Synthesis and polymerization of vinyl esters and vinyl ethers having bulky substituents

In order to clarify the effect of bulky substituents on the stereoregulation of vinyl monomers, vinyl-1-adamantanecarboxylate and vinyl 1-adamantyl ether were synthesized and polymerized by radical and cationic mechanisms, respectively. As open-chain models of the adamantyl group, vinyl trialkylacetate (alkyl = ethyl, n-propyl, and n-butyl) and vinyl tri-n-propylcarbinyl ether were also synthesized and polymerized. The adamantly group in the vinyl ester favored syndiotactic propagation in a manner similar to the trimethylcarbinyl group. Higher homologs of tri-n-alkylcarbinyl group showed higher syndiotacticity but this effect was saturated in higher members of the series. The effect of the adamantyl group in vinyl ether was similar to that of the tert-butyl group, leading to high isotacticity on cationic polymerizations in nonpolar solvents and to atactic polymers in polar solvents, but the tri-n-propylcarbinyl group was found unique in leading to what was assumed to be a heterotactic polymer. Polymers with the adamantyl group showed much higher softening points than polymers of the corresponding open-chain groups.     

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     

Stereocontrol in radical polymerization

The stereospecific radical polymerization of vinyl esters, methacrylates, and alpha-substituted acrylates was studied. Fluoroalcohols, as a solvent, have remarkable effects on the stereoregularity of the radical polymerizations of vinyl acetate, vinyl pivalate, and vinyl benzoate, affording polymers rich in syndiotacticity, heterotacticity, and isotacticity, respectively. This method was successfully applied to the polymerization of methacrylates to give syndiotactic polymers. The steric repulsion between the entering monomer and the chain-end monomeric unit bound by the solvent through hydrogen bonding is important for the stereochemical control in these systems. Lewis acid catalysts, such as lanthanide trifluoromethanesulfonates and zinc salts, were also effective for the stereocontrol during the radical polymerization of methyl methacrylate, to reduce the syndiotacticity and alpha-(alkoxymethyl)acrylates to synthesize isotactic and syndiotactic polymers. Radical polymerization of the methacrylates bearing a bulky ester group, such as the triphenylmethyl methacrylate derivatives, gave highly isotactic polymers, as in the case of anionic polymerization. In addition, the control of one-handed helical conformation was attained in the radical polymerization of 1-phenyldibenzosuberyl methacrylate using chiral neomenthanethiol or cobalt(II) complexes as an additive.     

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.     

Tacticity control in the radical polymerization of 2,2,2-trifluoroethyl methacrylate with fluoroalcohol

The free-radical polymerization of 2,2,2-trifluoroethyl methacrylate (TEMA) was carried out in fluoroalcohols to achieve stereoregulation. The polymerization reactivity at low temperature and syndiotactic specificity were enhanced by the use of fluoroalcohol as a solvent. The polymer having triad syndiotacticity (rr) of 70% was obtained in perfluoro-t-butyl alcohol. It was noted that the stereochemistry was nearly independent of reaction temperature. The stereoeffect of fluoroalcohols seemed to be due to the hydrogen-bonding interaction between the alcohol and the monomer or growing species. The hydrogen-bonding formation was determined by FTIR. The copolymerization of TEMA with methyl methacrylate (MMA) in hexafluoroisopropanol afforded a copolymer with syndiotactic specificity. By this method, a cladding material for an optical fiber based on poly(methyl methacrylate) (PMMA) with high mechanical strength and low refractive index could be obtained.     

Stereoregulation in cationic polymerization by designed Lewis acids. II. Effects of alkyl vinyl ether structure

Stereoregulation in the cationic polymerization of various alkyl vinyl ethers was investigated with bis[(2,6‐diisopropyl)phenoxy]titanium dichloride ( 1 ; catalyst) in conjunction with the HCl adduct of isobutyl vinyl ether as an initiator in n‐hexane at −78 °C. The tacticities depended on the substituents of the monomers. Isobutyl and isopropyl vinyl ethers gave highly isotactic polymers (mm = 83%), whereas tert‐butyl and n‐butyl vinyl ethers resulted in lower isotactic contents (mm ∼ 50%) similar to those for TiCl₄, a conventional Lewis acid, thus indicating that the steric bulkiness of the substituents was not the critical factor in stereoregulation. A statistical analysis revealed that the high isospecificity was achieved not by the chain end but by the catalyst 1 or the counteranion derived therefrom. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1060–1066, 2001     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.     

