Anionic polymerization of 1,3-pentadiene in toluene: homopolymer, alternating and block copolymers

Abstract

The anionic polymerization of (E)-1,3-pentadiene (EP) and (Z)-1,3-pentadiene (ZP) was performed in aromatic solvent using n-butyllithium as initiator. In contrast to poly(ZP)s, the resulting poly(EP)s possessed the predicted molecular weights and narrow molecular weight distributions (Đ ≤ 1.15). However, THF as polar additive has proved its validity to reduce the molecular weight distribution of poly(ZP) from 1.55 to as low as 1.16. The ¹H NMR and ¹³C NMR results indicated that both poly(EP) and poly(ZP) consist of 1,4-addition & 1,2-addition units, without 3,4-addition units. Moreover, a novel, well-defined styrene–pentadiene alternating copolymer has been synthesized in toluene via living anionic copolymerization and the NMR spectra demonstrated that the resulting copolymers obtained possess a strictly alternating structure and much narrower polydispersity (Đ ≤ 1.17). Finally, the living nature of the polymerization was also inspected via sequential copolymerization with methyl methacrylate (MMA) and 2-vinylpyridine (2VP) to yield well-controlled diblock copolymers (poly(EP)-b-poly(MMA) & poly(EP)-b-poly(2VP)).

---

Introduction

It is well known that 1,3-pentadiene is a large-scale byproduct of the processes of hydrocarbon cracking or isoprene production with properties similar to isoprene and cyclopentadiene.¹ However, 1,3-pentadiene has mainly been utilized to produce low value-added hydrocarbon resins via uncontrolled cationic polymerization until now. Actually, a rather limited number of authors have focused on the potential application of poly(1,3-pentadiene) in the domain of elastomers and synthetic fibers.^2,3 Therefore, the synthesis of well-defined poly(1,3-pentadiene) to develop high value-added and novel industrial products is of great significance.

In principle, 1,3-pentadiene has two isomeric forms (cis- and trans-configuration, i.e. EP and ZP), which can be polymerized to exhibit a variety of isomeric structures (such as cis-1,4, trans-1,4, cis-1,2, trans-1,2, and 3,4-units as well as their combinations). Since Natta et al. firstly synthesized a polymer from 1,3-pentadiene in 1955,⁴ a large number of studies, most of which concentrate on the stereospecific coordination polymerization and cationic polymerization, were devoted to investigate the polymerization of this conjugated 1,3-diene.^5–10 The stereospecific coordination polymerization of ZP or EP monomer can generate several kinds of stereoregular polypentadienes with predominant cis-1,4,⁵ trans-1,4,⁶ cis-1,2 (ref. 7) or trans-1,2 (ref. 8) units. However, nearly all stereospecific catalyst systems were unable to afford polymers with predictable number-average molecular weights and rather broad molecular weight distributions.^5–9 In addition, the cationic polymerization of 1,3-pentadiene inevitably gave uncontrolled insoluble fraction derived from the serious side reactions, such as chain transfer, intramolecular cyclization and double bond isomerization.¹⁰ In fact, the first case of living well-defined polypentadiene (Đ ∼ 1.05) synthesized by anionic polymerization in saturated hydrocarbon solvent reported by our groups.¹¹

As we all know, living anionic polymerization is a powerful tool for the synthesis of a variety of model materials with well-defined molecular characteristics.^12–14 Furthermore, the living anionic polymerization systems also allow the syntheses of tailored multi-block copolymers by the sequential copolymerization. Since block segments are generally thermodynamically incompatible, they are phase-separated at the molecular level, followed by self-organizing to assemble, to form three-dimensional periodic nanostructures and supramolecular assemblies. Such self-assemblies have attracted considerable attention due to the ability to arrange functional domains at the nanoscale, which are expected to play an important role in molecular devices with many potential applications in the fields of nanoscience and nanotechnology.¹⁵ However, it is difficult to synthesize the living well-controlled diblock polymer derived from 1,3-diene and polar monomer in nonpolar solvents alone due to the conflicting solvent requirements. Some undesirable reactions can occur with certain polar monomers, even these monomers can be used to prepare the last block in a sequence.¹⁶ Some literature reports several examples of the syntheses of the well-defined diene–alkyl methacrylate or diene-2-vinylpridiene block copolymers. For example, the poly(butadiene)-block-poly(methyl methacrylate) (PB-b-PMMA) diblocks have been made by polymerizing butadiene in benzene, then adding THF, and finally adding the MMA at −78 °C after capping the chains with 4-vinylpyridine.¹⁷ Poly(isoprene)-block-poly(methyl methacrylate) (PI-b-PMMA) diblocks have been prepared by firstly polymerizing isoprene in n-hexane using sec-BuLi as initiator, then removing the n-hexane under vacuum and replacing with THF prior to the addition of DPE together with LiCl, and finally adding MMA at −90 °C.¹⁸ The diblock copolymers of isoprene and 2VP were also prepared in benzene solution by using sec-BuLi as initiator in the presence of LiCl.¹⁶

