Block Copolymerization of Lactide and an Epoxide Facilitated by a Redox Switchable Iron‐Based Catalyst

A cationic iron(III) complex was active for the polymerization of various epoxides, whereas the analogous neutral iron(II) complex was inactive. Cyclohexene oxide polymerization could be “switched off” upon in situ reduction of the iron(III) catalyst and “switched on” upon in situ oxidation, which is orthogonal to what was observed previously for lactide polymerization. Conducting copolymerization reactions in the presence of both monomers resulted in block copolymers whose identity can be controlled by the oxidation state of the catalyst: selective lactide polymerization was observed in the iron(II) oxidation state and selective epoxide polymerization was observed in the iron(III) oxidation state. Evidence for the formation of block copolymers was obtained from solubility differences, GPC, and DOSY‐NMR studies.     

Ionic polymerization under an electric field. III. Cationic polymerizations of α-methylstyrene and styrene

Cationic polymerizations of α-methylstyrene and styrene were carried out in an electric field with iodine as a catalyst and ethylene dichloride as the solvent. The effects of the field on the rate of polymerization and the degree of polymerization were studied. It was found that the field increased the rate of polymerization of α-methylstyrene and, also slightly increased the degree of polymerization, whereas the field had no influence on these quantities in the case of styrene. The expressions for the rate of polymerization and the degree of polymerization, which were derived in a previous paper and refined in the present paper, show that these quantities are generally a function of the degree of dissociation of ion pairs at growing chain ends. For a comparatively large degree of dissociation, these expressions can account for the field effect as was observed on α-methylstyrene, if one assumes that the degree of dissociation in the presence of an electric field is larger than that in its absence, and that the free-ion propagation proceeds much faster than the ion-pair propagation. For a small degree of dissociation, however, these expressions become practically independent of the degree of dissociation so that a possible increase due to the presence of an electric field gives rise to no observable effect on the polymerization. This situation may be interpreted as corresponding to the case of styrene. In other words, the polymerization of α-methylstyrene has more free ionic character than that of styrene.     

Anionic heterogeneous polymerization of methacrylonitrile by n-butyllithium

The anionic heterogeneous polymerization of methacrylonitrile by butyllithium in petroleum ether was investigated. The polymerization was of the “living” type, as seen from the linear dependence of the molecular weights on [MAN]/[BuLi]. This behavior was further supported by block polymerization experiments in which the monomer was added in two portions and the molecular weights obtained were directly proportional to the total monomer concentration. The initiator efficiency was low, and initiator consumption was only about 2%. This fact, together with the results of the block polymerizations showed that there was preferential addition of monomer to the growing chain ends rather than to the initiator. The molecular weights were independent of the rate of monomer addition. This as well as the “living” behavior of the polymerization of methacrylonitrile on a wide range of monomer and catalyst concentrations and the absence of chain transfer to monomer was essentially different from that of the similar heterogeneous polymerization of acrylonitrile by butyllithium previously investigated. This is due to the absence of an α-acidic hydrogen in methacrylonitrile.     

Polymerization of Cyclic Ethers

Abstract

In the present series of studies on the cationic polymerization of cyclic ethers, the reactivities of cyclic ethers were quantified and the effect of the catalyst upon the polymerization kinetics was revealed. These kinetic analyses were successfully performed by means of our “phenoxyl end-capping method”. The change of the reactivity by the ring size of the monomer was interestingly demonstrated. In addition, it is emphasized that the frequency factor as well as the activation energy influence the rate constant of propagation. As to the effect of catalyst upon the polymerization kinetics, the most important conclusion is that the rate constant of propagation changes very little according to the changes of the catalyst components. Variation of the conversion rate by a change of catalyst is due to differences in the rates of the initiation and the termination reactions.     

非MAO的茂钛均相催化体系催化苯乙烯间规聚合——[Cp~*TiMe_3]/[Ph_3C]~+[B(C_6F_5)_4]~-催化体系

非ＭＡＯ的茂钛均相催化体系催化苯乙烯间规聚合———［ＣｐＴｉＭｅ３］／［Ｐｈ３Ｃ］＋［Ｂ（Ｃ６Ｆ５）４］－催化体系许光学林尚安（中山大学高分子研究所广州５１０２７５）关键词茂钛络合物，茂金属催化剂，苯乙烯，间规聚苯乙烯间规聚苯乙烯（ｓＰＳ）由于具...     

