Preparation of ultrahigh molecular weight syndiotactic poly(vinyl alcohol) microfibrillar fibers by low-temperature solution polymerization of vinyl pivalate in tertiary butyl alcohol and saponification

Vinyl pivalate (VPi) was solution polymerized in tertiary butyl alcohol (TBA) and in dimethyl sulfoxide (DMSO) with a low chain transfer constant using a low temperature initiator, 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN). The effects of polymerization temperature and initiator concentration were investigated in terms of polymerization behavior and molecular structures of poly(vinyl pivalate) (PVPi) and its Saponification product poly(vinyl alcohol) (PVA). TBA was absolutely superior to DMSO in increasing the syndiotacticity and molecular weight of PVA. In contrast, TBA was inferior to DMSO in causing conversion to polymer, indicating that the initiation rate of VPi production in TBA was lower than that in DMSO. These effects could be explained by a kinetic order of ADMVN concentration, calculated by the initial rate method. Low-temperature solution polymerization of VPi in TBA or DMSO by adopting ADMVN proved to be successful in obtaining PVA of ultrahigh molecular weight [maximum number-average degree of polymerization (P_n): 13,500-17,000] and of high yield (ultimate conversion of VPi into PVPi: 55-83%). In the case of bulk polymerization of VPi at the same conditions, maximum P_n and conversion were 14,500-17,500 and 22-36%, respectively. The P_n and syndiotactic diad content were much higher and the degree of branching was lower with PVA prepared from PVPi polymerized at lower temperatures in TBA. Moreover, PVA from the TBA system was fibrous, with a high degree of orientation of the crystallites, indicating the syndiotactic nature of TBA polymerization.