Stereocontrol in the free‐radical polymerization of methacrylates with fluoroalcohols

The free‐radical polymerization of methyl methacrylate (MMA), ethyl methacrylate (EMA), isopropyl methacrylate (IPMA), and tert‐butyl methacrylate (t‐BuMA) was carried out under various conditions to achieve stereoregulation. In the MMA polymerization, syndiotactic specificity was enhanced by the use of fluoroalcohols, including (CF₃)₃COH as a solvent or an additive. The polymerization of MMA in (CF₃)₃COH at −98 °C achieved the highest syndiotacticity (rr = 93%) for the radical polymerization of methacrylates. Similar effects of fluoroalcohols enhancing syndiotactic specificity were also observed in the polymerization of EMA, whereas the effect was negligible in the IPMA polymerization. In contrast to the polymerizations of MMA and EMA, syndiotactic specificity was decreased by the use of (CF₃)₃COH in the t‐BuMA polymerization. The stereoeffects of fluoroalcohols seemed to be due to the hydrogen‐bonding interaction of the alcohols with monomers and growing species. The interaction was confirmed by NMR measurements. In addition, in the bulk polymerization of MMA at −78 °C, syndiotactic specificity and polymer yield increased even in the presence of a small amount {[(CF₃)₃COH]/[MMA]_o < 1} of (CF₃)₃COH. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4693–4703, 2000     

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     

Poly(vinyl alcohol) obtained through polymerization of some vinyl esters

Bulk and/or solution polymerizations of a series of vinyl esters, e.g., vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pivalate, and vinyl benzoate were conducted. Iodine-coloration, 1,2-glycol structure, molecular weight, and tacticity (triad and pentad) were measured for the resulting poly(vinyl alcohol)s (PVAs). The iodine-coloration abilities of PVAs, derived from poly(vinyl ester)s that were obtained through bulk polymerization at 60°C, depended on the starting monomer, increasing in the following order: vinyl benzoate < vinyl acetate < vinyl propionate < vinyl butyrate < vinyl pivalate. In solution polymerizations of vinyl propionate and vinyl butyrate, it was revealed that the tacticity of the derived PVAs apparently depended on the type and amount of polymerization solvent employed, as found previously in the case for vinyl acetate. The iodine-coloration of these PVA samples varied in the same order as their syndiotactic content, while no relationship was observed toward their 1,2-glycol content. The probabilities of the syndiotactic propagation at 60°C were estimated as 0.49 (benzoate), 0.54 (acetate), 0.55 (propionate), 0.56 (butyrate), and 0.60 (pivalate), respectively.     

Effect of tetraalkylammonium salts on the tacticity of poly(methyl methacrylate), 2.. Initiation by potassium tert-butoxide

The anionic polymerization of methyl methacrylate was carried out in the presence of potassium tert-butoxide (t-BuOK)/quaternary ammonium salts (QAS) in toluene and tetrahydrofuran at −60°C. It was found that in toluene some QAS additives substantially increase the syndiotacticity of poly(methyl methacrylate). Two types of QAS were distinguished, quite different in their action. The addition of QAS with one or two longchain alkyl groups (>C₁₂), does not change significantly the mode of the monomer addition, whereas the polymerization in the presence of tetraalkylammonium salts with four equal substituents and dimethyldidodecylammonium bromide yields predominantly a syndiotactic polymer with high conversion and comparatively low polydispersity (M̄_w/M̄_w = 1.3−1.5). In some cases QAS additives are more effective modifiers than cryptand [2.2.2].     

