa-Cp-Flu metallocene catalysts polymerize propylene syndiospecifically via 1,2-insertion regiochemistry and catalyst-site-control stereochemistry. Chart 3 depicts the proposed propagating species and stereocontrol that involves the alternating, migratory insertion of propylene at the enantiotopic sites of the C_s-ligated cationic metal complex. Several pathways can introduce stereoerrors, including enantiofacial misinsertion (mm type) as the predominant source of stereodefects, site epimerization (m type), “back-side” misinsertion (mr type), and chain epimerization (m or mm type).⁴⁶ Hence, the resulting polymer syndiotacticity is sensitive to polymerization conditions (e.g., solvent polarity, temperature, monomer concentration) and ion-pairing characteristics (e.g., cation and anion structure, coordination, and dynamics).

Chart 3 Proposed propagating species and catalyst-site stereocontrol in the syndiospecific polymerization of propylene by C_s-ligated catalysts.

2. Polystyrene

In contrast to PP, syndiotactic PS has a higher rate of crystallization than that of other tactic PS configurations and also a considerably higher T_m (by ∼40 °C) than that of it-PS.⁴⁷ Thus, st-PS, while maintaining a near constant T_g of 100 °C, exhibits a T_m ranging from 265 °C to 275 °C, depending on the polymorphic phase(s) in presence; indirectly, this is dependent on catalyst structure and polymerization conditions. Highly syndiotactic PS is also much more resistant to common solvents,⁴⁸ and the insolubility of st-PS in boiling ketones is often used as a technique to demonstrate that the polymer is highly syndiotactic (rrrr > 98%).²⁶ The combination of heat and chemical resistance with good electrical properties (low dielectric constant and dissipation factor), coupled with a low polymer density, makes st-PS an attractive material for applications in automotive, electrical and electronics, as well as industrial and consumer uses.^49,50 Although st-PS was first synthesized in 1985,²⁹ these attractive polymer properties have upheld strong motivation in developing new catalysts or approaches towards its synthesis, and thus the field has grown rapidly since that time, with half-titanocene complexes activated with MAO achieving the majority of the success.

In their seminal report, Ishihara and co-workers described the synthesis of st-PS through the use of CpTiCl₃ and a large excess of MAO as cocatalyst or activator.^30,31 The polymerization in toluene at 50 °C proceeded to a quantitative monomer conversion, but the activity decreased during the course of polymerization (20 min, TOF = 3.37 × 10⁴ h⁻¹; 180 min, TOF = 1.34 × 10⁴ h⁻¹), indicative of catalyst deactivation. The PS produced is highly syndiotactic (98 wt% insoluble in boiling methyl ethyl ketone) and has a high T_m of 266 °C and a M_w of 63.3 kDa (PDI = 1.99), Table 2; the M_n of the polymer remained nearly constant at different monomer conversions, due to chain transfer reactions present in this polymerization. Among a number of metal complexes screened, only titanium-based complexes can produce st-PS. Under these conditions, homoleptic non-titanocene compounds gave low monomer conversions (<10%), but half-titanocene(III), CpTiCl₂, achieved a much higher conversion of 44%, while half-titanocenes(IV), CpTiCl₃ (10) and Cp^*TiX₃ (11, X = Cl, Cp^* = pentamethyl cyclopentadienyl, Chart 4), enabled quantitative monomer conversion and produced highly syndiotactic PS.³¹ Sandwich titanocenes such as Cp₂TiCl₂ and Cp^*₂TiCl₂ offered only very low yield (≤2%) of st-PS.

Chart 4 Representative discrete catalyst systems that produce highly syndiotactic PS.

Table 2 Characteristics of styrene polymerization by highly syndiospecific catalysts

Catalyst	Cocatalyst (equiv)	T p (°C)	TOF (h^−1)	M w (kg mol^−1)	PDI (Mw/Mn)	SY %^a (rrrr %)	T m (°C)	ref.
a Syndiotactic yield, measured by wt% insoluble st-PS (rrrr > 98%) in boiling ketones such as 2-butanone. The r pentads in the parenthesis were determined by ^13C NMR.
10	MAO (600)	50	33,700	63.3	1.99	(98)	266	30,31
11-Cl	MAO (300)	50	140	169	3.6	n.r.	275	51
11-F	MAO (300)	50	6,640	660	2.0	n.r.	275	51
11-Me	B(C6F5)3	25	2,080	268	2.6	n.r.	n.r.	53
11-Bz	B(C6F5)3	25	900	122	1.7	n.r.	n.r.	53
11-Me	Ph3CB(C6F5)4	50	3,430	n.r.	n.r.	96	n.r.	55
12	MAO (4000)	50	15,690	720	n.r.	98.2	271	56,57
13-H	MAO (4000)	50	71,920	545	n.r.	92.5	270	58
13-Me	MAO (4000)	50	76,150	424	n.r.	92.8	275	58
14	MAO (4000)	75	317,310	130	n.r.	92	265	59
15	MAO (500)	50	11,730	324	1.90	>99	n.r.	62
16	MAO (500)	50	29,520	400	2.17	>99	n.r.	62
17	none	60	16,440	93.4	1.73	(>99)	264	63
19	Ph3CB(C6F5)4	25	130,940	519	1.37	(>99)	273	66
21	MAO (150)	80	17,120	269	4.8	>95	268	70

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Upon activation with excess MAO, half-titanocene complexes of a general formula CpTiX₃ (X = halides, alkoxides, alkyls, etc.) are highly active, similarly syndiospecific styrene polymerization catalysts, but the X ligand can significantly impact the polymerization activity and the resulting polymer MW, with fluorides being typically the most active, followed by alkoxides and then chlorides.²⁶ For instance, the polymerization activity of Cp^*TiF₃ (11, X = F) is enhanced by 46-fold over Cp^*TiCl₃ (11, X = Cl) under the same conditions, with both producing highly syndiotactic PS having a T_m of 275 °C (Table 2).⁵¹ In addition, the MW (M_w = 660 kDa, PDI = 2.0) of the st-PS produced by Cp^*TiF₃ is ∼ four times higher than the MW of the polymer produced by the chloride analog. The observed significant role of the X ligands in the styrene polymerization activity and the polymer MW is presumably attributed to the relative abstractive kinetics (activation by MAO), stability of the resulting cationic catalyst (which is influenced by the coordinating nature of the counterion [X–MAO]⁻), and propensity towards β-H elimination. Alkyl derivatives such as Cp^*TiMe₃ and Cp^*TiBz₃ need only a stoichiometric amount of molecular cocatalysts such as B(C₆F₅)₃ and [Ph₃C][B(C₆F₅)₄] to generate the corresponding syndiospecific cationic catalysts (Table 2).^52–55 The structural characterizability of the cationic catalyst resulted from the above abstractive activation using a stoichiometric amount of the molecular cocatalyst has also provided important information about the mechanistic aspects of the polymerization.

Extending the supporting ligand from Cp to Ind (Ind = η⁵-indenyl), Chien, Rausch and co-workers observed the enhanced polymerization activity (by 50–100%) and higher syndiotactic yield of the bulk PS when comparing (Ind)TiCl₃ (12) with CpTiCl₃ (10) under identical polymerization conditions (including the amount of MAO, temperature, time, and concentrations of the monomer and catalyst).^56,57 The authors attributed the observed higher activity and syndiotactic yield (up to 98.2%) by (Ind)TiCl₃ to the greater electron-donating ability of the indenyl ring relative to the Cp ring. The more thermally robust catalyst supported by Ind also enabled its polymerization at high temperature (75 °C) without noticeable loss in activity and syndiotactic yield. Since this finding, a large number of half-titanocene catalysts supported by substituted indenyl derivatives have been developed,²⁶ with catalysts 13⁵⁸ and 14⁵⁹ standing out for their significantly improved activity (Table 2). As in the case of Cp^*-based complexes, (Ind)TiF₃ shows significant enhancements in polymerization activity as well as the resulting polymer MW and T_m, relative to (Ind)TiCl₃.⁶⁰ Naturally extending the supporting ligand to η⁵-fluorenyl (Flu) is of interest, but (Flu)TiCl₃ is thermally unstable.⁶¹ Knjazhanski and co-workers solved this issue by preparing stable Flu-based mono- and bis-Flu titanium isopropoxide complexes [(η¹-Flu)Ti(OⁱPr)₂(μ-OⁱPr)]₂ (15) and (η¹-Flu)(η⁵-Flu)Ti(OⁱPr)₂ (16).⁶² Both catalysts are highly active, with the η⁵-Flu-ligated catalyst 16 being 2.5 times more active than the η¹-Flu-ligated catalyst 15, reaching a TOF of 2.95 × 10⁴ h⁻¹ (Table 2). Both Flu-based catalysts produce st-PS with a near quantitative syndiotactic yield of >99%, slightly outperforming the Cp and Ind isopropoxide derivatives, which produced PS with a syndiotactic yield of 95% and 98%, respectively, in a comparative study carried out under the same conditions.