Recently, one of our research has been focused on the living anionic polymerization of 1,3-pentadiene and found that the RLi initiators, the solvents as well as the polar additives have a great influence on the anionic polymerization of 1,3-pentadiene isomers.^11,19 According to the results, the toluene has proved to be a perfect candidate as a typical aromatic hydrocarbon solvent in the case of anionic polymerization of 1,3-pentadiene. More importantly, polar monomers (such as MMA) as well as its homopolymers have well solubility in toluene even at −78 °C. Therefore the well-defined amphiphilic block copolymers can be easily achieved by using toluene as solvents without considering the procedure of removing the hydrocarbon solvent under high vacuum before polar monomer polymerization. Herein this paper describes the anionic homopolymerization of 1,3-pentadiene, alternating copolymerization of 1,3-pentadiene with styrene as well as blocking copolymerization of 1,3-pentadiene with MMA and 2VP using n-BuLi as initiator in toluene or in toluene/THF mixed solvents and discusses the microstructure of the resulting products.

Experimental

Materials

All reagents were purchased from Aldrich unless otherwise specified. THF (99%) and toluene (99%) were refluxed over sodium/diphenylketyl and then distilled under dry nitrogen before use. Methyl methacrylate (MMA, 99%) and 2-vinylpyridine (2VP, 97%) were distilled over CaH₂ under reduced pressure and finally distilled over triethylaluminum on the vacuum line. 1,1-Diphenylethylene (DPE) was distilled over a small amount of n-BuLi under high vacuum. Lithium chloride (99.99%) was dried in the oven at 120 °C under vacuum for 24 h. n-BuLi (1.6 mol L⁻¹ in n-hexane) was used without further purification. Methanol (99.5%) was sparged with nitrogen for 10 min to remove dissolved oxygen.

Styrene (St; Polymer grade), supplied by Yueyang Baling Petrochemical Chem. Co. Ltd., was distilled from CaH₂ under vacuum and then stored at 0 °C under nitrogen. (E)-1,3-Pentadiene (≈93.2%, containing 2.1% (Z)-1,3-pentadiene, 0.5% isoprene, 0.5% 1-pentine, 1.2% cyclopentadiene, 2.5% cyclopentane), (Z)-1,3-pentadiene (≈98.2%, 1.8% cyclopentane) are supplied as components from C₅ fractions by ShangHai Petrochemical Chem. Co. Ltd. All 1,3-dienes were purified by refluxing over CaH₂ for 4 h and distilled under nitrogen.

Purification of 1,3-pentadiene

The purification of 1,3-pentadien is a long-term and highly complex procedure. The main purpose of the thorough procedure was to effectively inhibit the initiator deactivation which can be caused by the trace amounts of cyclopentadiene and C₅ alkynes (>50 ppm). The procedure used for the purification of 1,3-pentadiene consists of the following several steps: (a) stand overnight with stirring in silver ammonia solution at room temperature; (b) repeat step (a) at least three times till the precipitate is no longer observed, followed by the distillation under a dry nitrogen atmosphere; (c) stand overnight with stirring in CaH₂ at room temperature; (d) stand overnight with stirring in n-BuLi at room temperature (the slow propagation reaction at room temperature allows this relatively long-term exposure); (e) if necessary, repeat step (d); (f) distill and collect the final purified monomer into the Schlenk tube under dry nitrogen.

Step (b) was of necessity to minimize the concentration of terminal alkyne and cyclopentadiene. Step (d) was crucial as it decreases the percent of the cyclopentadiene residue and unreacted alkyne to as low as 10 ppm or less, as detected by gas chromatography spectroscopy (GC) on HP-1 capillary column (60 m × 0.32 mm × 0.25 μm).

Anionic polymerization of 1,3-pentadiene and alternating copolymerization of 1,3-pentadiene with styrene

All operations were carried out in glove box (H₂O, O₂ < 1 ppm) at room temperature, and the typical synthesis is as follows: 3 mL anhydrous solvent was added to a 10 mL Schlenk tube equipped with a magnetic stir bar using 5000 μL pipette. The measured amount of initiator was then supplied to this solution with 200 μL pipette. To start the polymerization, the purified 1,3-pentadiene (including EP and ZP, wt% = 10–15%) was introduced to this mixture, then the Schlenk tube was transferred from glove box to oil bath at setting temperature. After the polymerization, the living polymerization was terminated by adding a few drops of degassed MeOH, followed by transferring this solution to a 10 mL centrifuge tube. The solution was washed several times by a large volume of MeOH to precipitate out the polymer. The separated product was dried under vacuum at 40 °C for 48 h. The polymerization of mixtures of 1,3-pentadiene and styrene were carried out according to the homopolymerization procedure.

To investigate the kinetics of 1,3-pentadiene homopolymerization and copolymerization, the polymerization mixture was needed to stand for a prescribed polymerization time and quenched with degassed MeOH. All conversions were determined by weighting capacity unless otherwise stated.