聚合温度对聚氯乙烯树脂结构和性能的影响

采用凝胶渗透色谱法、¹³H NMR和DSC法测定了悬浮聚合PVC树脂的平均分子量、间规度、结晶度和玻璃化温度.结果表明:聚合温度下降,PVC的平均分子量上升,分子量的对数值与聚合温度的倒数成正比;PVC的间规度和结晶度均增大,在形成结晶的间规链段的最短长度(ξ_min)为6时,由间规度计算的结晶度理论值与测定值接近;分子量和结晶度增加的共同作用,使PVC的玻璃化转变温度上升.     

Dual roles of alkyl alcohols as syndiotactic‐specificity inducers and accelerators in the radical polymerization of N‐isopropylacrylamide and some properties of syndiotactic poly(N‐isopropylacrylamide)

The effects of simple alkyl alcohols on the radical polymerization of N‐isopropylacrylamide in toluene at low temperatures were investigated. We succeeded in the induction of syndiotactic specificity and the acceleration of polymerization reactions at the same time by adding simple alkyl alcohols such as 3‐methyl‐3‐pentanol (3Me3PenOH) to N‐isopropylacrylamide polymerizations. The dyad syndiotacticity increased with a decrease in the temperature and an increase in the bulkiness of the added alcohol and reached up to 71% at ?60 °C in the presence of 3Me3PenOH. With the assistance of NMR analysis, it was revealed that the alcohol compounds played dual roles in this polymerization system; an alcohol compound coordinating to the N? H proton induced syndiotactic specificity, and that hydrogen‐bonded to the C?O oxygen accelerated the polymerization reaction. The effect of syndiotacticity on the properties of poly(N‐isopropylacrylamide)s was also examined in some detail. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4450–4460, 2006     

Effect of stereosequences on crystallinity and properties of zone‐drawn poly(vinyl alcohol) microfibrils

To effectively orient the molecular chains of novel syndiotactic poly(vinyl alcohol) (PVA) microfibrillar fiber (PVA fibril), a high‐temperature zone‐drawing method was adopted. The PVA fibrils were directly prepared from the saponification and in situ fibrillation without a spinning procedure. The maximum draw ratio of the PVA fibril increased with a decrease in the syndiotactic diad (r‐diad) content, indicating that the deformability of PVA molecules was lowered in higher syndiotactic PVA. Degree of crystal orientations up to 0.990 were achieved by stretching the PVA fibril with the r‐diad content of 65.1% and the original degree of crystal orientation of 0.902 at 250 °C close to its crystal melting temperature (T_m). When the same draw ratio was applied to the fibrils, a higher crystal orientation was achieved for the fibrils having higher syndiotacticity. Wide‐angle X‐ray data show that the longitudinal crystal sizes of drawn PVA fibrils were larger in higher syndiotacticities. The degree of crystal orientation, crystallinity, T_m, longitudinal crystal size, and tensile strength of the maximum drawn PVA fibril with a r‐diad content of 65.1% were 0.99, 0.97, 279 °C, 187 Å, and 4.66 N/tex, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1263–1271, 2001     

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.     

Stereoregularity of poly(alkyl α-chloroacrylates) and mechanism of isotactic polymerization

The catalytic activity of the complexes prepared by the reaction of Grignard reagents with ketones, esters, and an epoxide as polymerization catalysts of methyl and ethyl α-chloroacrylates was investigated. The modifiers which gave isotactic polymers were α,β-unsaturated ketones such as benzalacetophenone, benzalacetone, dibenzalacetone, mesityl oxide, and methyl vinyl ketone, and α,β-unsaturated esters such as ethyl cinnamate, ethyl crotonate, and methyl acrylate. Catalysts with butyl ethyl ketone, propiophenone, and propylene oxide as modifiers produced atactic polymers but no isotactic polymers. It was revealed that the complex catalysts having a structure ? C?C? O? MgX (X is halogen) gave isotactic polymers. The mechanism of isotactic polymerization was discussed. In addition, for radical polymerization of ethyl α-chloroacrylate, enthalpy and entropy differences between isotactic and syndiotactic additions were calculated to give ΔH_i^* ? ΔH_s^* = 910 cal/mole and ΔS_i^* ? ΔS_s^* = 0.82 eu.     

