Preparation of syndiotacticity-rich high molecular weight poly(vinyl alcohol) microfibrillar fiber by photoinitiated bulk polymerization and saponification

Vinyl pivalate (VPi) was polymerized in bulk by ultraviolet-ray initiation at low temperatures using 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN) and 2,2'-azobis(isobutyronitrile) (AIBN) as photoinitiators. High molecular weight (HMW) poly(vinyl pivalate) (PVPi), having a number-average degree of polymerization (P_n) of 13,000–28,000, was obtained at conversions below 30% and converted by saponification to a syndiotacticity-rich HMW poly(vinyl alcohol) (PVA) microfibrillar fiber with P_n of 7300–18,300, syndiotactic diad (S-diad) and triad contents of ∼ 64% and ∼ 39%, respectively, and crystal melting temperature (T_m) of ∼ 249°C. ADMVN gave higher P_n than AIBN. On the other hand, conversion was smaller with the former than with the latter, and it was found that the initiation rate of ADMVN was lower than that of AIBN. P_n of PVA was constant while P_n of the precursor PVPi increased with increasing conversion. The syndiotacticity, T_m and thermal stability of PVA obtained from PVPi were much superior to those of PVA derived from poly(vinyl acetate) prepared under the same polymerization conditions. Polymerization of VPi at lower temperatures gave PVA with higher syndiotacticity. © 1997 John Wiley & Sons, Inc.     

Preparation of water‐soluble,syndiotacticity‐rich,low molecular weight poly(vinyl alcohol) microfibrils in high yields with the low‐temperature polymerization of vinyl pivalate in tetrahydrofuran and saponification

To prepare water‐soluble, syndiotacticity‐rich poly(vinyl alcohol) (PVA) microfibrils for various industrial applications, we synthesized syndiotacticity‐rich, low molecular weight PVA by the solution polymerization of vinyl pivalate (VPi) in tetrahydrofuran (THF) at low temperatures with 2,2′‐azobis(2,4‐dimethylvaleronitrile) (ADMVN) as an initiator and successive saponification of poly(vinyl pivalate) (PVPi). Effects of the initiator and monomer concentrations and the polymerization temperature were investigated in terms of the polymerization behaviors and molecular structures of PVPi and the corresponding syndiotacticity‐rich PVA. The polymerization rate of VPi in THF was proportional to the 0.91 power of the ADMVN concentration, indicating the heterogeneous nature of THF polymerization. The low‐temperature solution polymerization of VPi in THF with ADMVN proved to be successful in obtaining water‐soluble PVA with a number‐average degree of polymerization (P_n) of 300–900, a syndiotactic dyad content of 60–63%, and an ultimate conversion of VPi into PVPi of over 75%. Despite the low molecular weight of PVA with P_n = 800, water‐soluble PVA microfibrillar fibers were prepared because of the high level of syndiotacticity. In contrast, for PVA with P_n = 330, shapeless and globular morphologies were observed, indicating that molecular weight has an important role in the in situ fibrillation of syndiotacticity‐rich PVA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1103–1111, 2002     

Free-radical polymerization of vinyl esters using fluoroalcohols as solvents: Effect of monomer structure on stereochemistry

Free-radical polymerization of vinyl esters including vinyl propionate (VPr), vinyl isobutylate (ViBu), vinyl 2,2-dimethylbutylate (VDMB), vinyl 2,2-dimethylvalerate (VDMV), vinyl 2,2-bis(trifluoromethyl)propionate (VF6Pi), and vinyl benzoate (VBz) was carried out using fluoroalcohols as solvents, and the tacticity of the obtained polymers was determined by NMR analysis of the produced poly(vinyl alcohol) (PVA). The polymerization of VPr, ViBu, VDMB, and VDMV, which are bulkier than VAc, in fluoroalcohols afforded polymers rich in heterotacticity (up to mr = 61%) similar to that of vinyl pivalate (VPi) whereas VAc is known to give a syndiotactic polymer under the reaction conditions used here. The polymerization of VF6Pi, which is the bulkiest among the monomers used in this study, gave a polymer rich in syndiotacticity in bulk and in fluoroalcohols regardless of the structure of the solvents. On the other hand, the polymerization of VBz in fluoroalcohols gave polymers with a higher isotacticity (up to mm = 33%) than bulk polymerization. Thus the monomer structure strongly affected the stereochemistry of the free-radical polymerization of vinyl esters in fluoroalcohols. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2677–2683, 1999     

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

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 °C 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 °C were fibrous, with a high degree of crystallinity and orientation of the crystallites.     