Block copolymerization of EP with St

Using n-BuLi as an initiator, EP was polymerized in toluene at 80 °C for 2 h. A small aliquot was removed from the solution and rapidly quenched in methanol for analysis. Subsequently, styrene was added and allowed to polymerize for 4 h at 50 °C. The block copolymers were recovered by precipitation in methanol. They were dried under vacuum at 40 °C until constant weight.

Block copolymerization of EP with MMA

Using n-BuLi as an initiator, EP was polymerized in toluene at 80 °C for 2 h. A small aliquot was removed from the solution and rapidly quenched in methanol for analysis. The living poly(EP) anions were end-capped with 1,1-diphenylethylene (DPE) at room temperature for 1 h. LiCl containing THF ([LiCl]/[living sites] = 6) was then added to the reactor, with formation of a toluene/THF(50/50, v/v) mixture (deep red color), to which MMA was finally added with the immediate disappearance of the red color and polymerized at −78 °C. The block copolymers were recovered by precipitation in methanol. They were dried under vacuum at 40 °C until constant weight.

Block copolymerization of EP with 2VP

Block copolymerization of EP with 2VP was carried out following the above procedure for the block copolymerization of EP with MMA. No color change was noticed after the addition of 2VP. Methanol was used for terminating the polymerization and the resulting colorless solution was poured into water to precipitate the block copolymer. The block copolymer was dried under vacuum at 60 °C for 24 to 48 h and the yield was typically quantitative.

Measurements

The concentration of impurity in 1,3-pentadiene was measured in a HP gas chromatograph (GC) equipped with FID detector (180 °C) and a HP-1 column at oven temperature 18–20 °C (first step: 20 °C remaining for 15 min; two step: rise to 180 °C at ramp rate of 10 °C min⁻¹).

The number-average molecular weights (M_n) and molecular weight distributions (Đ) of the polymer samples were determined using a Waters GPC liquid chromatograph (Waters, USA) equipped with gel columns (300 × 7.8 mm). THF was used as the eluent, and the flow rate was 1.0 mL min⁻¹ at room temperature. A molecular weight calibration was established using polystyrene (PS) standards.

¹H NMR spectra of the polymer samples were recorded with a Varian INOVA-400 spectrometer at room temperature. ¹³C NMR measurements were performed on a Varian INOVA-400 spectrometer using an “Inverse Gate” (delay time = 5 s) procedure, allowing a quantitative determination. Chemical shifts were recorded in ppm downfield relative to CDCl₃ (δ = 7.26 ppm) and CDCl₃ (δ = 77.2 ppm) for ¹H NMR and ¹³C NMR as standard, respectively.

The IR spectra were recorded on a IR Affinity-1 instrument (with resolution of 2 cm⁻¹) using polymer films on KBr disks. The films were obtained by deposition from polymer solutions in CS₂ solvent.

T_g values of polymer samples were measured by DSC using a NETZSCH instrument DSC200F3 apparatus under nitrogen. The polymer samples were first heated to 150 °C, cooled to −100 °C, and then scanned at a rate of 10 °C min⁻¹.

Results and discussion

Anionic polymerization of 1,3-pentadiene

In order to obtain basic information concerning the polymerization reaction, the anionic polymerization of 1,3-pentadiene was carried out with n-BuLi under various conditions as shown in Table 1. The polymerization of EP was successfully performed with n-BuLi in toluene and the complete conversion of EP was achieved within 2 h (run 2). The yield of the polymer was quantitative and the gel permeation chromatography (GPC) showed that the resulting poly(EP) maintained a unimodal and narrow shape (Fig. 1a). The polydispersity index, M_w/M_n, was always around 1.09, indicating the narrow molecular weight distribution (MWD). On the other hand, when using THF as the solvent, the existence of the chain-transfer and chain-termination side reaction gave rise to a low yield (<60%) and much broader MWD (run 6). Similar phenomenon (run 4, 5 and 7, Fig. 1b) was observed in the case of ZP under the same condition, only the MWD of poly(ZP) (Đ > 1.5) obtained in toluene was rather higher than that of poly(EP). All these results indicated that the chain-termination and chain-transfer reaction or at least the relatively slow initiation process may exist when the polymerization of ZP was carried out in toluene. Therefore, increasing the initiation reaction rate will contribute to solve this problem. To prove this point, the anionic polymerization of ZP in toluene was carried out by using THF as polar additive for the purpose of forming loosen ion pair with much faster initiating reactivity. Actually it gave a relatively higher yield at low temperature (40 °C) and kept the molecular weight distribution relatively stable and narrow (1.16 < Đ < 1.40) by adding measured amount of THF as polar additive within a certain range (THF/Li < 50) (Table 2 and Fig. 1c). Of course, the yield of the poly(ZP) was decreased with the increasing THF/n-BuLi molar ratio, as shown in Table 2. These results indicated that the broad MWD of poly(ZP) may be attributed to the relatively slow initiation process as well as the rapid propagation compared with the chain-termination or chain-transfer reaction.