Departing from the intensively investigated half-titanocene-type catalysts, in 2004 Carpentier and co-workers synthesized ansa-lanthanocene allyl complexes (17, Ln = Nd; 18, Ln = Y, La, Sm) incorporating the C_s-symmetric Me₂C(Cp)(Flu) ligand set, which had been proven effective in the syndiospecific polymerization of propylene, and probed their effectiveness as single-component (i.e., without cocatalysts) syndiospecific styrene polymerization catalysts.⁶³ The styrene polymerization was conducted under neat conditions or in toluene at 60 °C, with all lanthanide catalysts producing highly syndiotactic PS (rrrr ≥ 99% by ¹³C NMR). The polymerization activity follows the order Nd ≫ Sm > La > Y, with the most active Nd catalyst (17) reaching a TOF of 1.64 × 10⁴ h⁻¹ (Table 2). Soluble syndiotactic oligostyrenes were prepared recently using the Nd catalyst in combination with ⁿBu₂Mg that promotes a coordinative chain transfer polymerization, thus producing up to 130 PS chains per Nd.⁶⁴ The syndiospecific polymerization by 17 was shown proceed predominantly by a chain-end control mechanism, and the copolymerization of styrene and ethylene by the Nd catalyst afforded st-PS-co-PE copolymers composed of st-PS segments randomly separated by isolated ethylene units.⁶⁵ On the other hand, Hou and co-workers found that neutral half-sandwich lanthanocenes, (C₅Me₄SiMe₃)Ln(CH₂SiMe₃)₂(THF) (19, Ln = Sc; 20, Ln = Y, Gd, Lu), are inactive for styrene polymerization at room temperature in toluene; however, upon activation with 1 equiv of Ph₃CB(C₆F₅)₄, all resulting cationic complexes are active for the polymerization, producing quantitatively syndiotactic PS with T_m = 268–273 °C.⁶⁶ Within this series, the cationic Sc catalyst derived from the precursor 19 exhibits the highest activity (TOF of 1.31 × 10⁵ h⁻¹ at 25 °C), which is comparable with the most active half-titanocene catalysts and higher than the other lanthanide-based catalysts derived from 20 in the series by about three orders of magnitude. This half-sandwich cationic Sc catalyst also exhibits some “living” polymerization characteristics, as evidenced by a nearly linear increase in polymer MW as an increase in the monomer-to-catalyst ratio while the polymer MWD remaining narrow. Uniquely, catalyst system 19 also rapidly and selectively copolymerizes styrene and ethylene to produce blocky st-PS-b-PE copolymers that contain large amounts (up to 87 mol%) of racemically enchained styrene sequences (blocks) connected by repeated ethylene units incorporated only in small amounts.⁶⁶ Recent variations of Cp^*-type half-sandwich Sc catalyst precursors included bis(borohydrido)⁶⁷ and bis(silylamido)⁶⁸ Sc complexes which, upon activation with Al(ⁱBu)₃/Ph₃CB(C₆F₅)₄, also produce highly syndiotactic PS. Turning to non-metallocene catalysts, chelating bis(phenolate) titanium complexes, upon activation with MAO, also exhibit good activity for the synthesis of highly syndiotactic PS.⁶⁹ For instance, titanium complex 21, activated with only 150 equiv of MAO, polymerized styrene at 80 °C with a TOF of 1.71 × 10⁴ h⁻¹, producing PS with a syndiotactic yield of >95% and a T_m of 268 °C.⁷⁰

Investigations into the nature of active species derived from the half-titanocene + activator (MAO, B(C₆F₅)₃, [HNMe₂Ph][B(C₆F₅)₄]) system by ESR,^71–73NMR,⁷⁴ as well as structural model and polymerization⁷⁵ studies have led to a general consensus that the active species responsible for the formation of st-PS is a cationic Ti^III, represented by a general structure of [Cp'TiR]⁺. Although reactions involving Ti/Al ligand exchange, Ti ligand abstraction by the Lewis acid, and Ti(IV) reduction to Ti(III) can be envisioned to participate in the catalyst formation, the fundament reaction steps leading to the catalyst are still unclear. As the stereomicrostructure of the st-PS produced by CpTiCl₃/MAO consisted of long sequences of racemic dyads with only isolated meso dyads, …rrrrrrrmrrrr…, the polymerization is described to proceed via a chain-end control mechanism. A key feature of this chain-end stereocontrol is the steric repulsion between the phenyl groups of the coordinated monomer and the last inserted monomer, which leads to the syndiotactic configuration.²⁶ More specifically, a syndiospecific insertion step involves η²-styrene coordination, ηⁿ (n ≥ 3) benzyl coordination of the last inserted monomer unit of the growing chain, cis-addition to the double bond, secondary (2,1-) styrene insertion, and inversion of the chirality at Ti after each insertion step (Chart 5).^76,77 Stereocontrol of the C_s-symmetric ansa-lanthanocene catalyst system 17 was recently investigated by DFT calculations, which showed that the control by the thermodynamics is a result of combined steric repulsions between styrene–styrene and the last-inserted styrene (phenyl ring)–the fluorenyl ring.⁷⁸

Chart 5 Proposed propagating species and chain-end stereocontrol in the syndiospecific polymerization of styrene by half-titanocene catalysts.

3. Poly(methacrylate)s

3.1 Radical polymerization

Successful examples utilizing radical polymerization techniques to produce highly syndiotactic methacrylic polymers are scarce and usually require low to extremely low temperatures to overcome the poor stereochemical control generally associated with radical polymerization mechanisms. In 1970, Farmer and co-workers reported the syndiospecific radical polymerization of methacrylic acid (MAA), initiated by cobalt 60 γ radiation.⁷⁹ The PMAA samples were esterfied by reaction with diazomethane to afford PMMA for NMR analysis. Investigating solvent interactions and effects of the solvent structure, the authors showed that hydrogen bonding between alcohol solvents and the acidic monomer and the polymer radical provide large steric effects that are amplified at low temperatures, effectively dictating the stereoregularity of the resulting polymer. Specifically, at −78 °C, the polymerization in methanol, 1-propanol, and 2-propanol resulted in polymers with rr = 83.8, 87.1, and 95.0%, respectively. Lowering T_p to −115 °C in 1-propanol increased the syndiotacticity to rr = 92.0%. Chovino and Gramain synthesized poly(methacrylic tetraethyl ammonium salt) using the radical polymerization initiated by 4,4′-azobis(4-cyanovaleric acid) in water.⁸⁰ Impressively, at a relatively high temperature of 5 °C, highly syndiotactic (rr = 92.0%) polymer was synthesized, which the authors attributed to repulsion between the negatively charged carboxylate groups of the incoming monomer and growing polymer chain, leading to racemic monomer insertion. Okamoto and co-workers utilized fluoroalcohols to influence the stereospecificitiy in the radical polymerization of MMA at low temperatures, initiated by ⁿBu₃B/air.⁸¹ Thus, the MMA polymerization in (CF₃)₃COH solvent at −78 °C produced highly syndiotactic PMMA (rr = 91%) in quantitative yield, with M_n = 117 kDa and a relatively high PDI of ∼4. Lowering the reaction temperature to −98 °C only slightly increased the syndiotacticity (rr = 93%), but drastically reduced the polymer yield to only 27%, affording a polymer with M_n = 227 kDa and a very high PDI of ∼9. The authors also showed that addition of even a small amount of (CF₃)₃COH (0.1 to 1 equiv) to the bulk MMA polymerization was as effective in controlling tacticity as using (CF₃)₃COH as solvent, and demonstrated that this control over syndiotacticity was largely steric based, due to the H-bonding interaction of the fluoroalcohol with the carbonyl oxygen of the monomer and the propagating species. More recently, Okamoto, Kakuchi and co-workers reported atom-transfer radical polymerization of MMA in fluoroalcohol solvent ((CF₃)₂CHOH) at low temperatures (–20 °C to −78 °C), affording st-PMMA (rr up to 84%) with M_n ∼ 13 kDa and PDI = 1.17–1.31.⁸²

