A phenol copolymer was synthesized by the free-radical copolymerization of p-hydroxybenzaldehyde (HBA) and p-phenolsulfonate with horseradish peroxidase (HRP)/H2O2 as an initiator. The synthesis process included aldehyde protection via acetalization, enzymatic polymerization, and aldehyde deprotection via hydrolysis of acetals. The effects of the monomer ratio and the HRP and H2O2 composites on the properties of poly(p-hydroxybenzaldehyde-co-p-phenolsulfonate) [poly(HBA-co-PS)] were investigated by the determination of the shrink temperature (T-s) of tanned leather. The poly(HBA-co-PS) structure was characterized by Fourier transform infrared spectroscopy, NMR, UV spectroscopy, and gel permeation chromatography. The mechanism of enzymatic copolymerization, including initiation, H transfer, radical transfer, coupling, termination, differed from that of traditional free-radical polymerization. A remarkable characteristic was that poly(HBA-co-PS) had a large conjugated structure in its molecules that markedly improved the activity of the aldehyde and hydroxyl groups and showed easy crosslinking in the leather fibers. The applied results showed that the T-s of tanned leather with poly(HBA-co-PS) reached 83.6 degrees C. Poly(HBA-co-PS) could be used as agent for tanning leather. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 523-529, 2013