Tailoring chain structures of l-lactide and ε-caprolactone copolyester macromonomers using rac-binaphthyl-diyl hydrogen phosphate-catalyzed ring-opening copolymerization with monomer addition strategy

Abstract

Copolymerizations involving polyester macromonomers (MMs) generated from biomass provide a new route for introducing high biomass content into existing polymeric products. Stannous 2-ethylhexanoate-catalyzed ring-opening copolymerization (ROcoP) is commonly utilized to synthesize MMs, but this approach generates polymer chains containing terminal metal residues and limits control of MM chain structures due to the presence of transesterification side reactions (TSRs). Here, rac-1,1′-binaphthyl-2,2′-diyl-hydrogenphosphate (rac-BNPH) was used for the 2-hydroxylethyl methacrylate (HEMA) initiated ROcoP of L-lactide (L-LA) and ε-caprolactone (ε-CL) to produce the well-defined MMs. The copolymerization kinetics and monomer feeding strategies, batch and semibatch, were studied, and the influence on MM chain structures was investigated using both ¹H and ¹³C NMR analysis. The rac-BNPH was identified as an effective catalyst for the ROcoP of L-LA and ε-CL, producing narrowly dispersed MMs with a 96% retention of terminal vinyl groups associated with HEMA. The ε-CL was more reactive than L-LA, and the reactions exhibited characteristics of living polymerization. The TSRs could be significantly suppressed using batch operation or semibatch with fast ε-CL addition. It was found that slowing the ε-CL addition generated more randomly and uniformly distributed comonomers along MM chains. In general, it is demonstrated here that the rac-BNPH catalyzed semibatch ROcoPs is an effective means for tailoring the chain structures of MMs.