A polymer comprising units of formula (1-1) or (1-2) and having a weight average molecular weight of 1,000 to 500,000 is provided. R1 is an acid labile group, R2 is H or straight or branched C1-4 alkyl, Z is a tetravalent C2-10 hydrocarbon group, and k=0 or 1. A resist composition comprising the polymer as a base resin has significantly improved sensitivity, resolution and etching resistance and is very useful in microfabrication.