Hydrolyzable groups X designate independently from each other and without limitation alkoxy groups -0-R 1 , wherein R 1 preferably represents a linear or branched alkyl group, preferably a CrC 4 alkyl group or an alkoxyalkyl group, an acyloxy group -0-C(O)R 3 wherein R 3 is an alkyl group, preferably a d-C 6 alkyl group, better an ethyl or methyl group, halogens such as Cl or Br, amino group optio