Ion‐trapping in cationic polymerization, 2. Relative rates of trapping and relative chemical shifts for structurally differing phosphines as trapping agents

The reaction of ethyl trifluoromethanesulfonate with phosphines to form phosphonium salts was used as a model reaction for the “trapping” of the various propagating species by phosphines in the polymerization of THF and other cyclic ethers. In dichloromethane at 25°C, the values of the bimolecular rate constant ktr were determined for various phosphines. For (C6H5)3P:5.10−3 mol−1.l.s−1, for (C6H5)(CH3)2P:0,22 mol−1.l.s−1, for (C6H5)(n‐C3H7)‐(CH3)P:0,13 mol−1.l.s−1, and for (n‐C4H9)3P: 1,3 mol−1.l.s−1. It was found that the ratio ktr/ka (ka is the rate constant of the ethylation of THF by ethyl trifluoromethanesulfonate) changes from 40 for triphenylphosphine to 104 for tributylphosphine. Differences of chemical shifts, measured for various quaternary phosphonium cations differing in one substituent, were observed for ions with various substituents (e.g. based on parent methylpropylphenylphosphine).