wherein m and n are an integer such that m+n is 38 on average, and R2 is a divalent group represented by the formula: ##STR37## 1.96 g of cyclohexanone (0.02 mol in terms of a carbonyl group); 1.98 g of cyclohexylamine (0.02 mol in terms of an amino group); 0.5 g of dibutyltin dilaurate; 8 g of fumed silica; and 2 g of methyltri(methylethylketoxyimino)silane were mixed in an anhydrous state to pre