Synthesis of high-molecular-weight poly(vinyl alcohol) with high yield by novel one-batch suspension polymerization of vinyl acetate and saponification

Generally, owing to tautomerism of vinyl alcohol monomer, poly(vinyl alcohol) (PVA) cannot be obtained by direct polymerization but it can be obtained by the saponification of poly(vinyl ester) precursors such as poly(vinyl acetate) (PVAc). In this study, to obtain high-molecular-weight (HMW) PVA with high yield through a one-batch method, we tried continuous saponification of PVAc prepared by suspension polymerization of vinyl acetate (VAc). We controlled various polymerization conditions, such as polymerization temperature, initiator concentration, suspending agent concentration, agitation speed, and VAc/water ratio, and obtained PVAc with a maximum conversion of VAc into PVAc of over 95-98%. PVA beads having various molecular parameters were prepared by continuous saponification of PVAc microspheres. Despite our employing a one-batch process, a maximum degree of saponification of 99.9% could be obtained. Continuous heterogeneous saponification of prepared PVAc yielded HMW PVA having a number-average degree of polymerization of 2,500-5,500, a syndiotactic diad content of 51-52%, and degree of saponification of 85.0-99.9%.     

Preparation of ultrahigh‐molecular‐weight syndiotactic poly(vinyl pivalate) monodisperse microspheres by low‐temperature suspension polymerization of vinyl pivalate

The particle size distributions of poly(vinyl pivalate) (PVPi) produced from low‐temperature suspension polymerization of vinyl pivalate (VPi) with 2,2′‐azobis(4‐methoxy‐2,4‐dimethylvaleronitrile) (AMDMVN) as an initiator have been studied. By controlling various synthesis parameters, near‐monodisperse PVPi microspheres from 100 to 400 μm were obtained that are expected to be precursors of near‐monodisperse syndiotactic poly(vinyl alcohol) (PVA) microspheres for biomedical embolic applications. The mean particle diameter follows the relationship: the volume average diameter, D_vad ∝ Y^0.26[VPi]^0.52[AMDMVN]^?0.25[PVA]^0.40T^?8.35Rpm^?0.67, where Y, [VPi], [AMDMVN], [PVA], T, and Rpm are the fractional conversion, concentrations of VPi, AMDMVN, and suspending agent, polymerization temperature, and agitation speed during the polymerization of VPi, respectively. The polydispersity of the particle size distribution of PVPi decreased with decreasing conversion, [AMDMVN], T, and Rpm and with increasing [VPi]. In the case of [PVA], optimization of the suspension stability led to a narrow particle size distribution. Ultrahigh‐molecular‐weights PVPi and PVA (number‐average degrees of polymerization of PVPi (25,000–32,000) and PVA (14,000–17,500), of high syndiotactic diad content (63%), and of high ultimate conversion of VPi into PVPi (85–95%) were obtained by suspension polymerization at 10 °C, followed by saponification. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 789–800, 2005     

Free‐radical copolymerization of vinyl esters using fluoroalcohols as solvents: The solvent effect on the monomer reactivity ratio

Free‐radical copolymerizations of vinyl acetate (VAc = M₁) and other vinyl esters (= M₂) including vinyl pivalate (VPi), vinyl 2,2‐bis(trifluoromethyl)propionate (VF6Pi), and vinyl benzoate (VBz) with fluoroalcohols and tetrahydrofuran (THF) as the solvents were investigated. The fluoroalcohols affected not only the stereochemistry but also the polymerization rate. The polymerization rate was higher in the fluoroalcohols than in THF. The accelerating effect of the fluoroalcohols on the polymerization was probably due to the interaction of the solvents with the ester side groups of the monomers and growing radical species. The difference in the monomer reactivity ratios (r₁, r₂) in THF and 2,2,2‐trifluoroethanol was relatively small for all reaction conditions and for the monomers tested in this work, whereas r₁ increased in the VAc‐VF6Pi copolymerization and r₂ decreased in the VAc‐VPi copolymerization when perfluoro‐tert‐butyl alcohol was used as the solvent. These results were ascribed to steric and monomer‐activating effects due to the hydrogen bonding between the monomers and solvents. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 220–228, 2000     

Preparation of poly(vinyl pivalate) microspheres by dispersion polymerization in an ionic liquid and saponification for the preparation of poly(vinyl alcohol) with high syndiotacticity