一氯代八甲基环四硅氧烷阳离子聚合动力学

合成了八甲基环四硅氧烷的一氯取代物,并以其为单体,98%浓硫酸为催化剂,采用称重法研究了一氯代八甲基环四硅氧烷阳离子开环的本体聚合动力学,探讨了温度,催化剂浓度对其聚合速率常数的影响.研究表明:温度,催化剂浓度对一氯代八甲基环四硅氧烷的聚合速率有显著影响,其聚合活化能是40.37kJ.mol-1;与八甲基环四硅氧烷阳离子开环聚合相比,氯代甲基使硅氧烷的开环聚合速率减慢,聚合活化能升高.     

Effects of pulses of current on the cationic electropolymerization of isobutylvinylether

The cationic electropolymerization of isobutyl vinyl ether was induced and controlled by sequences of pulses of current of various lengths and of similar or opposite polarity. A remarkable degree of control over the rate of the reaction was demonstrated for a system that is subject to “flash” polymerization by conventional chemical initiators. The mechanism is shown definitively not to correspond to the electric field effects proposed by Tanaka, Ise and Sakurada as applicable to all such electropolymerizations.     

Changes in concentration of tetraoxane produced during the solution polymerization of trioxane catalyzed by BF3·O(C2H5)2

The behavior of tetraoxane produced during the polymerization of trioxane was investigated kinetically. In the polymerization of trioxane, a short induction period for the formation of methanol-insoluble polymer was observed and during the induction period a certain amount of tetraoxane, depending on the polymerization conditions, was produced. This amount was independent of the initial concentration of catalyst but increased with an increase in the polymerization temperature and in the initial concentration of trioxane. So this amount was found to be determined by the equilibrium between the formation and consumption of tetraoxane. On the other hand, in the early stage of polymerization of trioxane, the formation of an appreciable amount of soluble polymer was estimated. Consequently the formation of tetraoxane was explained in terms of the “back-biting” reaction of the soluble growing chain with depolymerization.     

Molecular weight distributions in radiation-induced polymerization. VII. γ-ray-induced polymerization of “wet” styrene in the solid state

In the present paper kinetic and molecular weight distribution results are reported for the γ-ray-initiated polymerization of styrene in the solid state. “In-source” polymerization over the temperature range ?35°C to ?55°C and post-polymerization at ?35°C have been investigated for “wet” styrene samples (water concentration ≈ 10^?3 mole/l.). An interesting feature of the solid-state polymerization of styrene is the bimodal nature of the molecular weight distribution. On a qualitative basis the results resemble those obtained previously for the polymerization of rigorously dried (“dry”) styrene. However, there are noticeable differences on a quantitative basis resulting from the considerable difference in the water content between wet and dry samples. On the basis of these studies, the kinetic and molecular weight distribution data have been interpreted as being indicative of polymerization occurring simultaneously via free-radical and cationic mechanisms.     

Dependence of the conformation of nascent poly(oxymethylene) on the conditions of cationic polymerization of trioxane in solution

The conformational structure of nascent poly(oxymethylene) (POM) obtained by cationic polymerization of trioxane in nitrobenzene was investigated by i.r. spectroscopy. It was found that the conformational order of this POM depends considerably on the conditions of preparation. Under conditions of simultaneous polymerization and crystallization, “A” polymer, with long sequences of monomer units in regular G conformation, is obtained. Under conditions of successive polymerization and crystallization, the formation of conformational defects in the helical POM chains is favoured. Then, depending on the supersaturation, we obtained either (a) POM of “B” form with short sequences of monomer units in G conformation, or (b) POM of a mixed type, the i.r. spectrum being describable as a superposition of “A” and “B” spectra. The results indicate that, at comparatively high catalyst concentration, the thermodynamical approach for regulation of supermolecular structure of polymers can be applied successfully for regulation of the conformational order of nascent POM also. At lower concentration of active centres in the polymerizing system, kinetic factors affect considerably the conformational structure of nascent POM.     

Polymerization of 1,3,5-Tri(1,3,5,7-tetra)-methyl-1,3,5-tri(1,3,5,7-tetra)-10-carbomethoxydecylctri(tetra)siloxane

The cationic and anionic polymerization of 1,3,5-tri(1,3,5,7-tetra)methyl-1,3,5-tri (1,3,5,7-tetra)-10-carbomethoxydecylcyclotri (tetra) siloxane, catalyzed by sulfuric acid and alkali metal naphthalenes, respectively, was studied. With sulfuric acid the polymer yield increased with increasing catalyst concentration, while the molecular weights decreased. With potassium naphthalene the polymerization reaction was first order to monomer, and the molecular weights increased linearly with increasing the percent conversion in accordance with a “living” polymerization. In both cases the polymerization was an equilibrium reaction and the conversion was about 85%. Only low molecular weight polymers were obtained due to steric effects of the bulky long-chain substituents.     

