Trapping polystyrene radicals using nitrones: Synthesis of polymers with mid-chain alkoxyamine functionality

Polystyrene radicals were formed in the presence of NtBPN under a variety of reaction conditions and reactant ratios, forming polymer dimers of twice the molecular weight of the monobrominated polystyrene (PStBr) precursors. The polystyrene radicals were generated by the activation of the monohalogentated polystyrene precursors, prepared by atom transfer radical polymerization (ATRP). The extent of radicals trapped by the nitrone, and therefore containing mid-chain alkoxyamine functionality, was determined by thermolysis of the polymer dimers, with the C-O bond in this functionality being cleaved and reverting the chains back to the approximate size of the precursors. Polymer dimers could also be formed simply by radical-radical combination of the chain-ends, which is conventional atom transfer radical coupling (ATRC) and thus contains a head-to-head C-C bond, rendering them inert to thermolysis under conditions that cleave the C-O bond. It was found that near quantitative alkoxyamine mid-chain functionality could be achieved by activating the PStBr in the presence of 10 equivalents of nitrone, 5 equivalents of copper bromide, and 2 equivalents of copper metal. Further reducing the amount of copper metal led to incomplete coupling, while increasing the equivalents beyond 2 generated polymer dimers with less than quantitative mid-chain functionality. Monochlorinated polystyrene (PStCl) precursors gave much poorer coupling results compared to reactions with PStBr, which is consistent with the stronger C-Cl bond resisting activation and the formation of the polystyryl radicals.