We report here a successful free-radical dispersion polymerization of vinyl pivalate (VPi) in an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][TFSI]) using poly(vinyl pyrrolidone) (PVP) as a stabilizer. Morphological analysis by FE-SEM revealed that poly(vinyl pivalate) (PVPi) obtained from dispersion polymerizations were in the form of spherical particles. Micron-sized, PVPi particles with a number-average molecular weight (M_n) of 166,400 g/mol could be obtained using 5% stabilizer (w/w to monomer) at 65 °C for 20 h. The effects of varying concentration of stabilizer, initiator and monomer upon polymer yield, molecular weight, and morphology of PVPi were also investigated. Analogous polymerizations in dimethyl sulfoxide (DMSO) and bulk served as references. In addition, the preparation of poly(vinyl alcohol) (PVA) by saponification of the resultant PVPi was described.     

Emulsion Polymerization of Vinyl Acetate at Low Temperatures

Polymerization of vinyl acetate (VAc) was carried out with potassium persulfate (KS) and ammonium sulfite as redox initiator in the presence of acetonitrile for the purpose of preparation of polyvinyl alcohol (PVA) of desired degree of polymerization (DP). The PVA obtained by this method at 0°C showed higher DP and lower solubility in water. It was observed that acetonitrile was a useful chain transfer agent in this polymerization system. It was also observed that ZnCl₂ accelerated the polymerization of VAc and the over-all activation energy of polymerization was 9.73 Kcal/mol. However, the stereoregularity of PVA obtained by the saponification of PVAc in this system was not improved by the presence of ZnCl₂.     

Low temperature suspension polymerization of vinyl acetate using 2,2′-azobis(2,4-dimethylvaleronitrile) for the preparation of high molecular weight poly(vinyl alcohol) with high yield

 To obtain high molecular weight (HMW) poly(vinyl acetate) (PVAc) with high conversion and high linearity for a precursor of HMW poly(vinyl alcohol) (PVA), vinyl acetate (VAc) was suspension-poly-merized using a low-temperature initiator, 2,2′-azobis (2,4-dimethyl-valeronitrile) (ADMVN), and the effects of polymerization conditions on the polymerization behavior and molecular structures of PVAc and PVA prepared by saponifying PVAc were investigated. On the whole, the experimental results well corres-ponded to the theoretically predicted tendencies. Suspension polymerization was slightly inferior to bulk polymerization in increasing molecular weight of PVA. In contrast, the former was absolutely superior to the latter in increasing conversion of the polymer, which indicated that the suspension polymerization rate of VAc was faster than the bulk one. These effects could be explained by a kinetic order of ADMVN concentration calculated by initial-rate method and an activation energy difference of polymerization obtained from the Arrhenius plot. Suspension polymerization at 30 °C by adopting ADMVN proved to be successful in obtaining PVA of HMW (number-average degree of polymerization (P _n)): (4200–5800) and of high yield (ultimate conversion of VAc into PVAc: 85–95%) with diminishing heat generated during polymerization. In the case of bulk polymerization of VAc at the same conditions, maximum P _n and conversion of 5200–6200 and 20–30% was obtained, respectively. The P _n, lightness, and syndiotacticity were higher with PVA prepared from PVAc polymerized at lower temperatures. Received: 10 February 1998 Accepted: 15 April 1998     

衰减链转移(DT)聚合醋酸乙烯酯的动力学

在醋酸乙烯酯的普通自由基聚合体系中加入少量碘（质量分数为0.57%～0.86%）,用偶氮二异丁腈作引发剂合成聚醋酸乙烯酯,对其聚合反应的动力学及反应机理进行了研究。 考察了碘质量分数对聚合反应速率、聚合物分子量及分子量分布的影响,发现随着碘浓度的增加,聚合物分子量及分子量分布得到更好的控制;对聚合过程进行了核磁跟踪,考察了聚合过程中几种化合物的变化情况,特别是初级自由基与碘生成的加合物A-I（A来自引发剂分裂后产生的自由基）及单体加合物A-Mn-I（M代表单体单元）的变化情况;对聚合物结构作了详细的1H NMR分析,结果表明,聚合过程中分子量随时间延长而逐渐增大,分子量分布随单体转化率增加而变窄,聚合终期,单体转化率达到80%左右时,所得聚合物分子量分布窄（Mw/Mn≤1.41）,且含有碘端基。该方法的自由基聚合具有活性/可控的性质。     