The syndiospecific radical polymerization at elevated temperatures is difficult, but Akashi et al. developed a novel template strategy to synthesize highly isotactic and syndiotactic PMMA, at high temperatures, utilizing the affinity for isotactic and syndiotactic PMMA to form a stereocomplex (Chart 6).⁸³ In their report, the authors prepared silica particles that were coated with porous thin films of either highly isotactic or syndiotactic PMMA, and performed the stereospecific radical polymerization in the presence of these templates; subsequent selective solvent extraction can separate the desired tactic polymer. To synthesize highly syndiotactic PMMA, the authors first polymerized MAA, initiated by 2,2′-azobis(N,N-dimethyleneisobutyramidine)dichloride in water at 40 °C in the presence of silica particles coated with isotactic PMMA, followed by methylation to afford syndiotactic PMMA with rr as high as 98%, but with a rather high PDI of ∼2. Another example of the syndiospecific radical polymerization at elevated temperatures is the radical polymerization of 9-fluorenyl methacrylate, initiated by AIBN. The polymerization was carried out at 60 °C, producing a high MW polymer (M_n = 123.8 kDa, PDI = 1.98) with syndiotacticity rr = 89%.⁸⁴

Chart 6 Synthesis of highly syndiotactic PMMA by stereocomplex template approach.

3.2 Anionic polymerization

The stereospecificity in the typical anionic polymerization of (meth)acrylates is extremely sensitive to variables such as T_p and solvent, as demonstrated in an early report by Evans and co-workers who investigated the effects of solvent on the anionic polymerization of MMA initiated by fluorenyllithium.⁸⁵ In mixed toluene-diethyl ether solvent, the polymerization produces isotactic polymers; in mixed toluene-tetrahydrofuran solvent, at low tetrahydrofuran ratios isotactic polymer is formed, whereas when higher tetrahydrofuran ratios (15 vol %) are employed, syndiotactic PMMA is formed. Methacrylates bearing bulky side groups, such as 9-fluorenyl methacrylate, follow this pattern: the polymerization initiated by ^tBuLi at −78 °C gave an isotactic polymer (m = 90%) in toluene, but a syndiotactic polymer (rr = 89%) in THF.⁸⁴ Overall, the sensitivity of the resulting polymer tacticity to temperature is due to the chain-end control nature of the typical anionic polymerization, while the solvent effect is attributed to competition between counterion coordination to chain-end (penultimate ester group) and monomer vs. solvation.⁸⁶ Thus, polar coordinating solvents (e.g., THF, DME) strongly solvate the counterion prohibiting it from exerting an influence on monomer enchainment because it is largely unassociated with the propagating enolate chain end when approaching the monomer, thereby favoring syndiotactic placement for steric reasons, whereas nonpolar solvents (e.g., toluene) typically favor isotactic placement through a rigid propagating chain model involving monomer pre-coordination to the counterion which is associated with the propagating chain end and additionally coordinated to the penultimate ester group.

Major efforts made to the anionic polymerization initiated by classical initiators such as organolithium reagents have been to render the polymerization controlled or living by shutting down or significantly suppressing side and/or termination reactions.^2,3,87–89 One strategy, developed by Kitayama, Hatada and co-workers, combines alkyl lithium reagents with bulky organoaluminum compounds, which in particular has proven extremely versatile and attractive due to its high degree of control over both polymerization and stereochemistry.^90,91 For instance, the MMA polymerization by ^tBuLi at −78 °C in toluene yielded it-PMMA (mm = 78%) with a high PDI of 3.10 as expected, but highly syndiotactic PMMA (rr = 92%) with a low PDI of 1.13 when combining ^tBuLi with 3 equiv of trioctylalumunium [Al(ⁿOct)₃], Table 3. Decreasing T_p to −93 °C, coupled with 5 equiv of Al(ⁿOct)₃, resulted in a noticeable increase in syndiotacticity to rr = 96% (PDI = 1.18).⁹⁰ The polymerization activity was rather low with TOF ≤ 2 h⁻¹, and the polymerization of other alkyl methacrylates (R = ethyl, isopropyl, and ^n/ibutyl) by the ^tBuLi + Al(ⁿOct)₃ system also produces highly syndiotactic polymers with narrow MWD's,⁹¹Table 3. Aluminum phenoxides, RAl(2,6-^tBu₂C₆H₃O)₂ (22, Chart 7), can also profoundly affect the stereochemical control of the polymerization by ^tBuLi at low temperatures. Thus, the polymerization of MMA by ^tBuLi/22a (5 equiv) at −78 °C afforded ht-PMMA (67.8% mr)⁹² when R = Me, or st-PMMA (89.1% rr) by ^tBuLi/22b (5 equiv) when R = Et.⁹³ This strategy was also applicable to the syndiospecific polymerization of trimethylsilyl methacrylate by ^tBuLi at −78 °C in toluene, yielding a highly syndiotactic polymer with rr = 93.4% or 96.4% when 3 or 5 equiv of 22a was combined with the initiator.⁹⁴

Table 3 Anionic initiator systems producing highly syndiotactic poly(alkyl (R) methacrylate)s

Initiator/catalyst	R, [M]/[I]	T p (°C)	TOF (h^−1)	M n (kg mol^−1)	PDI (Mw/Mn)	rr (%)	T g (°C)	ref.
^t BuLi/3Al(^nOct)3	Me, 50	–78	1.9	4.87	1.13	92	n.r.	90
^t BuLi/3Al(^nOct)3	^i Bu, 50	–78	1.9	8.69	1.06	93	n.r.	90
^t BuLi/5Al(^nOct)3	Me, 50	–93	1.6	4.92	1.18	96	n.r.	90
^t BuLi/5 22b	Me, 50	–78	1.7	4.45	1.10	89	n.r.	93
^t BuLi/5 22b	Et, 50	–78	2.0	6.49	1.09	92	n.r.	93
^t BuLi/5 22a	TMS, 50	–78	2.0	7.03	1.16	96	n.r.	94
23a/2Al(C6F5)3	Me, 200	–78	53	37.0	1.35	94	138	19
^t BuLi/2Al(C6F5)3	Me, 200	–78	42	59.8	1.33	95	140	98
MTS/0.05Tf2NH	Me, 100	–55	<1	14.0	1.04	90	n.r.	99
Ph3P/AlEt3	Me, 40	–93	∼1	19.4	2.03	95	135	100
24	Me, 46.7	–78	521	300 (Mw)	n.r.	93	n.r.	102
25	Me, 46.7	–78	532	120 (Mw)	n.r.	93	n.r.	102
26	Me, 10	–98	<1	17.3	1.20	94	n.r.	103
26	Me, 10	–110	<1	14.0	1.19	97	n.r.	103
27	Me, 400	–30	2,280	∼40	∼1.1	92	135	104

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Chart 7 Structures of catalysts or anionic initiators that produce highly syndiotactic PMMA.

Lithium ester enolates, Me₂C=C(OR)OLi (alkyl α-lithioisobutyrate, 23), should be, in principle, ideal initiators for the polymerization of (meth)acrylates. As the propagating species in the anionic polymerization of (meth)acrylates by organolithium initiators are lithium ester enolates, rates of initiation and propagation should be nearly identical, giving rise to polymers with narrow MWD's. In actuality, lithium ester enolates have a strong tendency for aggregation (n = 2–6) in hydrocarbon solvents, and the existence of various aggregated ester enolates creates significant problems in controlling the polymerization rate and the polymer MWD.⁹⁵ Additionally, lithium ester enolates are unstable, even in the solid state, and subject to decomposition to ketenes and lithium alkoxides and β-keto ester enolates.⁹⁶ Addressing these obstacles, Rodriguez and Chen extended the concept of adding bulky aluminum Lewis acids to organolithium initiators towards the polymerization of methacrylates initiated by lithium enolates and found that the added bulky aluminum species, such as MeAl(BHT)₂ (BHT = butylated hydroxytoluene = 4-Me-2,6-^tBu₂C₆H₂O), serve as both the catalyst for monomer activation and deaggregator for converting the oligomeric, multi-site enolate species to a monomeric enolaluminate active species.⁹⁷ The end result is the termed “single-site anionic polymerization” that propagates in a controlled, bimetallic fashion, Chart 10, producing st-PMMA (71% rr) with narrow MWD (1.12) at 23 °C. It should be noted that lithium ester enolaluminates are less reactive than the parent lithium ester enolates, but more selective with preferential addition to the activated monomer in a syndioselective fashion (Chart 8).

