and mixtures of said compounds; wherein each R1 is independently a hydrocarbon group of from 1 to 18 carbon atoms or R1 is an organofunctional hydrocarbon group of from 1 to 12 carbon atoms, said functional group being amino, carboxylic acid, carbinol ester or amido; each X is independently chosen from halogen, amino, alkoxy groups of from 1 to 12 carbon atoms and acyloxy groups of from 1 to 12 ca