Core‐shell microspheres with surface grafted poly(vinyl alcohol) as drug carriers for the treatment of hepatocellular carcinoma

Core(polyvinyl neodecanoate‐ethylene glycol dimethacrylate)‐shell(polyvinyl alcohol) (core (P(VND‐EGDMA))‐shell(PVA)) microspheres were developed by seeded polymerization with the use of conventional free radical and RAFT/MADIX mediated polymerization. Poly(vinyl pivalate) PVPi was grafted onto microspheres prepared via suspension polymerization of vinylneodecanoate and ethylene glycol dimethacrylate. The amount of grafted polymer was found to be independent from the technique used with conventional free radical polymerization and MADIX polymerization resulting into similar shell thicknesses. Both systems—grafting via free radical polymerization or the MADIX process—were found to follow slightly different kinetics. While the free radical polymerization resulted in a weight gain linear with the monomer consumption in solution the growth in the MADIX controlled system experienced a delay. The core‐shell microspheres were obtained by hydrolysis of the poly(vinyl pivalate) surface grafted brushes to form poly(vinyl alcohol). During hydrolysis the microspheres lost a significant amount of weight, consistent with the hydrolysis of 40–70% of all VPi units. Drug loading was found to be independent of the shell layer thickness, suggesting that the drug loading is governed by the amount of bulk material. The shell layer does not appear to represent an obstacle to the drug ingress. Cell testing using colorectal cancer cell lines HT 29 confirm the biocompatibility of the empty microspheres whereas the clofazimine loaded particles lead to 50% cell death, confirming the release of the drug. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3256–3272, 2007     

粒径可控的聚乙烯醇交联微球VA/DVB的制备

以醋酸乙烯酯(VAc)为主单体,二乙烯基苯(DVB)为交联剂,聚乙烯醇(PVA)为分散剂,采用悬浮聚合法制备了交联微球VAe/DVB.重点考察了分散剂用量、搅拌速率、油水两相比例、NaCl用量等因素对交联微球的形成及其粒度的影响.使用甲醇对微球VAc/DVB进行醇解反应,制得了聚乙烯醇交联微球VA/DVB.结果表明:交联微球VA/DVB的物理形态决定于前驱体微球VAc/DVB的形貌与粒径.在悬浮聚合体系中,分散剂用量、搅拌速率与油水两相比是影响交联微球制备的主要因素,当分散剂用量太少(<0.3%)、搅拌速率太慢(<200 r/min)与油水两相体积比太大(>l:4)时,共聚合体系中均不能发生成球过程.控制悬浮聚合的反应条件,可以制备出球形度好、粒径可调控的交联微球VA/DVB.影响醇解反应的主要因素是反应温度,适宜的温度是40℃,反应15 h醇解度可达92%.     

醋酸乙烯酯的可控/活性自由基聚合

概述了醋酸乙烯酯单体可控/活性自由基聚合的现状.总结了氮氧化合物存在下的聚合、原子转移自由基聚合、可逆加成断裂链转移聚合以及含碘化合物的衰减链转移聚合这四种活性自由基聚合方法用于醋酸乙烯酯聚合的研究结果,并对这四种方法作了简要的比较.     

RAFT/MADIX copolymerization of vinyl acetate and 5,6‐benzo‐2‐methylene‐1,3‐dioxepane

The synthesis of well‐defined degradable poly(vinyl acetate) analogues is achieved by RAFT copolymerization of 5,6‐benzo‐2‐methylene‐1,3‐dioxepane (BMDO) and vinyl acetate (VAc) using methyl (ethoxycarbonothioyl)sulfanyl acetate (MEA) as controlling agent. Several monomer mixtures with low BMDO contents (<30 mol %) are employed to prepare different copolymers. In all the cases, the evolution of molar masses and the dispersity values (<1.26) confirm the controlled feature of the polymerization. The livingness of the obtained chains is demonstrated by successful chain extension experiments with VAc, although the presence of dead chains is also shown. The introduction of ester groups into the main chain of these P(VAc‐co‐BMDO) copolymers allows their degradation when treated with a mixture of KOH/MeOH in reflux during 2.5 h. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 104–111     

醋酸乙烯(VAc)的活性/可控自由基聚合

本文综述了醋酸乙烯(VAc)单体的活性/可控自由基聚合研究进展.醋酸乙烯是一种重要的单体,是生产聚醋酸乙烯(PVAc)和聚乙烯醇(PVA)的原料.传统的自由基聚合方法如溶液、乳液、悬浮和分散等都可以用来实现VAc的聚合,得到不同分子量的PVAc和PVA.由于醋酸乙烯增长自由基的高活性,存在向聚合物链的链转移从而导致聚合物的分子量分布比较宽,为了得到分子量分布更窄的聚合物,活性可控聚合方法也被用来实现VAc的聚合.     

