C-13 n.m.r. spectroscopy was used to differentiate between block and statistical copolymers of hydroxymethylthiirane and methylthiirane. Block copolymers could be exclusively obtained if hydroxymethylthiirane constituted the first sequence; however, if the first sequence consisted of methylthiirane, the synthesis of block copolymers failed because of a termination reaction. We demonstrate that the termination reaction is partially avoided in a protic medium.