CO2-Tuned Sequential Synthesis of Stereoblock Copolymers Comprising a Stereoregularity-Adjustable Polyester Block and an Atactic CO2-Based Polycarbonate Block

Stereoblock copolymers composed of atactic CO2-based polycarbonate and various stereoregular polyester blocks with cis-2,3-(exo, exo), cis-2,3-(endo, endo), or partly trans-2,3-(exo, endo) repeating units were successfully synthesized via sequential ring-opening copolymerization (ROCOP) of cyclohexene oxide (CHO) with norbornene anhydride (NA) and then ROCOP of CHO with CO2 using a salcyCrCl/ PPNCl binary catalyst. The structure of the block copolymers was confirmed by NMR and GPC. Incorporation of carboxyl groups by the thiol-ene reaction of the pendant norbornenyl groups further confirmed the polymer structures. The geometric structure of polyester units in the block copolymers is tailored simply by varying the NA isomer and the monomer feed ratio of CHO to NA. Notably, CO2 suppressing the configuration transformation from cis-(exo, exo) to trans-(exo, endo) of polyester block has been revealed in the sequential ROCOP. Using this strategy, the stereospecific polyester with an dominant cis-2,3-(exo, exo)-poly(NA-alt-CHO) units, the thermodynamically less stable isomeric form, was obtained in the ROCOP of exo-NA with an excess of CHO in the presence of CO2. The role of CO2 in restraining the geometric transformation from cis-(exo, exo) to trans-(exo, endo) was proposed in the ROCOP of exo-NA with an excess of CHO catalyzed by PPNCl.