toms and X is aliphatic acyl containing up to 8 carbon atoms or cyano; and R is an alkyl group containing up to 10 carbon atoms, a cycloalkyl group containing up to 10 carbon atoms, a phenyl group or RaO C CH2X where R is an alkylene group containing up to 10 carbon atoms and X is aliphatic acyl containing up to 8 carbon atoms or cyano, provided that one and only one of R and R is always --Ro-O( 3