通过自缩合乙烯基氧阴离子聚合制备超支化聚甲基丙烯酸羟丙酯

由甲基丙烯酸羟丙酯通过自缩合乙烯基氧阴离子聚合(self-condensing vinyl oxyanionic polymerization)制备了端羟基的超支化聚甲基丙烯酸酯. 以氢化钾(KH)和冠醚的复合物为引发剂时, 可以得到高分子量的聚合物. 用¹H NMR和¹³C NMR谱图证实了聚合物的超支化结构. 由于在聚合过程中存在质子转移反应, 引发剂与单体的摩尔比会影响所得聚合物的结构. 超支化聚合物的玻璃化转变温度在58.1～81.4 ℃之间, 且随着引发剂与单体的比例的减小而降低. 当引发剂与单体等摩尔比时, 所得聚合物的支化度为0.49.     

Synthesis of Poly(Vinyl Alcohol) and/or Poly(Vinyl Acetate) Particles with Spherical Morphology and Core-Shell Structure and its Use in Vascular Embolization

Poly(vinyl alcohol), PVA, is the most frequently used material in embolization of tumors, aneurisms and arteriovenous malformations due to its low toxicity, good biocompatibility and desirable physical properties. It is well known that PVA particles cannot be prepared by direct polymerization of vinyl alcohol. Its synthesis is typically performed by the suspension polymerization of vinyl acetate to produce poly(vinyl acetate), PVAc, followed by the saponification of the PVAc particles. This work shows that, using the suspension polymerization technique, it is possible to obtain spherical particles with a core-shell structure of PVA/PVAc with regular morphology, instead of particles with irregular shapes and sizes, as usually found in many commercial embolization products. Therefore, this work presents the production of PVA/PVAc spherical particles that can be used to occlude blood vessels, eliminating the disadvantages of commercial PVA. In vivo clinical tests with white “New Zealand” rabbits undergoing kidney inflammation reaction have shown that these spherical particles are much more efficient for vascular embolization.     

Characterization of poly(vinyl alcohol) during the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as emulsifier

During the emulsion polymerization of vinyl acetate (VAc) using poly(vinyl alcohol) (PVA) as stabilizer and potassium persulfate as initiator, the VAc reacts with PVA forming PVA-graft-PVAc. When the grafted polymer reaches a critical size it becomes water-insoluble and precipitates from the aqueous phase contributing to the formation of polymer particles. Since particle formation and therefore the properties of the final latex will depend on the degree of grafting, it is important to quantify and to characterize the grafted PVA. In this work, the quantitative separation and characterization of the grafted water-insoluble PVA was carried out by a two-step selective solubilization of the PVAc latex, first with acetonitrile to separate PVAc homopolymer, followed by water to separate the water-soluble PVA from the remaining acetonitrile-insoluble material. After the separation, the water-soluble and water-insoluble PVA were characterized by Fourier Transform Infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance (NMR) analyses, from which the details of the PVA-graft-PVAc structure were obtained. © 1996 John Wiley & Sons, Inc.     

Iodine-Transfer Polymerization (ITP) of Ethylene and Copolymerization with Vinyl Acetate

Controlled radical polymerization of ethylene using different commercially available, cheap, and non-toxic iodo alkyls is performed by iodine transfer polymerization (ITP) under mild conditions (≤100 °C and ≤200 bar). The formed well-defined iodo end-capped polyethylene (PE−I) species is very stable upon storage. Narrow molar-mass distributions (dispersities around 1.6) were obtained up to number average molar masses of 7300 g mol⁻¹. The ethylene copolymerization by ITP (ITcoP) with vinyl acetate allowed to form a broad range of poly(ethylene-co-vinyl acetate) (EVA) containing from 0 to 85 mol % of VAc unit. In addition, EVA-b-PE block copolymers or EVA-b-EVA gradient block copolymers with different content of VAc in the blocks were obtained for the first time using ITP. Finally, reactivity trends were explored by a theoretical mechanistic study. This highly versatile synthetic platform provides a straightforward access to a diverse range of well-defined PE based polymer materials.