Chart 8 Single-site, syndioselective anionic polymerizationvia bimolecular propagation regulated by aluminum Lewis acids.

Among a series of bulky aluminum Lewis acid catalysts examined, the alane Al(C₆F₅)₃ not only generates the most active system (having the highest degree of activation towards monomer), but also renders the system the highest degree of control over polymerization initiated by Me₂C=C(OⁱPr)OLi (23b) at ambient temperature or 0 °C.⁹⁷ At −78 °C, the MMA polymerization by a combination of 23a with 2 equiv of Al(C₆F₅)₃ produced highly syndiotactic PMMA with rr = 94% and T_g = 138 °C.¹⁹ Similarly, a combination of ^tBuLi with 2 equiv of Al(C₆F₅)₃ also afforded highly syndiotactic PMMA (95% rr) at T_p of −78 °C (Table 3). The use of more stable alkyl α-lithioisobutyrates such as 23b, coupled with Al(C₆F₅)₃, affords a more controlled polymerization system than 23a/2Al(C₆F₅)₃, and addition sequence is important for the polymerization by ^tBuLi + 2 Al(C₆F₅)₃ as premixing these two reagents before addition of MMA generates a hydride-bridged bis(aluminate) initiator, Li[Al(C₆F₅)₃–H–Al(C₆F₅)₃]⁻, which also affords highly syndiotactic PMMA (94% rr) at T_p of −78 °C.⁹⁸ Kakuchi et al. recently employed a Brønsted acid, trifluoromethanesulfonimide (Tf₂NH), as activator for the anionic polymerization of MMA initiated by 1-methoxy-1-trimethylsilyloxy-2-methylpropene (MTS), which produced st-PMMA (rr = 90%) at −55 °C in DCM.⁹⁹ This polymerization has a high degree of control, but its activity is very low (TOF < 1 h⁻¹); increasing T_p to 27 °C drastically increased the rate of polymerization (TOF = 10 h⁻¹), at the expense of syndiotacticity (rr = 72%).

In 1992, Hatada and co-workers reported using tertiary phosphines coupled with triethylaluminum for the syndiospecific polymerization of MMA in toluene at low temperatures (–78 to −93 °C).¹⁰⁰ Most notably, the Ph₃P-Et₃Al system polymerized 40 equiv of MMA at −93 °C to st-PMMA (M_n = 19.4 kDa, PDI = 2.03, and rr = 95%) in 70% yield after 24 h. With a needed high 2.5 mol % catalyst loading, the polymerization was sluggish (TOF ∼ 1 h⁻¹) and inefficient (I^* < 15%). Performing the polymerization at −78 °C decreased syndiotacticity to 89% rr and diminished the initiator efficiency (I^* ∼5%). Utilizing Et₃P at −93 °C or −78 °C afforded PMMA with rr = 89% or 81%, with significantly enhanced initiator efficiencies. This system was also shown to polymerize other methacrylates to syndiotactic (∼90% rr) homo- and copolymers.

There has been growing interest in utilizing organo magnesium and calcium compounds in catalysis,¹⁰¹ and in some cases, such species have found success in producing highly syndiotactic PMMA. An early report by Joh and Kotake disclosed that Mg initiators incorporating a piperidine moiety, such as ethylpentamethyleneiminomagnesium (24) and bis(pentamethylenimino)magnesium (25) produced st-PMMA with rr = 93% at −78 °C in toluene.¹⁰² Hatada et al. reported a Grignard reagent, m-vinylbenzylmagnesium chloride (26), polymerized MMA in THF from −78 °C to −110 °C, affording highly syndiotactic polymers with rr = 90.6–96.6% with relatively narrow MWD's from 1.30 to 1.19 for the methanol-insoluble fraction (35–75 wt%, depending on T_p).¹⁰³ More recently, Gibson and co-workers synthesized a discrete magnesium ketone enolate dimer, [(BDI)Mg(μ-OC(=CH₂)-2,4,6-Me₃C₆H₂)]₂ [BDI = HC(C(Me)=N-2,6-ⁱPr₂C₆H₃)₂] (27, Chart 7), and found it to be highly active for MMA polymerization (TOF = 2,280 h⁻¹) in a living fashion at −30 °C in toluene or chloroform, leading to highly syndiotactic PMMA (92% rr) with a high T_g of 135 °C (Table 3).¹⁰⁴ As addition of donor ligands such as THF readily breaks the dimer to form the monomeric Mg enolate complex, it can be envisioned that the MMA polymerization by the dimeric enolate complex 27 proceeds via the monomeric enolate complex in the presence of MMA. The stereocontrol ability of this catalyst system relies on the steric bulk of the ligand rendered by the N-aryl groups; thus, changing from 2,6-ⁱPr₂C₆H₃ to 2,6-Et₂C₆H₃ and 2,6-Me₂C₆H₃N-aryl groups resulted in a substantial reduction of the syndiotacticity of the PMMA produced at −30 °C from 92% rr to 78% rr and 73% rr, respectively.¹⁰⁵ The analogous monomeric magnesium alkyl, aryl, or amide complexes are also highly active catalysts for MMA polymerization, producing st-PMMA with ∼90% rr at −30 °C. Owing to the ligand-assisted, chain-end control nature of the polymerization by these discrete magnesium complexes, the syndiotacticity erodes drastically to only 75% rr for the polymerization carried out at ambient temperature (23 °C). Allen and co-workers reported in 1974 that organocalcium compounds, including (Flu)CaCl, (Ind)₂Ca, Cp₂Ca, and Ph₂Ca, in polar DME at low temperatures converted MMA to st-PMMA; in the case of Cp₂Ca, the syndiotacticity was reported to be 94% rr at T_p = 0 °C, but achieving only 8% conversion.¹⁰⁶ This claim was later disputed by a more recent report revealing that the PMMA formed had only a modest syndiotacticity of rr = 80% when the purified Cp₂Ca was used for the polymerization.¹⁰⁷

3.3 Coordination polymerization

Similarly to the stereospecific polymerization of α-olefins, heterogeneous Ziegler–Natta type catalysts were the first transition-metal catalysts to conquer the field of stereospecific coordination polymerization of MMA. In 1965, Abe and co-workers reported the use of TiCl₄/AlEt₃ mixtures as effective polymerization catalysts of MMA at temperatures below 0 °C in toluene.¹⁰⁸ It was found that Al/Ti ratios between 3–5 were optimal, and most successfully, with Al/Ti = 5 and MMA/Ti = 26 at −78 °C, the monomer conversion was 88.5% after 18 h (TOF = 1.3 h⁻¹). The resulting PMMA had a high syndiotacticity of rr = 92% and a high MW (Table 4), giving a minimum initiator efficiency of <1%. The same authors later investigated this polymerization system in more detail and observed that the polymerization can also be mediated in a similar fashion by other trialkyl aluminum species such as triisobutylaluminum and trihexylaluminum, although there was a slight decrease in syndiotacticity (rr = 87–90).¹⁰⁹ With AlEt₃, the syndiotacticity of the resulting PMMA reported in this full paper was somewhat higher (94% rr) at T_p of −78 °C.