Complexation du chlorure de vinyle par le tetrahydrofuranne et le n-butyraldéhyde: Étude par résonance magnétique nucléaire haute résolution

Examination of the HR-NMR spectra of vinyl chloride in the presence of tetrahydrofuran and n-butyraldehyde suggests the formation of donor-acceptor type complexes. The equilibrium quotients and NMR constants were estimated by using the Benesi-Hildebrand method. The existence of these complexes would explain the anomalies observed: first, in the anionic polymerization of vinyl chloride initiated by tert-butylmagnesium chloride is tetrahydrofuran. The initiation step seems to be governed by this complexation phenomenon; second, in the radical polymerization of the same monomer in n-butylaldehyde the obtained poly(vinyl chloride) is more syndiotactic than the radical polymer prepared in bulk by the usual methods.     

High molecular weight syndiotacticity-rich poly(vinyl alcohol) gel in aging process

The properties of the aged gels of high molecular weight syndiotacticity-rich poly(vinyl alcohol)s (HMW S-PVAs) with different syndiotactic diad (s-diad) contents were investigated. HMW S-PVA gels with s-diad content of 61.5% and 58.2% showed the rapid increases of the syneresis and the turbidity from the early stage of aging time, which is ascribable to the phase separation, while that with s-diad content of 55.7% did not. From the morphological study, it was confirmed that the phase separation in HMW S-PVA gel with s-diad content of 61.5% occurred without the liquid-liquid phase separation in sol state, whereas both the liquid-liquid phase separation in sol state and the subsequent phase separation in gel state occurred in the case of HMW S-PVA gel with s-diad content of 58.2%. On the other hand, HMW S-PVA gel with s-diad content of 55.7% showed neither the liquid-liquid phase separation in sol state nor the phase separation in gel state in the long period of time. It was also confirmed from wide angle X-ray diffractogram that the crystallization was accompanied by the phase separation in gel state in the aging process of PVA gel. However, the crystallization was hindered by the fast network formation at the initial stage of time. Later the syndiotacticity promoted the crystallization. The tensile modulus of HMW S-PVA gel with higher syndiotacticity increased more significantly with time. Received: 2 December 1999/Accepted: 12 July 2000     

Synthesis and micellar behavior of poly(vinyl alcohol-<Emphasis Type="Italic">b</Emphasis>-styrene) copolymers containing PVA blocks with different syndiotacticity

Poly(vinyl alcohol-b-styrene) (poly(VA-b-St)) diblock copolymers with different syndiotacticity of poly(vinyl alcohol) (PVA) block were synthesized via consecutive telomerization, atom transfer radical polymerization, and saponification. These amphiphilic block copolymeric micelles were prepared by dialysis against water. Dynamic light scattering and transmission electron micrograph measurements confirmed the formation of a micelles, and the size of a micelle was less than 100 nm and increased with the molecular weight of polystyrene (PS) block. From the fluorescence emission spectrum measurements using pyrene as a fluorescence probe, the copolymers formed micelles with critical micelle concentration (CMC) in the range of 0.125–4.47 mg/l. The CMC values increase with decrease of the molecular weight of the PS block and increase of the syndiotacticity of PVA block. Kinetic stability study of micelles showed increased stability for block copolymers containing PVA block with higher syndiotacticity.     

Conformational analysis of poly(2-hydroxyethyl methacrylate)

The poly(2-hydroxyethyl methacrylate) (PHEMA) is a disubstituted vinyl chain in which the substituents CO₂CH₂CH₂OH and CH₃ differ in size and shape. In order to verify the various characteristics of the PHEMA chain, the conformational energy calculations for meso and racemic diads, which are the segments consisting of the stereoregular isotactic and syndiotactic chains, respectively, were carried out using ECEPP/2 potential. From these calculations, the averaged geometry and the statistical weights were obtained in a local minima. The characteristic ratio, C∞ = (〈r²〉_o/nl²)∞, was determined from the statistical weights and geometries. The calculated C∞ for the isotactic and syndiotactic chain are 10.2 and 2.3, respectively. The characteristic ratio for isotactic chain is larger than that for syndiotactic chain. This shows that the syndiotactic chain is more folded than the isotactic chain is, and that the calculated tendency is in reasonably agreement with the experimental tendency of acrylate polymers.