Table 1 Anionic polymerization of 1,3-pentadiene under various conditions^a

Run	Solvent	Monomer	Tp (°C)	Time (h)	Yield (%)	Mn^b (kg mol^−1)		Đ^b
						Calcd^b	Obsd^c
a Polymerization was carried out under dry nitrogen in Schlenk tube; monomer/solvent = 0.2–0.4 g/3.0 mL, [n-BuLi]0 = 5.0–5.2 × 10^−3 mol L^−1.b Mn(calcd) = (Mw of monomer) × [M]/[I].c Mn(obsd) and Đ(Mw/Mn) were determined by GPC calibration using polystyrene standards in THF.
1	Toluene	EP	40	24	92	16.1	17.1	1.09
2	Toluene	EP	80	2	100	17.0	17.2	1.09
3	Toluene	EP	80	4	100	36.5	38.5	1.07
4	Toluene	ZP	40	2	83	29.5	41.1	1.55
5	Toluene	ZP	60	20 min	63	22.4	34.0	1.45
6	THF	EP	80	12	58	11.3	5.3	1.55
7	THF	ZP	70	12	60	11.7	6.1	1.66

---

Fig. 1 GPC curves of (a) poly(EP) prepared with n-BuLi in toluene (Table 1, run 1, M_n,obsd = 17.1 kg mol⁻¹, Đ = 1.09), (b) poly(ZP) prepared with n-BuLi in toluene (Table 1, run 4, M_n,obsd = 41.1 kg mol⁻¹, Đ = 1.55) and (c) poly(ZP) prepared with n-BuLi/THF (1.00/1.00) system in toluene (Table 2, run 8, M_n,obsd = 37.7 kg mol⁻¹, Đ = 1.16).

Table 2 Effect of THF on the anionic polymerization of ZP in toluene at 40 °C for 24 h^a

Run	Initiator system ([n-BuLi]/[THF])	Yield (%)	Mn^b (kg mol^−1)	Đ^b
a Polymerization was carried out under dry nitrogen in Schlenk tube at 40 °C for 24 h. Monomer/solvent = 0.4 g/3.0 mL, [n-BuLi]0 = 4.0 × 10^−3 mol L^−1.b Mn and Đ (Mw/Mn) were determined by GPC calibration using polystyrene standards in THF.
8	1.00/1.00	100	37.7	1.16
9	1.00/5.00	98	34.1	1.21
10	1.00/10.00	94	31.2	1.28
11	1.00/25.00	93	30.7	1.30
12	1.00/50.00	90	28.9	1.35

---

Microstructure analysis

When the anionic polymerization of 1,3-diene is studied, the microstructure analysis and control of the resulting polymer is of great importance since the physical properties of the polymer strongly depend on the corresponding microstructure.²⁰ Herein the microstructure of the resulting polypentadiene was analyzed by ¹H and ¹³C NMR spectroscopy.

Fig. 2 shows a typical ¹H NMR spectrum of poly(EP) obtained by anionic polymerization of EP with n-BuLi in toluene. The complete assignment and method to calculate individual content of cis-1,4, trans-1,4, cis-1,2 and trans-1,2-units have been reported in our previous study.

Fig. 2 ¹H NMR spectrum of the poly(EP) obtained with n-BuLi in toluene.

Besides ¹H NMR, ¹³C NMR spectrum was used to give further information on the sequence distributions in the main chain. As shown in Fig. 3a, the spectra of samples exhibit a large number of peaks both in the aliphatic carbon regions (10–45 ppm) and olefinic methylene (–CH=CH₂) signals between 110 and 141 ppm. Actually, the analyses to DEPT135 spectrum of poly(ZP) have the same results regardless of polymerization temperature. Thus, combined with the ¹H NMR and ¹³C NMR spectrum, it can be easily concluded that the poly(EP) and poly(ZP) samples (shown in Table 3) contained predominantly trans-1,4 units, as well as the combination of the cis-1,4, trans-1,2, cis-1,2 units with the absence of 3,4 units.

Fig. 3 ¹³C NMR: (a) ordinary complete decoupling and (b) DEPT135 spectra of poly(EP) prepared at 40 °C in toluene.

Table 3 The microstructure and diad sequence distributions of the polypentadiene^a

Run	Microstructure^b (%)
	trans-1,4	cis-1,4	trans-1,2	cis-1,2	3,4-
a For reaction conditions see Table 1.b Determined by ^1H NMR and ^13C NMR.
1	63.1	20.9	7.6	8.4	0
2	58.9	22.3	7.9	10.9	0
4	63.1	13.4	16.4	7.1	0
5	66.7	10.3	17.8	5.2	0

---

Meanwhile, the polymerization temperature has less effect on the microstructure of the polymer chain.

