Fragmentation pathways of methacrylic homopolymers with labile trialkylsilyl ester side-groups-A mass spectrometric investigation of the RAFT process

Fragmentation pathways of methacrylic polymers containing labile silyl ester pendant groups were characterized upon collisional activation of electrosprayed oligomer adducts. Dissociation of lithiated oligomers mainly generated low mass product ions containing the a end-group and arising from backbone bond homolytic cleavages, as typically observed for poly(alkylmethacrylate)s. Combined with the sum of the end-group masses as determined from MS data, this main reaction allowed both end-groups to be validated. In contrast, loss of HCOOR (with R = triisopropylsilyl) or elimination of alkene species from alkylsilyl groups, such as reported from poly(butylmethacrylate), was not observed here, probably due to steric hindrance of the pendant moieties. Activation of ammonium oligomer adducts was found to provide highly complementary information, since they exhibited a similar behavior as compared to poly(methacrylic acid), with serial eliminations of a triisopropylsilanol neutral conducting to the determination of the polymerization degree of the dissociating precursor ion. It should be noted however that this useful pathway was not always observed to efficiently compete with dissociation of reactive omega end-groups. Based on these fragmentation rules, five poly(triisopropylsilyl methacrylate)s were identified in a sample obtained via reversible addition-fragmentation chain transfer (RAFT) polymerization using cyanoisopropyl dithiobenzoate as the chain transfer agent. Their structure could be rationalized with regards to RAFT processes: (i) dormant chains initiated by a cyanoisopropyl group and terminated by a dithiobenzoate group, as the main species: (ii) proton- and (iii) vinyl-terminated dead chains produced by disproportionation termination events: (iv) alpha,omega-cyanoisopropyl-terminated polymer chains arising from a combination termination between two macromolecular propagating radicals, and (v) dormant chains containing methacrylic acid monomer units within the chain. (C) 2011 Elsevier B.V. All rights reserved.