rtiary-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 non-biodegradable polymer units and has an average molecular weight of about 1,000 to 500,000 daltons, m and n