Sequence Analysis of Styrenic Copolymers by Tandem Mass Spectrometry

Abstract

Styrene and smaller molar amounts of either <i>m</i>-dimethylsilylstyrene (<i>m</i>-DMSS) or <i>p</i>-dimethylsilylstyrene (<i>p</i>-DMSS) were copolymerized under living anionic polymerization conditions, and the compositions, architectures, and sequences of the resulting copolymers were characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and tandem mass spectrometry (MS²). MS analysis revealed that linear copolymer chains containing phenyl–Si­(CH₃)₂H pendants were the major product for both DMSS comonomers. In addition, two-armed architectures with phenyl–Si­(CH₃)₂–benzyl branches were detected as minor products. The comonomer sequence in the linear chains was established by MS² experiments on lithiated oligomers, based on the DMSS content of fragments generated by backbone C–C bond scissions and with the help of reference MS² spectra obtained from a polystyrene homopolymer and polystyrene end-capped with a <i>p</i>-DMSS block. The MS² data provided conclusive evidence that copolymerization of styrene/DMSS mixtures leads to chains with a rather random distribution of the silylated comonomer when <i>m</i>-DMSS is used, but to chains with tapered block structures, with the silylated units near the initiator, when <i>p</i>-DMSS is used. Hence, MS² fragmentation patterns permit not only differentiation of the sequences generated in the synthesis, but also the determination of specific comonomer locations along the polymer chain