Copolymerization of l-lactide/trimethylene carbonate by organocatalysis: controlled synthesis of comb-like graft copolymers with side chains with different topologies

Abstract

A series of linear-comb poly(trimethylene carbonate)-block-poly(L-lactide) [lcP(TMC-b-LLA)] with polybutadiene (PB) as backbone and a P(TMC-b-LLA) diblock copolymer as side chain were obtained using controlled synthesis and evaluated as thermoplastic elastomers. The comb-like graft copolymer was achieved using sequential ring-opening polymerization of trimethylene carbonate (TMC) and L-lactide (L-LA) at room temperature (RT) using an organocatalyst, associated with the multi-hydroxyl PB (PB–OH) as a macroinitiator. The thermal and mechanical properties of these graft copolymers were investigated using differential scanning calorimetry, dynamic mechanical analysis and tensile testing. The Young's modulus (E) and the elongation at break (ε_b) followed predicted trends as the monomer composition changed. Furthermore, well defined linear-comb PLLA-gradient-PTMC [lcP(LLA-grad-TMC)] and linear-comb PLLA-random-PTMC [lcP(LLA-ran-TMC)] were also synthesized for comparison. Results from ¹H and ¹³C-NMR spectroscopy, and gel permeation chromatography confirmed the formation of the chain microstructures. It was found that the properties of the copolymers depended not only on the comonomer content but also on their topologies. The results showed that the gradient and random structure of the side chain could yield unusual properties: the lcP(LLA-grad-TMC) copolymer possessed a distinctive, broad glass transition temperature in comparison with the other two graft copolymers. The lcP(LLA-ran-TMC) random copolymer performed like a typical rubber with a significant high ε_b value (ε_b = 1800%) which was even larger than the pure linear-comb PTMC (ε_b = 865%). Furthermore, this simple “one-pot” method using an organocatalyst to synthesize comb-like graft copolymers with “block”, “gradient” and “random” side chains is demonstrated systematically in this work.