Kinetic studies

In the previous study, it was found that the living controlled anionic polymerization of ZP was not successful when carried out in toluene. Thus, we compared polymerization behavior of ZP with that of EP in order to estimate the polymerization rate of them. The results of kinetic investigations with 1,3-pentadiene at different temperatures ranging from 40 °C to 70 °C were shown in Fig. 4a and b. The experiments showed that the first-order reaction rates in toluene under the same conditions are listed in the following order: ZP > EP. For the anionic polymerization of EP, the initiator system induced relatively slow and the conversion can hardly reach up to 90% at low temperature (T < 50 °C). However, the conversion can easily exceed 90% within 2 h at high temperature (T ≥ 60 °C). It can be explained that the polymerization rate of EP (T_b ≈ 42 °C) strongly depends on the absorption and diffusion efficiency of monomer in toluene solution. In addition, it is very clear that there is a relative long stationary-conversion platform (i.e., induction period of polymerization) during the initial polymerization reaction at low temperature (T < 50 °C), which is not observed in the anionic polymerization ZP in toluene under the same condition.

Fig. 4 Time-conversion curves for the anionic polymerization of EP (a) and ZP (b) with n-BuLi in toluene; first-order time-conversion plots of EP (c) and ZP (d) with n-BuLi in toluene; Arrhenius plot of the apparent rate constant k′′_p for the anionic polymerization of EP (e) and ZP (f) with n-BuLi in toluene. Polymerization was performed under dry nitrogen in Schlenk tube at different temperature: monomer/solvent = 0.3 g/3.0 mL, [n-BuLi]₀ = 5.2 × 10⁻³ mol L⁻¹.

As shown in Fig. 4c and d, the logarithmic conversion data, ln(1−x) (x is the monomer conversion), plotted against time t, gave straight lines, which shows constant concentrations of the growing species during the polymerizations. From the slopes observed in Fig. 4c and d, we have estimated k′′_p (the apparent rate constant, which was determined from the slops in the case of curved first-order time-conversion plots) value at each temperature as shown in Table 4. The k′′_p values for EP, which were lower than that for ZP at temperatures below 70 °C under the same experiment condition, were strongly depending on the polymerization temperature. The Arrhenius plots (Fig. 4e and f) of the apparent rate constants, k′′_p, is linear in the range of 40–70 °C without considering the induction time under low temperature. The apparent values of the activation parameters are given as follows:

Table 4 Apparent rate constants of anionic polymerization of 1,3-pentadiene

Tp (°C)	k′′p × 10^−4 (L mol^−1 min^−1)
	EP	ZP
40	18.5 ± 0.6	195.5 ± 12.2
50	70.8 ± 4.5	311.1 ± 11.3
60	241.8 ± 15.1	465.3 ± 31.6
70	693.2 ± 65.5	671.3 ± 82.5

---

EP monomer A = 1.47 × 10¹⁵ E′′_a = 107.11 ± 2.07 kJ mol⁻¹.

ZP monomer A = 2.60 × 10⁴ E′′_a = 36.66 ± 0.69 kJ mol⁻¹.

Then the apparent activation energy E′′_a and the ln A value of the polymerization reaction of EP is significantly larger than that of ZP under the same experiment conditions. This all indicates that the temperature has much more effect on the k′′_p value for EP than that for ZP and supports the higher polymerizability and propagation rate of ZP, which may contribute to the broad MWD of poly(ZP).

As shown in Fig. 5, the polymerization kinetics study of EP isomer demonstrated that the conversion increased with polymerization time, and the number-average molecular weight (M_n) of the poly(EP) product increased linearly with the conversion while the molecular weight distribution remained much narrow(Đ ≤ 1.15). All these findings suggest that neither chain-termination nor chain-transfer reaction occur, i.e., the polymerization has both living and controlled character.

Fig. 5 M_n and Đ curves of poly(EP) obtained with n-BuLi in toluene at 70 °C: monomer/solvent = 0.3 g/3.0 mL, [n-BuLi]₀ = 5.2 × 10⁻³ mol L⁻¹.

Alternating copolymerization of 1,3-pentadiene and St

Near recently, we accidentally synthesized a series of alternating copolymers in one-pot method via living anionic copolymerization of 1,3-pentadiene with St with n-BuLi in cyclohexane.²¹ Herein we also found this interesting alternating phenomenon and synthesized the well-defined alternating copolymers in toluene. As shown in Fig. 6, anionic copolymerization of 1,3-pentadiene/St mixtures with various comonomer feeding ratios were carried out in toluene at 50 °C. According to the results of kinetic investigations, the feeding ratios affected the reaction rates and the copolymerization rate increased with the increasing amount of St. As summarized in Fig. 6c and d, when equimolar mixture of 1,3-pentadiene and St was allowed to react, the copolymer consisted of the two monomers with the ratio 1 : 1 irrespective of the reaction time, which means that the rate of consumption of two monomers are equal. Moreover, the obtained alternating copolymers exhibited a well-controlled M_n and much narrow MWD (in Fig. 6e and f). Specially, when the reaction was carried out in various feed ratios of St to ZP, the initial composition of the copolymers constantly remained as the ratio of 1 : 1 irrespective of the feed ratios. Notably, when one of the comonomers was consumed up, the living poly(St-alt-ZP)-b-poly(St) or poly(St-alt-ZP)-b-poly(ZP) product was finally obtained as shown in Fig. 7. However, the alternating tendency of copolymerization of St and EP was strongly depend on the feeding ratio, which yields slightly alternating or random copolymer with the molar ratio not being 1 : 1. It can be interpreted by the relatively strong electron-donating property of the terminal methyl group (δ = 13.07 ppm) and rigid-plan geometry of ZP molecular.²¹ Besides, we also used the ¹³C NMR spectra in detail to analyze the microstructure of alternating copolymer and it gave no evidence for either vinyl groups (δ = 124.3–125.0 ppm) or 3,4-units (δ = 115.0–116.5 ppm) (Fig. 7a). In conclusion, we have demonstrated the unprecedented anionic alternating copolymerization of 1,3-pentadiene with styrene in aromatic solvents.