Table 4 Coordination catalysts for the synthesis of highly syndiotactic MMA

Catalyst	[M]/[I]	T p (°C)	TOF (h^−1)	M n (kg mol^−1)	PDI (Mw/Mn)	I* (%)	rr (%)	ref.
TiCl4/5AlEt3	26	–78	1.3	255 (Mv)	n.r.	<1	92	108
28	500	–78	26.9	82.0	1.04	59	93	112
28	1000	–95	13.7	187	1.05	44	95	112
36	n.r.	–78	n.r.	2,270	n.r.	n.r.	94	115
Cp^*2YbAlH3·NEt3	1000	–40	n.r.	118	1.74	84	93	117
37	100	–78	3	157	1.11	4.2	91	118
38	200	–78	200	43.0	1.08	93	90	119
39/B(C6F5)3 (41)	100	25	814	16.1	1.23	60	94	133
39/B(C6F5)3 (41)	400	25	800	41.5	1.48	49	94	134
40/Ph3CB(C6F5)4 (42)	100	25	394	20.9	1.37	45	94	133
40/Ph3CB(C6F5)4 (42)	400	25	717	44.3	1.32	41	94	133
43/Ph3CB(C6F5)4	400	25	1,554	40.0	1.14	98	94	134
43/Ph3CB(C6F5)4	400	50	735	n.r.	n.r.	n.r.	93	134
44/Ph3CB(C6F5)4	400	25	1,557	n.r.	n.r.	n.r.	94	134
43/B(C6F5)3 (45)	400	25	260	40.1	1.39	98	91	135

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Yasuda's seminal work developed the highly stereospecific polymerization of MMA by organolanthanide catalysts, leading to polymers with unprecedentedly high MW's and narrow MWD's.^110,111 In 1992, Yasuda and co-workers reported the use of neutral, single-component, trivalent lanthanocenes, such as dimeric samarocene hydride [Cp^*₂SmH]₂ (28, Chart 9), as catalysts for living polymerization of MMA in toluene.¹¹² In particular, 28 at 0 °C was shown to rapidly polymerize MMA (0.033 mol % catalyst, 3 h, 98% conversion, TOF = 980 h⁻¹) and produce high MW PMMA (M_n = 563 kDa) with a narrow MWD (PDI = 1.04) and a moderate syndiotacticity (rr = 82.3%). Lowering T_p to −78 °C and −95 °C resulted in highly syndiotactic PMMA, with rr = 93.1% and 95.3%, respectively. Both the reactivity and catalyst efficiency are much higher than the classic heterogeneous catalyst (Table 4). It was also noted that several other analogous lanthanocenes, such as [Cp^*₂YMe]₂ (29), Cp^*₂SmMe₂AlMe₂ (30), and Cp^*₂LnMe(THF) (31, Ln = Sm, Yb, Lu; Chart 10), can equally afford syndiotactic PMMA (rr > 90%) with narrow MWD's (PDI = 1.03–1.05) at low temperatures (T_p < −78 °C). Overall, these lanthanocenes function as both initiator (to effect chain initiation and growthvia conjugate Michael addition) and catalyst (to activate monomer via monomer coordination to the highly Lewis acidic Sm center) in the polymerization of MMA. Chain initiation is proposed to involve nucleophilic attack of the Sm hydride to the coordinated (activated) MMA, followed by conjugate addition of the resulting ester enolate to a second MMA coordinated to Sm, giving rise to the eight-membered-chelate propagating species (Chart 9). Propagation proceeds via repeated intramolecular conjugate Michael additions through the Sm enolate–monomer complex (active species) to the eight-membered-ring intermediate (i.e., catalyst resting state) cycle.

Chart 9 Proposed chain initiation and propagation steps in the MMA polymerization by samarocene 28.

Chart 10 Coordination catalysts that produce highly syndiotactic PMMA.

Trimethylsilyl-substituted lanthanocene methyl complexes, [(Me₃SiC₅H₄)₂SmMe]₂ (32) and {[(Me₃Si)₂C₅H₃]₂LnMe}₂ (33, Ln = Sm, Nd; Chart 10) also initiate living polymerization of MMA at −78 °C with even higher TOFs than [Cp^*₂SmH]₂, but the PMMA syndiotacticity is somewhat lower (86–90% rr), as compared to 93% rr of the PMMA produced by [Cp^*₂SmH]₂ at −78 °C.¹¹³ Shen et al. investigated the polymerization of (dimethylamino)ethyl methacrylate by lanthanocene amide complexes.¹¹⁴ In particular, this methacrylate monomer was polymerized in a syndiospecific fashion by (MeC₅H₄)₂YbN(ⁱPr)₂(THF) (34) at −78 °C in toluene, affording a high MW (M_n = 260 kDa), syndiotactic polymer (rr = 92.8%). The catalyst had high activity with a TOF ∼ 245 h⁻¹, but the efficiency was low with I^* = 28%.

Yasuda et al. also investigated the polymerization of MMA in toluene by divalent lanthanocenes, Cp^*₂Ln(THF)₂ (35, Ln = Sm, Yb) and (Ind)₂Yb(THF)₂ (36).¹¹⁵ All three catalysts produced polymers with narrow MWD's (PDI = 1.09–1.11) at 0 °C. At −78 °C, 36 afforded ultra high MW PMMA (M_n = 2270 kDa) with rr = 93.8%; even at −40 °C, 35-Yb afforded PMMA with rr = 91.5%. The authors noted that, in contrast to the trivalent lanthanides, all the divalent lanthanides examined afforded lower initiator efficiencies (I^* < 40%). Later work by Boffa and Novak revealed that the propagating active species involved in the polymerization of MMA by divalent 35-Sm is a bimetallic samarium(III) species.¹¹⁶ This species was derived from a one-electron transfer from the samarium(II) complex to the monomer affording a samarium(III) cation and a MMA radical anion, which combine to form a samarium enolate radical. Two radicals then combine in a head-to-tail fashion, resulting in the bimetallic propagating species. Since two metal centers now form one polymer chain, the MW of the polymer will be at least twice as great as expected, and the observed initiator efficiency can never exceed 50% based on the unimetallic model. Knjazhanski and co-workers prepared a series of alane AlH₃ complexes of divalent lanthanocenes, which produced polymers with slightly different properties than their parent, non-alane complexes.¹¹⁷ Specifically, Cp^*₂YbAlH₃·NEt₃ afforded PMAA with rr = 93% in toluene at −40 °C. In comparison, under otherwise identical conditions, Cp^*₂Yb(THF) produces PMMA with a slightly lower syndiotacticity of rr = 89%.

Half-sandwich lanthanocene complex Cp^*La[CH(SiMe₃)₂]₂(THF) (37) polymerized MMA to highly syndiotactic (91% rr), high MW (M_n = 1.57 × 10⁵), and narrow MWD (1.11) PMMA at −78 °C, but with low activity (3 h⁻¹TOF) and I^* (∼4%).¹¹⁸ The polymerization at 25 °C was much more active (TOF = 200 h⁻¹), but the syndiotacticity dropped sharply to 74% rr. A divalent non-metallocene samarium complex supported by the bulky phenoxy ligand, (BHT)₂Sm(THF)₃ (38), promotes living polymerization of MMA in toluene at −78 °C, producing st-PMMA (M_n = 43 KDa, PDI = 1.08, rr = 90%) with good activity (TOF = 200 h⁻¹).¹¹⁹ The effect on the polymerization by adding an increasing amount of MeAl(BHT)₂ was performed; increasing the [Al]/[Sm] ratio from 0 to 5 led to a decrease in rr from 90 to 20%, accompanied by an increase in mm from 1 to 68%, and a nearly constant mr.

