Analysis of acrylic polymers by MALDI-TOF mass spectrometry

Abstract

Poly(methyl methacrylate) (PMMA) homopolymers synthesised using 'classical' anionic methods and subsequently studied by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) are discussed. Specifically, the attempts at different end-group functionalisation reactions, their varying degrees of success, and the characterisation of these functionalized polymers via MALDI are reported. Extra peaks were observed in the spectra of samples containing a tertiary amine end-group. A mechanism for the in situ elimination of H(_2)(g) involving these end-groups, which would fit the observations, is proposed. Two alternative, 'non-classical' routes to the desired materials were investigated, as difficulties in successfully performing capping reactions to give end functionalised PMMA were noted. The first method was a variation of standard anionic polymerisation that involved the use of lithium silanolates, which could be performed at a higher temperature than normal. The second was a controlled free-radical technique known as Reversible Addition-Fragmentation Chain Transfer (RAFT). A lack of control of the polymerisation to the desired degree was observed with the former method. A well-defined RAFT sample was observed to undergo in situ eliminadon also, for which a mechanism involving the dithioester end-group is proposed, and which is supported by MALDI-collision induced dissociation (CID) evidence. The synthesis of block copolymers of various compositions of MMA with r-butyl methacrylate (t-BMA) and hexyl methacrylate (HMA), along with their homopolymers, and their subsequent characterisation is reported. PHMA was analysed easily, in contrast to Pt-BMA. Only copolymers with a high PMMA content were analysed successfully and this has been rationalised in terms of the factors that affect cationisation. The characterisation of equimolar blends of various end-functionalised PMMA samples is reported also. Samples that favour the binding of a metal ion over protonation appear to have a higher ion yield. Once more, these observations are rationalised in terms of the factors that affect cationisation.