Fig. 6 (a) Time-conversion curves of St and ZP at 50 °C; (b) time-conversion curves of St and EP at 50 °C; (c) the mole ratio of St : ZP in copolymer (from ¹H NMR) at different conversion rates; (d) the mole ratio of St : EP in copolymer (from ¹H NMR) at different conversion rates; (e) GPC curves of poly(St-alt-ZP) obtained with n-BuLi in toluene at 50 °C; (f) GPC curves of poly(St-alt-EP) obtained with n-BuLi in toluene at 50 °C.

Fig. 7 (a) ¹³C NMR of poly(St-alt-ZP); (b) ¹H NMR of poly(St-alt-ZP)-b-poly(St); (c) ¹H NMR of poly(St-alt-ZP)-b-poly(ZP); (d) ¹H NMR of poly(St-alt-ZP).

Block copolymerization of EP

From the synthetic viewpoint, one of the most important advantages of living polymerization is to synthesize block copolymer with well-defined structures by sequential addition of two or more different monomers, which offers block copolymer with predictable molecular weight and narrow MWD. In this section, we performed the sequential copolymerization of EP with St, MMA and 2VP. After a complete conversion of EP, a small amount of the sample was withdrew for characterization and a known amount of St was added to resume the polymerization. The colorless of polypentadiene living species turned orange color immediately after the addition of St, and the polymerization was carried out under dry argon at room temperature for 12 h. As for MMA and 2VP, after a complete conversion of EP, a small amount of the sample was withdrew for characterization and a known amount of DPE ([DPE]/[n-BuLi] = 2) were added for 1 h to end-cap the poly(EP) anion. LiCl, as an additive to reduce side reaction,^18,22 containing THF was then added to the reactor, with formation of a toluene/THF mixture (deep red color). To restrict potential side reactions in the polymerization of MMA and 2VP, the reaction mixture was cooled down to −78 °C prior to the addition of MMA and 2VP. The poly(EP)-b-poly(St) and poly(EP)-b-poly(MMA) reaction mixture was then precipitated in methanol and the poly(EP)-b-poly(2VP) reaction mixture in water. The precipitated polymer was isolated and dried. Fig. 8 showed the ¹H NMR spectrum of the diblock polymers with poly(EP) as first segment. The aromatic proton signals of poly(St) were observed between 6.3 to 7.5 ppm. The signal around 3.6 ppm was attributed to the methyl group of poly(MMA). The adjacent proton to nitrogen in pyridine ring appeared between 8.1 to 8.4 ppm. Fig. 9 exhibited the clear shift of GPC profile from poly(EP) to poly(EP)-b-poly(St), to poly(EP)-b-poly(MMA) and to poly(EP)-b-poly(2VP) irrespectively. The diblock polymers with a small amount of homopolymer exhibited much narrow molecular weight distributions (Đ < 1.2), indicating the deactivation of partly living chain ends due to the impurities in THF solvent and the second monomer. Therefore, the polymerization of EP in toluene is thought to be a living controlled polymerization.

Fig. 8 ¹H NMR spectrum of block polymerization of EP with St, MMA and 2VP: (a) poly(EP)-b-poly(St); (b) poly(EP)-b-poly(MMA); (c) poly(EP)-b-poly(2VP).

Fig. 9 GPC curves of block polymerization of EP (first monomer) with MMA (second monomer) and 2VP (second monomer) in toluene: (a) poly(EP), M_n = 27 822 g mol⁻¹, M_w = 31 160 g mol⁻¹, Đ = 1.12; poly(EP)-b-poly(St), M_n = 51 553 g mol⁻¹, M_w = 58 255 g mol⁻¹, Đ = 1.13; (b) poly(EP), M_n = 9303 g mol⁻¹, M_w = 10 350 g mol⁻¹, Đ = 1.11; poly(EP)-b-poly(MMA), M_n = 12 531 g mol⁻¹, M_w = 14 348 g mol⁻¹, Đ = 1.14; (c) poly(EP), M_n = 17 677 g mol⁻¹, M_w = 19 718 g mol⁻¹, Đ = 1.11; poly(EP)-b-poly(2VP), M_n = 27 781 g mol⁻¹, M_w = 32 779 g mol⁻¹, Đ = 1.18.