Cationic polymerization of formaldehyde in liquid carbon dioxide. Part I

A cationic polymerization of formaldehyde which gave a high molecular weight polymer was studied in liquid carbon dioxide at 20–50°C. In the polymerization without any catalyst both the rate of polymerization and the molecular weight of the resulting polymer increased rapidly with a decrease in the loading density of the monomer solution to the reaction vessel, and also increased with an increase in the initial monomer concentration. From these results it was concluded that the initiating species could be ascribed to an impurity contained in the monomer solution. Both the rate of polymerization and the degree of polymerization of the polymer also increased with rising temperature. The carboxylic acid added acted as a catalyst in the polymerization because of increase in the polymer yield, the molecular weight of polymer formed, and the number of moles of polymer chain with increasing dissociation constant of acid used. It was concluded that the polymerization in liquid carbon dioxide proceeded by a cationic mechanism. Methyl formate had no influence on the polymerization, but methanol and water acted as a chain-transfer agent.     

Cocatalysis in Cationic Polymerization

Problems concerned with the principles of cocatalysis and coinitiation as a part of cationic polymerization are discussed. When the established concept of different reactive particles, i.e., contact and separated ion-pairs or free ions, is applied to cationic initiation and propagation centers, then common features for one general process can be drawn even in so diverse cases as “pseudocationic” polymerization, solvent “cocatalysis,” polymerization during condensation and induced by co-monomer addition. A special case of activation by solvent is “cocatalysis” by water.

The model for these cases was found during the interpretation of waves observed on styrene polymerization curves. The formation of ion-pairs proceeds spontaneously. The activation and deactivation of these ion-pairs is effected via coordination of suitable molecules with the former, i.e., by equilibrium shifts     

Polymerization of aromatic aldehydes. II. Cationic cyclopolymerization of phthalaldehyde

Phthalaldehyde was found to undergo cyclopolymerization with ease by several cationic catalysts and by γ-ray irradiation. The polymer was composed entirely of the dioxyphthalan unit, as confirmed by infrared spectroscopy and ready decomposition to monomer. The enhanced polymerizability of phthalaldehyde as compared with other aromatic aldehydes was explained in terms of the intermediate-type or, preferably, concerted propagation scheme. The conversion reached a saturation value of 87% in about 1 hr in methylene chloride at ?78°C, indicating an equilibrium polymerization. The ceiling temperature of the polymerization was ?43°C, as estimated from the relation between the saturation yield and polymerization temperature. The enthalpy and entropy of propagation were ?5.3 kcal/mole and ?23.0 eu, respectively. Since the molecular weight of the polymer was proportional to conversion, the propagating chain end was considered to be “living” in this system. The rate constant for propagation was calculated to be 0.18 1/mole-sec in methylene chloride at ?78°C with BF₃OEt₂ catalyst.     

Coordination polymerization in water affording amorphous polyethylenes

The coordination polymerization of ethylene in water as a reaction medium was studied. Rubbery amorphous branched polyethylene was obtained when a known cationic diimine-substituted methyl complex was employed as a catalyst precursor. High rates of up to 900 TOh(-1) (turnover frequency) were observed. In contrast to solution polymerization in an organic solvent, the rate of suspension polymerization in water increases greatly with ethylene pressure in the range up to 20 bar; this indicates control of the polymerization rate by the concentration of the olefin monomer at the catalytically active site. The effect and mode of mass transfer phenomena were studied. A high catalyst stability in the aqueous coordination polymerization was observed. It was found to be due to an "encapsulation" of the water-insoluble catalyst precursor in the hydrophobic amorphous polymer during the polymerization reaction, and this resulted in strongly restricted accessibility for the aqueous phase. Surprisingly, exposure of the water-stable catalyst precursor to ethylene monomer in solution in the presence of water resulted in immediate decomposition. Polymer microstructure, and thermal and mechanical properties were investigated. The different degree of branching, molecular weight, and corresponding macroscopic properties of the polymers obtained in water as a reaction medium versus solution polymerization in methylene chloride under the same conditions are due to the different phase behavior during polymerization (suspension vs. solution), as opposed to an effect of water on the catalytically active centers.     

