Synthesis of strictly alternating copolymers by living carbanionic copolymerization of diphenylethylene with 1,3-pentadiene isomers

Abstract

A well-defined strictly alternating copolymer was synthesized via the living anionic copolymerization of 1,3-pentadiene (PD) isomers with 1,1-diphenylethylene (DPE) in cyclohexane using n-BuLi as initiator. Intriguingly, the novel copolymer appeared to exhibit a much higher degree of alternation since neither DPE nor PD isomers are capable of forming homo-polymers within 24 h below 30 °C, and DPELi is also unable to cross-initiate the PD isomers due to its huge steric hindrance. Herein we investigated the DPE/PD copolymerization kinetic results, which are affected by solvent, temperature and PD isomer, as well as the comonomer feeding ratio. The results showed that the copolymer yield reached a maximum when an equimolar feeding ratio of the comonomer pair is maintained in a non-polar solvent. Meanwhile, the copolymerization rate of cis-1,3-pentadiene (CP) with DPE is much higher than that of the trans isomer (TP), as the formed zwitterion compound (DPC) derived from the DPE/CP pair shows higher reactivity when it is attacked by carbanionic species. In particular, the PD/DPE (1/1) copolymerization initiated by n-BuLi yielded living, well-defined, alternating (r_DPE·r_PD = 0, r_DPE = 0 and r_PD ≈ 0 below 30 °C) and quantitative copolymers with controllable M_n and low Đ_M (1.15–1.39). From a comparison of the apparent propagation rate constants k^app_p, the order of copolymerization activity was as follows: cis ≈ mixed isomer > trans. The values of the apparent activation energies E^app_a for the isomer copolymerizations were calculated to be 52.60 kJ mol⁻¹ and 47.95 kJ mol⁻¹ for the trans and cis isomers, respectively. The first-order kinetic resolution and NMR spectra demonstrated that the obtained copolymers possess strictly alternating repeating units containing 1,4-/1,2-chains in a ratio of 88%/12%, and no 3,4-units. The DSC results showed that the copolymers were amorphous with T_g of 93–97 °C, and that the non-crystallinity may have contributed to the 12% 1,2-units randomly incorporated into the macromolecular chain. FTIR spectroscopy showed that the 1,4-PD chains were predominantly trans-1,4 rather than cis-1,4 units. In general, the copolymerization behavior is somewhat similar to the free radical alternating copolymerization of CTC compounds. The formation of a macro zwitterion (DPC) through the distribution of electronic charges from the donor (PD isomer) to the acceptor (DPE) is proposed in the carbanion alternating copolymerization mechanism.