Differential scanning calorimetry (DSC)

DSC is a very useful technique to detect phase separation in binary blends and block copolymers containing immiscible polymeric components, provided that the individual glass transition temperatures (T_g) are sufficiently different from each other and the weight composition is far enough from extreme values.²³ Typical examples of various block copolymers recorded as a result of a second scan at 10 °C min⁻¹ were shown in Fig. 10a. The transition at low temperature (−48 to −49 °C) attributed to the glass transition of the soft poly(EP) phase were clearly observed for all the samples. This transition temperature was actually the low service temperature of the diblock copolymer. The second transition temperature for PS in poly(EP)-b-poly(St), P2VP in poly(EP)-b-poly(2VP) and PMMA in poly(EP)-b-poly(MMA) were 65, 95 and 114 °C, respectively. Observation of two glass transitions indicated that these materials were phase-separated.

Fig. 10 DSC traces of the diblock copolymers and alternating copolymers: (a) poly(EP)-b-poly(St) (Fig. 9a), poly(EP)-b-poly(2VP) (Fig. 9c) and poly(EP)-b-poly(MMA) (M_n(EP) = 17 164 g mol⁻¹, M_{n(poly(EP)-b-poly(MMA))} = 53 430 g mol⁻¹, Đ = 1.15); (b) poly(St-alt-ZP) (Fig. 6e), poly(St-alt-EP) (Fig. 6f). T_g values from the midpoint of the transion are listed around each curve.

DSC thermograms of St/1,3-pentadiene alternative copolymers with temperature range from −20 to 130 °C were shown in Fig. 10b. The St/ZP and St/EP alternative copolymers, without exception, showed a single phase transition corresponding to the glass transition, which was not a perfect platform and similar to a peak shape due to thermal aging. Compared to EP homopolymer with T_g of −48 °C, the St/1,3-pentadiene alternative copolymers with more rigid structure, originating from benzene in the side chains of the copolymer, possessed much higher T_g ranging from 17 to 28 °C. For the same reason, the T_g of the alternative copolymers were much lower than that of St homopolymers. Furthermore, the dependence of T_g on the molecular weight of St/1,3-pentadiene alternative copolymer can be seen in Fig. 10b. The T_g of the St/1,3-pentadiene alternative copolymers increased concomitantly with the molecular weight in the present work.

Mechanism

Though the microstructure of the polypentadiene was determined to be a mixture of 1,4- and 1,2-units, the mode of monomer addition was not clear. C₁ and C₄ of each monomer used in this study could be attacked by the propagating chain-end carbanion as shown in Scheme 1. However, due to the existence of 1,2-units and no evidence of 3,4-units, it is likely that the propagating carbanion attacked preferentially at C₁ on the monomer to form 1,4- and 1,2-anions. This may be explained by the occurrence of steric hindrance derived from the terminal methyl group, if the propagating carbanion attacks the C₄ on the monomer. In addition, Scheme 1 illustrates the formation of predominant 1,4- and 1,2-units as well as inexistence of 3,4-units. As proposed in this mechanism, methyl and methylene linked with C₄ anion and C₂ anion respectively belong to the electron-donating group, which can facilitate the transformation between the 1,4-anion and 1,2-anion. Moreover, the methine, C₄, linked with C₃ anion, promote the transformation from the 4,3-anion to 4,1-anion, which can't achieve in turn for the lack of electron-donating group linked to C₁ anion. Thus, the 4,1-anion should be more stable than the 4,3-anion and only the 1,4-, 1,2- and 4,1-anions participate in propagation, resulting in the formation of the observed 1,4 and 1,2 structures in the polymer.

Scheme 1 Expected structure of the propagating carbanion.

Conclusions

The anionic polymerization of EP and ZP was carried out in toluene using n-BuLi as an initiator. Rather than poly(ZP)s, the poly(EP)s with much narrow molecular weight distributions and predicted molecular weights were obtained. Microstructure analysis of the resulting polymers by NMR revealed that the poly(EP) and poly(ZP) samples have a great variety of isomeric structures, that is a mixture of trans-1,4, cis-1,4, trans-1,2, cis-1,2, and no 3,4-units was observed in the polymer chain. Meanwhile the detailed kinetic studies showed that the apparent polymerization activation energy as measured for EP (107.11 kJ mol⁻¹) is much higher than that for ZP (36.66 kJ mol⁻¹). Thus, the ratio of chain propagation rate to initiation rate for ZP is larger than that for EP, which directly gives poly(ZP) with broad molecular weight distribution (Đ > 1.5). Furthermore, the well-defined poly(EP-alt-St), poly(ZP-alt-St), poly(EP)-b-poly(St), poly(EP)-b-poly(MMA) and poly(EP)-b-poly(2VP) copolymers were prepared in toluene or toluene/THF solvent and characterized by NMR, GPC and DSC. Finally, an interpretation for microstructure of poly(1,3-pentadiene) was proposed considering the presence or absence of methyl group in propagating chain end greatly affecting formation of branched structure.

