Examples of the connecting group B include alkylene groups such as a methylene group, ethylene group, methylethylene group, 1,2-dimethylethylene group, propylene group, butylene group, methoxymethylethylene group and hexylene group, a polyether group represented by the formula: ##STR7## wherein t is an integer of 1 to 10, and a polyalkyleneamino group represented by the formula: ##STR8## wherein t