The photoinitiated copolymerization of vinyl pivalate (VPi) and vinyl acetate (VAc) was carried out in bulk. Varying monomer feed ratios of VPi/VAc (mol %/mol %) of 100/0, 90/10, 60/40, 50/50, and 30/70 were used. Polyvinyl alcohol (PVA) with s-diad contents ranging from 61.8% to 55.7% was obtained. The morphology and thermal properties of the different PVA samples were investigated. PVA prepared from PVPi homopolymer resulted in PVA with a well-aligned microfibrillar structure. With a decrease in the VPi content in the precursor polymer, the fibrous morphology was gradually lost, resulting in PVA with mixtures of fibrillar and spherical structures. When the PVPi content was 38.5 mol %, only spherical particles were observed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3460-3465, 2010