-thiazolindyl, pyridyl, and benzothiazolyl, provided that when R is p-nitrophenyl, R' is other than tertiary-butyl, benzyl or cyclohexyl, and when X is sulfur, R is other than benzyl;(b) a polymer of the formulawhereinP is a polymer comprising at least one unit of the formula (A.sub.m B.sub.n), wherein (A.sub.m B.sub.n) are substantially nonbiodegradable polymer units and has an average molecular