Mass spectrometric investigations into free radical polymerisation reaction mechanisms

Abstract

Contemporary mass spectrometry (MS) instrumentation featuring electrospray ionisation (ESI) or matrix-assisted laser desorption/ionisation (MALDI) ion sources were used to characterise the polymer distributions generated in various free radical polymerisations, allowing insights to be gained into the reaction mechanisms operating in these systems.In studying atom transfer radical polymerisation (ATRP) mediated star polymerisations of methyl acrylate (MA), ESI was found to be more effective in obtaining a comprehensive list of the distinct products present in the samples under investigation when compared to the employed MALDI technique. Furthermore, these studies showed that terminal Br losses observed at relatively high monomer to polymer conversions could be accounted for via mechanisms involving the acetone derived radicals (CH3)2ĊOH, ĊH3 and ĊH2COCH3.Through the use of ESI, it was found that for bulk polymerisations of MA and aqueous media polymerisations of N-isopropylacrylamide (NIPAAm) initiated using 60Co γ-irradiation and mediated via reversible addition-fragmentation chain transfer (RAFT), hydrogen radicals formed via the radiolysis of RAFT agent and/or monomer, and in the case of the NIPAAm system, water, are capable of initiating the polymerisations. In the NIPAAm polymerisations under scrutiny, it was also observed that hydroxyl radicals generated via the radiolysis of water may contribute towards the initiation process, and that propagating chains can potentially become terminated via trithiocarbonate cleavage reactions.By using ESI instruments to characterise oligomer samples produced via the free radical polymerisations of vinyl phosphonates, it was observed that chain propagations are initiated via activated monomer radicals, which likely form as a result of transfer reactions involving initiator fragments and vinyl phosphonate monomer units. Transfer to monomer reactions were suggested to limit chain growth in these systems, and evidence was also found for scission reactions involving alkoxy moieties which are formed via intramolecular methine carbon abstraction reactions.Characterisation of the polymer distributions generated in R-group approach RAFT MA star polymerisations using an ESI instrument allowed formation processes operating in acrylate star living/controlled radical polymerisations (CRPs) to be ascertained. Initiator fragment derived linear chains, ideal stars, star-star couples, and terminated star products formed as a result of disproportionation and combination reactions were detected. Evidence for mid-chain radical (MCR) derived reaction pathways was also observed; specifically, for termination reactions involving intermolecularly formed MCRs on both star arms and linear chains, and for re-propagation of intermolecularly formed MCRs on star arms