G and G' are preferably independently selected from the group consisting of reactive functional groups including hydroxyl groups (both alcoholic and phenolic) and esters thereof, carboxylic acid groups, thiocarboxylic acid groups, thiocarboxylic and carboxylic esters, preferably lower alkyl esters of from one to about 12 carbon atoms, such as methyl and ethyl esters, acyl halides such as chlorides