Synthesis of Block Copolymers of Varying Architecture Through Suppression of Transesterification during Coordinated Anionic Ring Opening Polymerization
Well-defined di- and triblock copolymers consisting of ε-caprolactone (CL), L-lactide (LA), and trimethylene carbonate (TMC) were synthesized via “PLA first route” in coordinated anionic ring opening polymerization/copolymerization (CAROP) with tin (II) octoate as catalyst. The desired block structure was preserved by use of protective additive α-methylstyrene by preventing the transesterification side-reactions. MALDI-TOF analysis revealed that the protection mechanism is associated with α-methylstyrene and tin (II) octoate complexation. Additionally, it was shown that use of α-methylstyrene in ring opening polymerization allowed the formation of polyesters with high molar mass. 1. Introduction Biodegradable block copolymers prepared from L-lactide, -caprolactone, 1,4-dioxan-2-one, and trimethylene carbonate have been synthesised and studied extensively during recent decades [1, 2]. The varying physical properties of block polymers allows for the combination of “soft” and “hard” polymers giving rise to copolymers that can be tuned for specific function such as elasticity [3, 4]. The sequence in which blocks are synthesised into block copolymers is specific and is determined by the choice of monomer and catalyst. For example, at the application of tin octoate as catalyst, the block copolymer structure polycaprolactone-polylactide (PCL-PLA) is formed if the ε-caprolactone is polymerized first, followed by polymerization of L-lactide. However, a random copolymer is obtained when PLA is initially synthesised followed by PCL [5, 6]. This is a result of transesterification of the PLA and segmentation of the block polymer. Two competing reactions during CAROP (coordinated anionic ring opening polymerisation) occur; (i) ring-opening of ester bonds in molecules of the initial cyclic monomer and (ii) cleavage of the ester bonds in the macromolecules of the polymer. These competing reactions depend on the choice of catalyst, the existing polymer, which acts as a macroinitiator, and the type of monomer added at the second stage of polymer synthesis. Atering the catalyst utilized allows the formation of varying sequences of blocks copolymers. For example, Y(CF3COO)3/Al(iso-Bu)3 catalyst or the complex [Y(L6)-{N(SiHMe2)2}(THF)] promotes the initial synthesis of the PLA block, followed by the PCL block [7]. Florczak et al. showed that selectivity of Al(OiPr)3 catalyst could be accomplished through coordination with SB(OH)2 ((S)-(t)-2,20-[1,10-binaphtyl-2,20-diylbis(nitrylomethilidyne)]diphenol), allowing synthesis of the block structure PCL-PLA-PCL, where the PLA
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