Group IV chiral metallocenes, especially in their cationic forms, have achieved phenomenal levels of success performing as homogeneous, single-site, stereospecific catalysts for polymerization of nonpolar α-olefins to stereoregular polymers with tailored stereomicrostructures.¹²⁰ Such successes have allowed for the establishment of a strong correlation between the symmetry of such chiral metallocenium catalysts and the stereomicrostructure of the resulting polymers. Thus, C₂-ligated catalysts readily produce it-PP, C_s-ligated catalysts produce st-PP, C₁-ligated catalysts afford either hemiisotactic or it-PP, and achiral catalysts generally produce at-PP. It seemed that, although there is a shift in mechanism from migratory insertion of α-olefins to coordinative-addition for polar conjugate olefins (acrylic monomers), such as MMA, these general rules held true, as C₂-ligated metallocenium catalysts, such as {rac-[C₂H₄(η⁵-Ind)₂]Zr(THF)[OC(OⁱPr)=CMe₂]}⁺[MeB(C₆F₅)₃]⁻,^121,122 at ambient temperature efficiently produce highly isotactic poly(methacrylate)s (>95% mm)^121,122 and poly[(meth)acrylamide]s (>99% mm).^123–127 However, when the C_s-ligated catalyst [Me₂C(Cp)(Flu)ZrMe]⁺, which is successful in synthesizing highly syndiotactic PP,^33,35,120 was used for the polymerization of MMA, no activity was observed. Although the inactivity issue was solved by using an ester enolate zirconocenium complex, {Me₂C(Cp)(Flu)Zr(THF)[OC(OⁱPr)=CMe₂]}⁺[MeB(C₆F₅)₃]⁻, the resulting PMMA was still only syndio-biased (rr = 64%).¹²⁸ A different type of C_s-ligated complex incorporating a linked dianionic Cp-amido ligand, Me₂Si(η⁵-(Me₄C₅)(^tBuN), affords PMMA with tacticity depending on metal. Thus, the zirconium enolate complex, [Me₂Si(η⁵-(Me₄C₅)(^tBuN)]Zr(L)[OC(O^tBu)=CMe₂]}⁺[B(Ar_F)₄]⁻ (Ar_F = 3,5-(CF₃)₂C₆H₃, L = neutral donor ligand such as THF or isobutyrate), reported by Collins et al.,¹²⁹ exhibited low activity (TOF = 9.4 h⁻¹) and afforded, unexpectedly (on the basis of its C_s ligation), highly isotactic PMMA (95.5% mm) via a site-control mechanism at low T_p (–60 °C and −40 °C) in a solvent mixture of toluene and CH₂Cl₂. On the other hand, Chen and co-workers found that the titanium complex bearing the same ligand set, [Me₂Si(η⁵-(Me₄C₅)(^tBuN)]TiMe⁺MeB(C₆F₅)₃⁻, effected living and syndioselective polymerization at ambient temperature, producing PMMA with syndiotacticity ∼80% rr and controlled MW and narrow MWD (PDI = 1.09).¹³⁰ The corresponding cationic titanium ester enolate complex, which simulates the structure of the active propagating species, behaves similarly to that of the alkyl complex, producing syndiotactic PMMA (80% rr, 18% mr, 2.0% mm) at ambient temperature with predominately isolated m meso dyad stereoerrors (…rrrrmrrrr…), again pointing to the apparent chain-end control nature of the titanium catalyst. Further studies by Chen, Cavallo and co-workers showed that catalyst site-epimerization corrects a stereomistake made in a previous enantiofacial misaddition, accounting for the formation of the predominately isolated m stereoerrors.¹³¹ Höcker and co-workers utilized the C_2v-ligated complex [Me₂CCp₂ZrMe(THF)]⁺[BPh₄]⁻ to produce PMMA with rr = 89.0% in CH₂Cl₂ at −45 °C, via chain-end control at low temperatures.¹³² The catalyst activity and efficiency were low (TOF ∼ 7 h⁻¹, I^* ∼16%).

The first catalyst-site-controlled syndiospecific polymerization of MMA was reported in 2008 by Ning and Chen, enabling the synthesis of highly syndiotactic PMMA at ambient or higher temperatures.¹³³ Neutral zirconocene mono(ester enolate) 39 and bis(ester enolate) 40 (Chart 10), supported by the rigid Ph₂C< bridged C_s-symmetric ligand, Ph₂C(Cp)(Flu), can be readily converted to the cationic catalystsviamethide and hydride abstraction, respectively. Specifically, activation of 39 is achieved by reaction with B(C₆F₅)₃·THF in CH₂Cl₂ or toluene to cleanly generate the cationic species 41, while the activation of 40 proceeds via H⁻ abstraction from the methyl group within the enolate moiety by Ph₃C⁺ forming Ph₃CH and isopropyl methacrylate coordinated to Zr; subsequent nucleophilic addition of another enolate ligand to this activated methacrylate monomer gives the cationic eight-membered-ring chelate 42, the active catalyst resting intermediate. Hence, the MMA polymerization at 25 °C in CH₂Cl₂ by bis(ester enolate) 40, upon in situactivation with Ph₃CB(C₆F₅)₄, produced highly syndiotactic PMMA (94% rr) with a high T_g of 139 °C via a predominately site-controlled mechanism. Likewise, catalyst 41, generated by in situactivation of precatalyst 39 with B(C₆F₅)₃·THF, rapidly polymerized MMA at 25 °C in CH₂Cl₂ with TOF reaching 814 h⁻¹ (95% conversion in 7 min in a [MMA]/[Zr] ratio of 100), yielding highly syndiotactic PMMA (94% rr). Catalyst 41 also polymerizes ⁿBu methacrylate to the corresponding syndiotactic polymer (94% rr) with M_n = 34.7 kDa and PDI = 1.30. Impressively, the syndiospecficity of this catalyst system proved robust at elevated temperature, producing PMMA with rr = 92.8% even at 50 °C in toluene. However, at higher monomer-to-catalyst ratios such as 400, both catalysts failed to achieve high monomer conversions (≤50%) and also exhibited relatively low catalyst efficiencies (<50%).¹³⁴ Subsequent catalyst structure-reactivity relationship studies by Chen and co-workers led to much more active and robust catalyst systems.¹³⁴ In particular, catalyst [Ph₂C(Cp)(2,7-^tBu₂-Flu)]Zr[OC(OⁱPr)=CMe₂]₂ (43), when activated with Ph₃CB(C₆F₅)₄in situ, rapidly polymerized 400 equiv of MMA at 50 °C in toluene to achieve 97% conversion in 15 min, giving a high TOF of 1554 h⁻¹. The PMMA produced exhibited a high syndiotacticity (rr = 94%) and a controlled MW (M_n = 40.0 kDa) with a narrow MWD (PDI = 1.14), thus giving rise to a nearly quantitative catalyst efficiency (I^* = 98%). Unlike the chain-end control polymerization, the high level of syndiospecificity of this catalyst-site-control polymerization is not markedly altered by changing the monomer-to-catalyst ratio (up to 2000 examined), solvent and temperature (93% rr at T_p of 50 °C). Introducing substituents to the Cp ring and the Flu rings at different patterns, or substituting the Ph₂C< bridge with other bridging moieties such as Me₂C<, Me₂Si<, or Ph₂Si<, decreases the catalyst syndiospecificity, with changing the bridging exerting the most pronounced effect.¹³⁴ On the other hand, placing a silyl group at the para-position of the bridging phenyl (i.e., 44) has virtually no impact on catalyst activity and syndiospecificity (44vs.43, Table 4).

Kinetic and mechanistic studies revealed that this polymerization is catalyst-site-controlled and proceeds with a monometallic, coordinative-addition mechanism, consisting of a fast intramolecular conjugate addition to form the eight-membered resting state intermediate, followed by the rate determining step (r.d.s) of opening the chelate by incoming monomer (Chart 11). Computational studies rationalized why the Ph₂C< bridged catalyst exhibits higher stereoselectivity than other catalysts with the Me₂C< or Me₂Si< bridge. Specifically, the Ph₂C< bridge rigidity pushes the η³-bound Flu ligand closer to the growing chain and the monomer, thereby increasing ΔE_Stereo between the competing transition states for the addition of a monomer molecule to the opposite (correct and wrong) enantiofaces of the enolate growing chain.¹³⁴

Chart 11 Propagation “catalysis” cycle (left) and transition state generating a stereomistake in the MMA polymerization by the cationic catalyst derived from 43.¹³⁴

Most recently, Chen and co-workers found that activation of C_s-ligated bis(ester enolate) metallocene precatalysts with the strong Lewis acid, B(C₆F₅)₃, affords quantitatively the corresponding isolable cationic eight-membered ester enolate metallacycles such as 45 (Chart 10).¹³⁵ This rapid, two-step reaction consists of vinylogous hydride abstraction to form the anion [HB(C₆F₅)₃]⁻ and nucleophilic addition of the second enolate ligand to the methacrylate resulted from loss of a hydride in the first enolate ligand to form the chelating cation. In contrast to ansa-Flu-Cp ligated, eight-membered chelating cations paired with more commonly used weakly coordinating anions, such as [MeB(C₆F₅)₃]⁻ or [B(C₆F₅)₄]⁻, the same cations paired with the anion [HB(C₆F₅)₃]⁻ show drastic activity differences in different solvents, with the activity in toluene being ∼16-fold lower than that in CH₂Cl₂. In comparison, the [HB(C₆F₅)₃]⁻ based catalysts also exhibit significantly lower activity (by 6–40 fold) and produce PMMA with noticeably lower syndiotacticity. Most uniquely, [HB(C₆F₅)₃]⁻ based catalysts effect substantial internal chain-transfer reactions, especially for polymerizations carried out in toluene and in the presence of excess B(C₆F₅)₃, thus releasing polymer chains with a terminal double bond and achieving a catalytic polymerization. The picture emerging from the combined experimental and theoretical study has led to a new hydride-shuttling chain transfer mechanism promoted by the hydridoborate anion, involving a hydride addition and abstraction sequence through the borane center.¹³⁵

