Disclosed are an optically active, ethylenically unsaturated monomer represented by the following structural formula: ##STR1## wherein: A stands for S, SO or SO.sub.2 ; X and Y each stand for a hydrogen atom or a substituent selected from the group consisting of alkyl groups and acyl groups; Z stands for a substituent selected from the group consisting of alkoxy, alkylamino, hydroxyl and silyloxy groups; and X and Y can be bonded together to form a ring with the nitrogen atom to which they are bonded or X and Y can be a single group bonded to said nitrogen atom through a double bond, with the proviso that at least one of X and Z is an alkenyl equivalent to said group, and polymers of this monomer. One typical instance of the monomer is benzyl 6-acrylaminopenicillanate.