Macromolecular engineering via ring-opening polymerization (1): L-lactide/trimethylene carbonate block copolymers as thermoplastic elastomers

Poly(L-lactide) (PLLA) is often regarded as tough and brittle while poly(1,3-trimethylene carbonate) (PTMC) is rather considered as a rubbery polymer. In an effort to improve the mechanical properties - especially ductility - of PLLA and thus to widen its field of applications, PLLA-PTMC diblock and triblock copolymers were synthesized through the sequential copolymerization of both L-lactide (L-LA) and trimethylene carbonate (TMC) using several catalytic systems. This process can be effectively catalyzed by inherently different systems ranging from a simple basic organocatalyst such as an amine (i.e., 4-N, N-dimethylaminopyridine, DMAP) or a phosphazene (i.e., 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro- 1,3,2-diazaphosphorine, BEMP), a simple Lewis acidic metallic salt such as aluminum triflate, or a more sophisticated discrete metallo-organic complex derived from a biofriendly metal, namely the beta-diiminate zinc complex [(BDIiPr)Zn(N(SiMe3)(2))] (BDI = CH(CMeNC6H3-2,6-iPr(2))(2)), [(BDIiPr) Zn(N(SiMe3)(2))]. Well-defined diblock PLLA-b-PTMC, triblock PLLA-b-PTMC-b-PLLA and 3-arm star GLY(PTMC-b-PLLA)(3) copolymers with controlled molecular features, i.e. controlled functional end-groups and molar masses, rather narrow dispersity values, were thus prepared. The thermo-mechanical properties of the resulting copolymers revealed that a minimal block size of the PTMC and of the PLLA segments within the copolymer of M-n,M-PTMC = ca. 10 000 g mol(-1) and M-n,M-PLLA = ca. 23 000 g mol(-1) enables significant improvement of the elongation at break (epsilon(b)) of PLLA up to 328%, while maintaining the Young's modulus (E = 2781 MPa) close to that of PLLA (E = 3427 MPa).