r 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 weight of about 1,000 to 500,000 daltons, m and n may be thesame or different and are about 5