4. Poly(cyclic ester)s

4.1 Polylactide

Polylactide (PLA) is one of the most commercially important polymers that exhibits both biodegradable and biocompatible properties, enabling a wide array of applications in packaging, microelectronics, and biomedical fields.^136,137PLA is conveniently synthesized by the ring opening polymerization (ROP) of the bioderived monomer, lactide.^138,139 Possessing two stereocenters, lactide exists as three different diastereomers: LL- (L-LA), DD- (D-LA), and DL- (meso-LA). It has been well documented that the ROP of the enantiomeric monomers, in absence of any site-epimerization, leads to polymers with quantitative isotacticity. To synthesize syndiotactic PLA, it is required that the stereoselective ROP of meso-LA be effectively carried out, namely exhibiting a large kinetic preference for attacking one of the diastereotopic sites in meso-LA. In 1999, Ovitt and Coates utilized enantiopure chiral aluminum salen isopropoxide complex 46 for the stereoselective ROP of meso-LA, affording highly syndiotactic PLA (r % = 96%), with the annealed polymer exhibiting a T_g of 34.1 °C and T_m of 152 °C (Chart 12).¹⁴⁰ Both T_g and T_m of st-PLA are considerably lower than those of typical it-PLA derived from the ROP of L-LA (T_g ∼ 50 °C and T_m up to 180 °C). Although the activity was low (1 mol % catalyst, 40 h, 94% conversion, TOF < 3 h⁻¹), the polymerization was controlled, producing st-PLA with a narrow MWD of 1.05 and M_n of 12.5 kDa, giving a high initiator efficiency of I^* = 88.8%. The polymerization was proposed to proceed through a catalyst site-control mechanism.

Chart 12 Stereospecific ROP of meso-LA to synthesize st-PLA by a chiral (salen)Al catalyst.

An analogous group 3 yttrium salen complex was found to be more active toward the polymerization of meso-LA, but producing only atactic PLA.¹⁴⁰ On the other hand, by recognizing that the catalysts that polymerize rac-LA to ht-PLA should also polymerize meso-LA to st-PLA, Okuda and co-workers employed heteroselective scandium catalysts (47), containing a 1,ω-dithia-alkanediyl-bridged bis(phenolato) ligand with bulky ortho-substituents for the stereoselectiv ROP of meso-LA to highly syndiotactic PLA (Chart 13).¹⁴¹ The polymerizations by such catalysts supported by the tetradentate [OSSO] ligand were carried out in toluene at 25 °C, affording polymers with the highest P_r (the probability of forming a new syndiotactic, racemic dyad) approaching 0.93. These catalysts exhibit much higher activity than the above salen Al catalyst, approaching quantitative monomer conversion (1 mol % catalyst) in 30 min, giving a TOF ∼ 200 h⁻¹. There was a great range in MWD of the polymers produced by the varying catalysts, ranging from broad (PDI = 2.15) to relatively narrow (PDI = 1.29), indicative of a varied degree of transesterification or chain transfer. The two most reactive and syndioselective catalysts with two cumyl substituents on the aromatic rings, depicted in Chart 13, also gave PLA with the measured M_n being much higher than the theoretical M_n, therefore yielding low initiator efficiencies of I^* = 25–32%. The st-PLA produced has a relative low T_m of 119 °C. A structurally related indium complex supported by the tetradentate [OSSO] ligand¹⁴² also produces st-PLA (P_r = 0.93%) at room temperature, but the MWD is much narrower (PDI = 1.05).¹⁴¹

Chart 13 Stereospecific ROP of meso-LA to st-PLA by tetradentate [OSSO]Sc catalysts.

4.2 Poly(β-hydroxyalkanoate)s

Poly(3-hydroxybutyrate) (PHB), a key member of naturally occurring biodegradable and biocompatible microbial aliphatic polyesters, poly(β-hydroxyalkanoate)s (PHAs), is an isotactic polymer (T_m = 180 °C) exhibiting properties similar to crystalline thermoplastic it-PP and produced by various bacteria and algae.^143,144 ROP of β-butyrolactone (_βBL) provides a convenient synthetic route to high molecular weight PHB.¹⁴⁵ While the polymerization of the enantiopure _βBL by achiral or chiral initiators leads to the corresponding isotactic polymer, the polymerization of racemic _βBL typically leads to an atactic polymer or a polymer with low stereoregularity. Carpentier and co-workers have developed discrete yttrium amide or alkoxide initiators (48) supported by tetradentate, dianionic amino-alkoxy-bis(phenolate) [O⁻,N,O,O⁻] ligands that promote syndiospecific polymerization of racemic _βBL to syndiotactic PHB (Chart 14) by a chain-end control mechanism.^146–148 The polymerization at 20 °C in toluene had living characteristics and was rapid, with TOF up to 24,000 h⁻¹; the stereoselectivity and activity of the yttrium catalyst system can be fine-tuned by the R substituents on the phenolate ring. The resulting polymers had narrow MWD's (PDI = 1.18–1.03) and exhibited P_r as high as 0.94 (89% rr) and T_m as high 183 °C.¹⁴⁷

Chart 14 Synthesis of syndiotactic PHB by living, syndiospecific ROP of rac-_βBL.

Thomas, Coates, and co-workers recently reported a new strategy for the synthesis of new sequentially controlled PHAs, which utilized a monomeric yttrium silylamide and a dimeric yttrium alkoxide (49, Chart 15) supported by a tetradentate phenoxyamine (salan) ligand to polymerize a 50 : 50 ratio of two different enantiomeric β-lactones with stereocenters of opposite configuration and bearing different substituents.¹⁴⁹ Both complexes enabled full conversion of 30–500 equiv of 50 : 50 mixtures of β-lactones in 5–120 min, with the highest TOF reaching 4700 h⁻¹. This polymerization system not only produced syndiotactic PHAs with narrow MWDs (PDI = 1.10–1.23 by 49), but additionally, the polymers were composed of an alternating monomer sequence (90–94% alternation). The T_m of the alternating copolymers with different β-side chains (R₁ and R₂) could be altered over a wide range of temperatures (47 °C to 210 °C), depending on the choice of enantiopure monomer building blocks.

Chart 15 Synthesis of sequence-controlled syndiotactic PHAs by alternating ROP of enantiopure β-lactones.

5. Miscellaneous polymers

5.1 Higher α-olefin polymers

C _s-symmetric, syndiospecific-propylene-polymerization catalysts such as 1 and its expanded derivatives have also been utilized to produce highly syndiotactic polymers derived from higher α-olefins.¹⁵⁰ For instance, Shiomura and co-workers synthesized a series of syndiotactic poly(1-olefin)s through the syndiospecific polymerization of 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene using catalyst 1 activated with MAO at 30 °C.¹⁵¹ In a kinetic study Deffieux et al. utilized the 1/1000MAO system to synthesize syndiotactic (89% rr) poly(1-hexene) in toluene at 20 °C and found that the polymer syndiotacticity is substantially lowered when carrying out the polymerization in polar chlorinated solvents such as o-dichlorobenzene and methylene chloride.¹⁵² Highly syndiotactic poly(1-pentene) (rrrr > 99%, M_n = 68.4 kDa, PDI = 1.84) was synthesized by the same catalyst system (1/700MAO) in toluene at 0 °C.¹⁵³ Hoff and Kaminsky synthesized syndiotactic poly(1-pentene), poly(1-hexene) and poly(1-octene) with syndiotacticity ranging from 88% to 91% rrrr using [Ph₂C(Cp)(2,7-^tBu₂-Flu)]ZrCl₂/7800MAO at T_p = 0 °C.¹⁵⁴ Highly syndiospecific propylene polymerization catalyst 6 is also effective in the syndiospecific polymerization of 4-methyl-1-pentene; at 0 °C the neat polymerization had TOF = 4.52 × 10³ h⁻¹ and produced a syndiotactic polymer (rrrr = 97%) with a high T_m of 215 °C.³⁹ Under similar conditions, catalyst 6 outperformed 1 in terms of syndioselectivity (TOF = 5.95 × 10³ h⁻¹, rrrr = 94%, T_m = 210 °C), while catalyst 4 was only active at 25 °C, producing an amorphous material. Zambelli et al. reported syndiospecific polymerization of β-branched 1-olefins such as (S)- and rac-4-methyl-1-hexene by 1/MAO, leading to the corresponding highly syndiotactic polymers.^155,156

