A phosphonium intermediate for cationic RAFT polymerization

Abstract

Phosphates and phosphinates were used as reversible chain-transfer agents for controlling the cationic polymerization of vinyl ethers initiated by a trace amount of triflic acid via the reversible addition–fragmentation chain-transfer (RAFT) mechanism with a phosphonium intermediate. The phosphorus chain-transfer agents, which were prepared by adding the corresponding phosphoric and phosphinic acids to vinyl ethers, enabled the synthesis of polymers with controlled molecular weights up to M_n ∼ 1 × 10⁵ and narrow molecular weight distributions (M_w/M_n ∼ 1.1). The obtained polymers are colorless and odorless, similar to the phosphorus chain-transfer agents. The phosphoric acids can also be directly used as precursors of the RAFT agents in conjunction with a small amount of TfOH and can control the molecular weights of the resulting polymers irrespective of the presence of an induction period. The phosphonium intermediate via the RAFT mechanism was also supported by ³¹P NMR analysis of the model reactions. The degenerative chain-transfer (DT) or RAFT mechanism was also confirmed by MALDI-TOF-MS analysis of the polymers obtained via crossover reactions between the two different reversible chain-transfer agents, i.e., phosphates and thioethers. The chain-transfer constants (C_tr) were calculated for various RAFT and DT agents in the cationic polymerization of isobutyl vinyl ether under the same conditions. The C_tr values decreased in the order of dithiocarbamate > phosphate ∼ phosphinate > trithiocarbonate > thioether > xanthate ∼ carbamate > acetate > chloride > trifluoroacetate.