Polymerization and degradation of aliphatic polyesters synthesized by atom transfer radical polyaddition

Abstract

Two AB*-type inimers (4-vinylbenzyl 2-bromo-2-isobutyrate (VBBiB) and 4-vinylbenzyl 2-bromo-2-phenylacetate (VBBPA)) were synthesized and used to obtain aliphatic polyesters through atom transfer radical polyaddition (ATRPA). First, we optimized the ATRPA of VBBiB at a moderate temperature (<25°C) using a commercially available CuBr 2 /Cu(0)/dNBpy system; however, the reaction run at 70°C exhibited self-degrading behavior, as revealed by molecular weight (MW) variations in GPC traces and evidence of a five-membered-ring lactone structure (5-(4-(bromomethyl)phenyl)dihydro-3,3-dimethylfuran-2(3H)-one) in 1 H NMR spectra. We suspected that the structural rearrangement occurred through ring-closure depolymerization of PVBBiB induced by the pendant bromine atoms serving as leaving groups. Accordingly, by varying the leaving group affinities, we performed a series of model reactions that guided us toward polymers with stable backbones. We thus synthesized three additional aliphatic polyesters PVBCiB (C: chloro), PVBBPA, and PVBAiB (A: azido) through ATRPA of VBBiB with halogen exchange, ATRPA of VBBPA, and azido substitution of bromine atoms, respectively. We confirmed the aliphatic polyester structures of PVBCiB and PVBBPA with high MWs (e.g., M w,PVBBPA = 25 550) which fulfilled our desire to obtain high-MW aliphatic polyesters through ATRPA method. From a comparison of the four aliphatic polyesters, PVBBiB and PVBCiB displayed significant self-degrading profiles over periods of several days, whereas PVBBPA and PVBAiB did not. This study unveils the limitations (relating to lactonization) of ATRPA involving AB*-type inimers and also widens the scope of the process.

Synthesis of Hyperbranched Polymers via Metal‐Free ATRP in Solution and Microemulsion

“…Among these highly branched polymers, hyperbranched polymers receive particular interest from both industries and academia because of their one‐pot inexpensive synthesis and retention of arborescent structures. Hyperbranched polymers typically could be produced in one pot via step‐growth polymerization of multifunctional AB m (m ≥ 2) monomers, [ 7–13 ] step‐growth copolymerization of A n and B m monomers (e.g., A 2 + B 3 ), [ 14–16 ] chain‐growth polymerization of AB m (m ≥ 2) monomers, [ 17–19 ] chain‐growth copolymerization of divinyl cross‐linkers, [ 20–26 ] self‐condensing vinyl polymerization (SCVP) of polymerizable initiators (inimers), [ 27–35 ] or polymerizable transfer agents (transmers). [ 36–40 ] It is important to note that any of these methods could produce hyperbranched polymers and randomly branched polymer as their difference lies in the fraction of branching unit incorporated into the polymer backbone.…”

Synthesis of Highly Branched Copolymers in Microemulsion

“…The ability to degrade these branched polymers under acidic condition represents an interesting property and has been achieved in hyperbranched polymers by incorporating various functional groups into the polymer architecture . Using an acetal‐containing inimer ABIEM, previously developed in our group, a copolymer was synthesized at ratios of [IEM] 0 :[ABIEM] 0 :[CuBr 2 ] 0 = 50:10:0.5.…”

“…Among the various synthetic methods, radical‐based self‐condensing vinyl polymerization (SCVP) of inimers (containing ini tiator fragment and mono mer vinyl group in one molecule) and/or transmers (containing chain trans fer group and mono mer vinyl group in one molecule) has become particularly popularly in the past two decades. All established reversible deactivation radical polymerizations (RDRP) techniques, including atom transfer radical polymerization (ATRP), nitroxide‐mediated polymerization (NMP) and reversible addition–fragmentation chain transfer (RAFT) polymerization have been intensively applied in the (co)polymerization of inimers and transmers, primarily in solution, for synthesis of branched polymers with various compositions. For instance, those synthesized by ATRP possess high density of chain‐end initiating groups and are easy to extend with vinyl monomers using the same technique .…”

Synthesis of Poly(N-vinylpyrrolidone)-Based Polymer Bottlebrushes by ATRPA and RAFT Polymerization: Toward Drug Delivery Application

“…In our previous study, we demonstrated a highly efficient ATRPA of AB type inimer containing an ethyl 2-bromo-2-phenylacetate (EBPA) based initiating site and a styrene group [56]. Accordingly, our previously reported 4-vinylbenzyl 2-bromo-2-phenylacetate (VBBPA) inimer possesses very high activation rate of the EBPA initiating site at the polyester chain end and high selectivity for the initiations between the chain end and the polyester backbone (i.e., benzyl halide initiating sites).…”

“…Based on the mechanism study, the copper catalysts possess high selectivity between the initiating site at chain end (EBPA) and the initiating site at backbone (PEBr) ( k act,EBPA / k act,PEBr = ca. 30,000) [56]. Therefore, we attained fast ATRPA and obtain linear type polyesters.…”