5.2 Conjugated diolefin polymers

The stereoselective polymerization of linear conjugated dienes leads to a variety of possible stereoregular, tactic polymer structures as a result of chemo-, regio- and stereoselectivities.¹⁵⁷ For instance, polymerization of 1,3-pentadiene (PD) can lead to one or more of the following 10 possible stereoregular polymers: 1,2-cis (iso- and syndiotactic), 1,2-trans (iso- and syndiotactic), 3,4-iso- and syndiotactic, 1,4-cis (iso- and syndiotactic), and 1,4-trans (iso- and syndiotactic) stereomicrostructures. Natta and co-workers reported in 1964 that homogeneous titanium alkoxide/aluminum alkyl catalyst systems converted 1,4-butadiene (BD) into syndiotactic 1,2-PBD (Chart 16), with concomitant formation of an amorphous polymer.¹⁵⁸ Many other catalyst systems, predominantly cobalt-based, have subsequently been developed for the synthesis of st-1,2-PBD, with a ternary system consisting of Co(acac)₃/organoaluminum/CS₂ being most effective, leading to st-1,2-PBD with high 1,2-selectivity (>99%), syndiotacticity (>99%), and crystallinity (>79%, T_m up to 216 °C).^159–161 Following the discovery of the syndiospecific polymerization of styrene by TiBz₄ and CpTiCl₃ (10) activated with MAO, the same catalyst systems were employed for the stereoselective polymerization of conjugated dienes. Zambelli,¹⁶² Oliva¹⁶³ and co-workers reported the synthesis of a syndiotactic 1,2-addition polymer from the polymerization of 4-methyl-1,3-pentadiene (4MPD) by TiBz₄/MAO and CpTiCl₃/MAO, respectively. The syndiotactic polymer produced by TiBz₄/100MAO at 20–30 °C contained ≥88% 1,2-units and is an amorphous materials, but catalytic hydrogenation afforded a fully saturated crystalline polymer with T_m = 186 °C.¹⁶² In comparison, the polymerization by CpTiCl₃/1000MAO at 20 °C or −18 °C led to ≥98% 1,2-syndiotactic P4MPD (Chart 16), and the syndiotactic polymer produced at −18 °C had a T_m of 95 °C.¹⁶⁴ Interestingly, the activity and stereoselectivity of the polymerization of 1,3-pentadiene by CpTiCl₃/500MAO are highly sensitive to T_p.¹⁶⁵ Thus, on going from T_p = 20 °C to 0 °C and −28 °C, the polymerization activity increased drastically from TOF = 15 h⁻¹ to 73 h⁻¹ and 154 h⁻¹, while the resulting PPD went from being ≥99% cis-1,4-isotactic to 72% cis-1,2-syndiotactic and ≥99% cis-1,2-syndiotactic (T_m ∼ 100 °C), respectively (Chart 16). Performing the polymerization of 1,3-pentadiene by CpTiCl₃/500MAO at −78 °C led to a polymer consisting exclusively of cis-1,2 units and having a syndiotactic structure, although the MW was still relatively low (M_n = 9.24 kDa and PDI = 1.7).¹⁶⁶ The crystalline trans-1,2-st-PPD (M_n = 211 kDa, PDI = 2.4, 63% rrrr, T_m = 164 °C) was synthesized with the CoCl₂(PⁱPrPh₂)₂–MAO system at −30 °C.¹⁶⁷ Natta et al. reported in 1962 the first synthesis of crystalline cis-1,4-st-PPD (T_m = 52–53 °C) using a cobalt catalyst system consisting of Co(acac)₃/AlEt₂Cl/H₂O–MAO.¹⁶⁸ The water in the system is expected to react with AlEt₂Cl to form an aluminoxane, the activator.

Chart 16 Structures of syndiotactic 1,2- and 1,4-conjugated diene polymers.

5.3 Bicyclic olefin polymers

Tactic polymers derived from the stereoselective ring-opening metathesis polymerization (ROMP) of bicyclic and polycyclic olefins have been recently reviewed,¹⁶⁹ and only several more recent examples are highlighted herein. Schrock and co-workers developed syndiospecific ROMP of dicarbomethoxy norbornadiene (DCMNBD) by a Z-selective, monoaryloxide-monopyrrolide (MAP) molybdenum imidoalkylidene catalyst (2 mol% 50a), affording a highly cis (>99%), syndiotactic (>99%) ROMP polymer (Chart 17).¹⁷⁰ Generation of such a stereomicrostructure for the ROMP polymer requires inversion at the Mo center with each forward metathesis step, namely through stereogenic catalyst-site control. Such MAP catalysts have been further utilized for the syndiospecific ROMP of 2,3-bis(trifluoromethyl)-bicyclo[2.2.1]hepta-2,5-diene and 3-methyl-3-phenylcyclopropene.¹⁷¹ Most recently, Schrock et al. synthesized cis,syndiotactic (>95%) ROMP polymers that incorporates alternating enantiomers through syndiospecific ROMP of racemic monomers, rac-endo-exo-5,6-dicarbomethoxynorbornene and rac-1-methyl-5,6-dicarbomethoxy-7-oxanorbornadiene, using 1 mol% MAP Mo catalyst 50b (Chart 17).¹⁷²

Chart 17 Synthesis of cis, syndiotacticROMP polymers containing alternating enantiomers.

5.4 (Meth)acrylamide polymers

Despite the structural similarities between (meth)acrylates and (meth)acrylamides and the successes in synthesizing highly isotactic polyacrylamides,^173,174 there have been very few reports on the syndiospecific polymerization of (meth)acrylamides. Okamoto and co-workers reported the syndiospecific radical polymerization of N-methyl methacrylamide.¹⁷⁵ Through screening a wide range of radical initiators and polymerization conditions such as solvent, temperature, and concentration, the authors found optimal conditions for high syndiospecificity. Thus, employing the ⁿBu₃B-air initiating system and performing the polymerization in (CF₃)₂CHOH at −78 °C with a [M]/[I] ratio of 50 resulted in only 7% polymer yield after 24 h, but the resulting polymer was highly syndiotactic (rr = 95%) and had M_n = 8 kDa and PDI = 2.4. Okamoto et al. carried out a similar study for the radical polymerization of N,N-dimethylacrylamide (DMAA) and N,N-diphenylacrylamide (DPAA) in fluoroalchols.¹⁷⁶ Interestingly, the radical polymerization of DMAA resulted in isotactic-biased polymers, whereas under similar polymerization conditions, polymerization of DPAA resulted in syndiotactic-biased polymers. On the other hand, the radical polymerization initiated by ⁿBu₃B-air, at −78 °C or −98 °C in THF, afforded highly syndiotactic PDPAA, with rr = 91% and 93%, respectively. The synthesis of highly syndiotactic PDMAA at much higher temperature (room temperature) has been recently achieved by catalyst-site-controlled coordination polymerization with C_s-ligated metallocenium catalysts.¹³⁴

6. Summary and outlook

As can be readily seen from the above survey, there has been an intense pursuit in the synthesis of highly syndiotactic polymers from a variety of monomer substrates. Among several mechanisms by which polymerization proceeds to yield highly syndiotactic polymers, coordination polymerization catalyzed by discrete metal complexes offers the capability for such synthesis to be carried out under economically viable conditions (e.g., industrial scale production of st-PP and st-PS). This trait is due to the ability of coordination polymerization to control the polymerization characteristics, especially its stereochemistry, at ambient and above temperatures, often in a catalytic fashion, by either site-control with chiral catalysts or chain-end control with steric interplay between the catalyst ligand, the coordinated monomer and the growing-chain end. It is also readily seen that the typical activity achieved for the coordination-insertion polymerization of nonpolar α-olefins is higher than that observed for the coordination-addition polymerization of polar conjugated olefins (acrylic monomers) by several orders of magnitude. Furthermore, the latter polymerization is typically carried out at low temperatures to achieve high syndiospecificity, until very recently, and is rarely catalytic. It is also important to recognize the fact that renewable monomer substrates derived directly from biomass are oxygenated, polar feedstocks. Hence, future research in this field will be largely directed at addressing these currently unmet challenges in the stereoselective polymerization of polar monomers, by developing stereoselective catalysts that can render high-speed coordination-insertion or -addition polymerization of polar feedstocks in a catalytic fashion, and exploring the synthesis of environmentally sustainable polymers, especially stereoregular ones, from renewable resources.

Acknowledgements

The authors are grateful for the financial support provided by the U. S. National Science Foundation (NSF-1012326).

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Footnote

† Current address: Division of Chemistry and Chemical Engineering, Caltech.

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