Low-temperature suspension polymerization of vinyl pivalate for the preparation of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) microfibrils with high yield

Abstract

To prepare ultrahigh molecular weight (UHMW) poly(vinyl pivalate) (PVPi) with high conversion and high linearity for a precursor of syndiotacticity-rich UHMW poly(vinyl alcohol) (PVA), vinyl pivalate (VPi) was suspension polymerized using a low-temperature initiator, 2,2'-azobis(2,4-dimethylvaleronitrile) (ADMVN), and the effects of polymerization conditions on the polymerization behavior and molecular structures of PVPi and PVA prepared by saponifying PVPi were investigated. Suspension polymerization was slightly inferior to bulk polymerization in increasing the molecular weight of PVA. In contrast, the former was superior in increasing the conversion of the polymer. Suspension polymerization of VPi at 25 degreesC by controlling various polymerization factors proved to be successful in obtaining PVA of UHMW (number-average degree of polymerization (P-n): 14,700-16,700), high syndiotactic diad content (62%), and of high yield (ultimate conversion of VPi into PVPi: 85-90%). In the case of bulk polymerization of VPi under the same conditions, maximum P-n, conversion of 15,800-17,000, and 25-35% were obtained, respectively. The degree of branching was lower and the P-n and syndiotacticity were higher with PVA prepared from PVPi polymerized at lower temperatures. All PVAs from PVPi suspension-polymerized at 25 degreesC were fibrous, with a high degree of crystallinity and orientation of the crystallites.