wherein R1 is each independently an organic group of 1 to 20 carbon atoms, preferably C1-C20 alkyl, C2-C20 alkenyl, C6-C20 aryl or C7-C20 aralkyl group; X is a halogen atom such as chlorine or an alkoxy group, typically C1-C4 alkoxy group; and n is 1, 2 or 3.