Han Do Ghim

Characterizations of poly(vinyl pivalate) polymerized in tertiary butyl alcohol and resulting syndiotactic poly(vinyl alcohol) microfibrils saponified

Abstract Vinyl pivalate (VPi) was solution-polymerized in tertiary butyl alcohol (TBA) and in dimethyl sulfoxide (DMSO) with low chain transfer constant using a low temperature initiator, 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN). TBA was absolutely superior to DMSO in increasing the syndiotacticity and molecular weight of poly(vinyl alcohol) (PVA). 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: 55–80%) to a much higher conversion than that from bulk polymerization. Moreover, PVA from TBA system were fibrous, with a high degree of orientation of the crystallites, indicating the syndiotactic nature of TBA polymerization.

Solution polymerization behavior of acrylonitrile by moderate temperature azoinitiator

Abstract Acrylonitrile (AN) was solution-polymerized in dimethyl sulfoxide (DMSO) and tertiary butyl alcohol (TBA) at 30, 40 and 50°C using a moderate temperature initiator, 2,2′-azobis(2,4,-dimethylvaleronitrile) (ADMVN); the effects of type and amount of solvent, polymerization temperature, and initiator concentration were investigated. On the whole, the experimental results corresponded to predictions. Moderate polymerization temperature using ADMVN proved to be successful in obtaining polyacrylonitrile (PAN) of high molecular weight and high conversion with smaller temperature rise during polymerization. The polymerization rates of AN in DMSO and in TBA were proportional to the 0.61 and 0.94 powers of ADMVN concentration, respectively. For the same polymerization conditions, DMSO was slightly superior to TBA in increasing molecular weight of PAN. On the other hand, DMSO was inferior to TBA in causing conversion to polymer, indicating that the initiation rate of AN in DMSO was less than that in TBA. These effects could be explained by an activation energy difference. For PAN produced in DMSO at 30°C, weight-average molecular weight of 865,000 was obtained, with a polydispersity index of 1.89.

Preparation and characterization of iodinated poly(vinyl alcohol) microfibril

Iodination of poly(vinyl alcohol) (PVA) microfibril, which was obtained from saponification of poly(vinyl pivalate), was conducted before and after zone drawing at various conditions. The resulting PVA microfibrils were characterized by differential scanning calorimeter and scanning electron microscopy. Surface morphologies of these PVA microfibrils showed some differences between PVA microfibril iodinated after and before drawing. Crude shapes of PVA microfibrils iodinated after drawing indicated that iodine decreased the structural regularity severely. On the other hand, PVA microfibrils iodinated before drawing showed relatively ordered surfaces. This was ascribed to the enhanced molecular ordering of PVA microfibrils due on zone drawing. Iodinated PVA microfibrils showed a decrease in crystal melting temperature of about 100 °C compared to the untreated sample. PVA microfibrils drawn after iodination showed relatively higher crystal melting temperature than those of microfibrils iodinated after drawing. These results were considered as the proofs of the changes in crystalline lattice of the PVA microfibrils. Effects of drawing temperature on sublimation of iodine were also evaluated.

A method to determine iodine desorption of high molecular weight syndiotactic poly(vinyl alcohol)/iodine complex film

Abstract A method to calculate the iodine desorption of high molecular weight syndiotactic PVA (s-PVA)/iodine film in water was suggested, which was achieved by measuring the weight loss of s-PVA/iodine film and the solubility of s-PVA film after immersion in water. The degree of solubility of s-PVA film in water at 80°C was below 3% and the iodine desorption of s-PVA/iodine film at the same temperature was limited to below 13%, while atactic PVA film is completely soluble over 50°C. The desorption of iodines in water was strongly affected by the dissolution of PVA. The longer soaking time gave the larger desorption of iodine regardless of immersion temperature. The iodine desorption of s-PVA/iodine film in hot water decreased with increasing drawing temperature and draw ratio.

Biomedical applications of stereoregular poly(vinyl alcohol) micro- and nanoparticles

Syndiotactic poly(vinyl alcohol) (PVA)/poly(vinyl pivalate/vinyl acetate) (P(VPi/VAc)) and atactic PVA/PVAc micro- and nanoparticles with skin/core structure have been prepared by heterogeneous saponification of P(VPi/VAc) and PVAc micro- and nanoparticles. Especially, to prepare P(VPi/VAc) and PVAc microparticles having various particle sizes and uniform particle size distribution, vinyl pivalate (VPi)/vinyl acetate (VAc) and VAc were suspension-polymerized using a low-temperature initiator, 2,2’-azobis(2,4-dimethylvaleronitrile). P(VPi/VAc) particles are promising precursor of stereoregular PVA embolic materials which can be introduced through catheters in the management of gastrointestinal bleeders, arteriovenous malformations, hemangiomas, and traumatic rupture of blood vessels. Monodisperse and/or nearly monodisperse P(VPi/VAc) and PVAc microparticles with various particle diameters were obtained by controlling suspension polymerization conditions. Monodisperse P(VPi/VAc) and PVAc microparticles having various particle sizes were partially saponified in the heterogeneous system. PVA/P(VPi/VAc) and PVA/PVAc microparticles having various tacticity and degree of saponification were produced by controlling various polymerization and saponification conditions. The coating of stereoregular PVA micro- and nanoparticles for drug release experiments was conducted with the strepo-avidin-alkaline phosphatase conjugate in variable conditions of pH value, coating buffer, and reaction temperature. Protein-coated syndiotactic PVA micro- and nanoparticles, which does not crosslinking, were more superior to controllability of drug release, durability, and dimensional stability to water and blood than atactic one.