八甲基环四硅氧烷正离子细乳液开环聚合动力学

以十二烷基苯磺酸钠(SDBS)为乳化剂,硫酸或盐酸为催化剂,八甲基环四硅氧烷(D4)为单体,十六烷为共稳定剂,超声预乳化,制备了聚硅氧烷细乳液,研究了超声时间、催化剂用量、乳化剂用量和温度对聚合动力学的影响.结果表明,在一定酸度范围内,聚合速度与硫酸浓度0.81次方、与盐酸浓度1.02次方、与乳化剂浓度-0.66次方成正比,反应的表观活化能为40.56kJ/mol.     

Stereospecific polymerization of methacrylonitrile. II. Influence of polymerization conditions on polymerization and on properties of polymers

Stereospecific polymerization of methacrylonitrile with diethylmagnesium has been studied. Polymerization temperature has an important effect on polymerization. The conversion, stereoregularity, and intrinsic viscosity of the polymer increased significantly with increasing polymerization temperature. Stereoregularity of the polymer improved with increasing the polymerization time and the monomer concentration, but it is independent of the catalyst concentration. Intrinsic viscosity of the crystalline polymer increased with increasing monomer concentration but is independent of the polymerization time and the catalyst concentration. It is suggested that two mechanisms are involved in this polymerization: coordinated anionic polymerization to from the crystalline polymer, and probably conventional anionic polymerization to form the amorphous polymer. It is found that crystalline polymer can also be obtained in homogeneous phase such as in tetrahydrofuran solvent.     

GROUP TRANSFER POLYMERIZATION OF ETHYL ACRYLATE WITH LEWIS ACID CATALYST

This paper reports the kinetics of group transfer polymerization (GTP)of ethyl acrylate (EA)with zinc iodide catalyst in 1,2-dichloroethane using dimethyl ketene methyl trimethylsilyl acetal (MTS) as initiator at 0℃and above 0℃. The amount of catalyst used was studied. When zinc iodide catalyst used is more than 10mol% relative to monomer, the rate of polymerization is proportional to the concentration of monomer, whereas zinc iodide catalyst used is less than 10 mol% of the monomer, the rate of polymerization is independent of the monomer concentration.In the GTP of EA an induction period was observed when the zinc iodide contents are less than l0mol%. If the reaction temperature is over 0℃, living species become unstable and diminish, leading to incomplete monomer conversion. The reaction curves equations are obtained. The polymers have narrow molecular weight distributions which are not changed as decreasing zinc iodide contents. The polydispersity is about 1.2.     

The “comonomer effect” in ethylene/α‐olefin copolymerization using homogeneous and silica‐supported Cp2ZrCl2/MAO catalyst systems: Some insights from the kinetics of polymerization,active center studies,and polymerization temperature

Homogeneous and silica‐supported Cp₂ZrCl₂/methylaluminoxane (MAO) catalyst systems have been used for the copolymerization of ethylene with 1‐butene, 1‐hexene, 4‐methylpentene‐1 (4‐MP‐1), and 1‐octene in order to compare the “comonomer effect” obtained with a homogeneous metallocene‐based catalyst system with that obtained using a heterogenized form of the same metallocene‐based catalyst system. The results obtained indicated that at 70 °C there was general rate depression with the homogeneous catalyst system whereas rate enhancement occurred in all copolymerizations carried out with the silica‐supported catalyst system. Rate enhancement was observed for both the homogeneous and the silica‐supported catalyst systems when ethylene/4‐MP‐1 copolymerization was carried out at 50 °C. Active center studies during ethylene/4‐MP‐1 copolymerization indicated that the rate depression during copolymerization using the homogeneous catalyst system at 70 °C was due to a reduction in the active center concentration. However, the increase in polymerization rate when the silica‐supported catalyst system was used at the same temperature resulted from an increase in the propagation rate coefficient. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 267–277, 2008     

The strange case of the “oscillating” catalysts

The field of stereoselective propene polymerization has been dramatically innovated by the discovery of homogeneous metallocene-based catalysts with well-defined and tunable molecular structure. Of all, “oscillating” metallocenes are probably the most ingenious and challenging example of catalyst design. Their catalytic species were built to “flip-flop” between a chiral and an achiral conformation, at a rate intermediate between those of monomer insertion and chain transfer. The result of this molecular switching would be a polypropylene with an isotactic/atactic stereoblock structure, performing as a thermoplastic elastomer. This essay discusses how the real polymerization mechanism differs from what the catalyst inventors had in mind, but also how - through fortunate circumstances -their optimism has been rewarded.