Acknowledgements

We thank ShangHai Petrochemical Chem. Co. Ltd. and YueYang BaLing Petrochemical Chem. Co. Ltd. for the 1,3-pentadiene and styrene as gifts. The authors are grateful to HongWen Liang and JianSong Fu for helpful discussion. The acknowledgements come at the end of an article after the conclusions and before the notes and references.

References

1. N. A. Plate and E. V. Slivinskii, Fundamentals of chemistry and technology of monomers: Manual. nauka, MAIK “nauka/interperiodika”, Moscow, 2002, pp. 3–696.
2. A. Carbonaro, N. Zamboni and G. Novajra, Rubber Chem. Technol., 1973, 46, 1274; G. Natta and L. Porri, in Polymer chemistry of Synthetic Elastomers, High Polymers, ed. J. P. Kennedy and E. G. M. Tornqvist, Interscience Publ., New York, vol. XXIII, part II, 1969, p. 597.
3. S. I. Shkurenko, T. A. Barakova, G. A. Gabrielyan and Z. A. Rogovin, Khim. Volokna, 1977, 4, 35.
4. G. Natta, L. Porri and G. Mazzanti, (Montecatini SpA). Italian Patent 538453, 1956Chem. Abstr. 1959, 53, 3756.
5. G. Ricci and S. Italia, Macromolecules, 1994, 27, 886; B. Purevsuren, G. Allegra, S. V. Meille, A. Farina, L. Porri and G. Ricci, Polym. J., 1998, 30, 431; G. Ricci and M. Battistella, Macromolecules, 2001, 34, 5766; C. Costabile, G. Guerra, P. Longo and S. Pragliola, Macromolecules, 2004, 37, 2016.
6. G. Natta, L. Porri, P. Corradini, G. Zanini and F. J. Ciampelli, Polym. Sci., 1961, 51, 463; S. Bruckner and G. D. Silvestro, Macromolecules, 1986, 19, 235; R. Napolitano, Macromolecules, 1988, 21, 622.
7. G. Ricci, E. Alberti, L. Zetta, T. Motta, F. Bertini, R. Mendichi, P. Arosio, A. Famulari and S. V. Meille, Macromolecules, 2005, 38, 8353.
8. C. Costabile, G. Guerra, P. Longo and S. Pragliola, Macromolecules, 2005, 38, 6327; G. Ricci, T. Motta, A. Boglia, E. Alberti, L. Zetta, F. Bertini, P. Arosio, A. Famulari and S. V. Meille, Macromolecules, 2005, 38, 8345.
9. G. Leone, A. C. Boccia, G. Ricci, A. Giarrusso and L. Porri, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 3227; X. Y. Jia, Y. M. Hu, Q. Q. Dai, J. F. Bi, C. X. Bai and X. Q. Zhang, Polymer, 2013, 54, 2973.
10. F. Duchemin, V. Bennevault-Celton, H. Cheradame, C. Merienne and A. Macedo, Macromolecules, 1998, 31, 7627; M. Delfour, V. Bennevault-Celton, H. Cheradame, F. Mercier and N. Barre, Macromolecules, 2003, 36, 991; V. A. Rozentsvet, N. A. Korovina, V. P. Ivanova, M. G. Kuznetsova and S. V. Kostujk, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 3297.
11. K. Liu, Q. He, L. Ren, L. G. Gong, J. L. Hu, E. C. Ou and W. J. Xu, Polymer, 2016, 89(20), 28.
12. Anionic Polymerization: Principles and Practical Applications, ed. H. L. Hsieh and R. P. Quirk, Marcel Dekker Inc., New York, 1996.
13. D. Baskaran and A. H. E. Müller, Prog. Polym. Sci., 2007, 32, 173.
14. A. Hirao, R. Goseki and T. Ishizone, Macromolecules, 2014, 47, 1883.
15. Y. Matsuo, R. Konno, T. Ishizone, R. Goseki and A. Hirao, Polymers, 2013, 5, 1012.
16. P. Q. Roderic and C. G. Sergio, Macromolecules, 2001, 34, 1192.
17. R. Fayt and P. Teyssié, Macromolecules, 1986, 19, 2077.
18. C. M. Fernyhough, R. N. Young and R. D. Tack, Macromolecules, 1999, 32, 5760.
19. K. Liu, Q. He, L. Ren, E. C. Ou, F. Xu and W. J. Xu, Living Anionic Polymerization of (E)-1,3-Pentadiene and (Z)-1,3-Pentadiene isomers, J. Polym. Sci., Part A: Polym. Chem., 2016 DOI:10.1002/pola.28100.
20. K. Takenaka, S. Amamoto, H. Kishi, H. Takeshita, M. Miya and T. Shiomi, Macromolecules, 2013, 46, 7282.
21. K. Liu, L. Ren, Q. He and W. J. Xu, Macromol. Rapid Commun., 2016, 37, 752.
22. J. W. Klein, J. P. Lamps, Y. Gnanou and P. Rempp, Polymer, 1991, 32(12), 2278.
23. J. M. Yu, P. Dubois and R. Jérôme, Macromolecules, 1997, 30, 4984.

---