le bonds of the main chain derived from the 1,4-addition units (side chain double bonds derived from 1,2-addition units/main chain double bonds derived from 1,4-addition units) is in the range of 10/90 to 99/1; wherein R1 and R2 are each independently a hydrogen atom, an alkyl group of 1 to 8 carbon atoms or an aryl group, and at least one of R1 and R2 is a hydrogen atom.