A polymer having a number average molecular weight (Mn) of from 10,000 to 5,000,000, as determined by gel permeation chromatography and reduced to polystyrene, and repeating units represented by the following formula (1): ##STR00001## wherein R.sup.1 and R.sup.2 may be the same with or different from each other and are substituted or unsubstituted alkyl groups having 1 to 12 carbon atoms, a is a number of 0, 1, 2 or a mixture thereof, and b is a number of 0, 1, 2 or a mixture thereof.