Kinetics of radical polymerization. XXX. Polymerization of styrene in solution

A study was made on the AIBN-initiated polymerization of styrene in benzene and dimethyl formamide at 50°C. The overall rate and the rate of initiation of the polymerization were determined and the number-average and weight-average molecular weights of the polymers formed were measured. The decrease in the rate and degree of polymerization with the decrease in styrene concentration was caused by the corresponding decreases in the rate constants of chain propagation and initiation steps. In the systems studied no chain transfer process occurred with an experimentally measurable rate.     

Cyclo- and Cyclized Diene Polymers. XIX. Polymerization of Butadiene with the C2H5AlCl2 + TiCl4 Catalyst

Abstract

The polymerization of butadiene with an EtAlCl₂-TiCl₄ catalyst system yields cyclopolybutadiene with varying amounts of trans-1, 4 units, depending upon the Al/Ti ratio and the solvent. Apparently different active centers are produced at Ti > Al and Al > Ti ratios. When the catalyst system has Ti > Al, there is a rapid decrease in the initial polymerization rate and the cyclopoly butadiene contains large amounts of methyl groups, 10–12% of trans-1, 4 units, 2–3% of 1, 2 units, and, when the polymerization is carried out in aromatic solvents, aromatic moieties are incorporated in the structure. When the catalyst system has Al > Ti, there is a very slow decrease of the initial polymerization rate, and the cyclopoly butadiene contains up to 40% of trans-1, 4 units, less than 1% of 1, 2 units, and methyl groups and solvent moieties are essentially absent even when the polymerization is carried out in aromatic solvents. Cocatalytic amounts of iodine greatly increase the initial rate of polymerization. The Ti > Al catalyst may promote 1, 3-cation-radical propagation with transoid monomer to yield a perhydrophenanthrene structure while the Al > Ti catalyst may promote 1,2 cation-radical propagation with cisoid monomer to yield a perhydroanthracene structure.     

Modeling of molecular weight development in atom transfer radical polymerization

A kinetic model has been developed for atom transfer radical polymerization processes using the method of moments. This model predicts monomer conversion, number‐average molecular weight and polydispersity of molecular weight distribution. It takes into account the effects of side reactions including bimolecular radical termination and chain transfers. The determining parameters include the ratios of the initiator, catalyst and monomer concentrations, as well as the ratios of the rate constants of propagation, termination, transfer and the equilibrium constant between radicals and their dormant species. The effects of these parameters on polymer chain properties are systematically simulated. The results show that an ideal living radical polymerization exhibiting a linear relationship between number‐average molecular weight versus conversion and polydispersity approaching unity is only achievable under the limiting condition of slow monomer propagation and free of radical termination and transfers. Improving polymerization rate usually accompanies a loss of this linearity and small polydispersity. For polymerization systems having a slow initiation, the dormant species exercise a retention effect on chain growing and tend to narrow the molecular weight distribution. Increasing catalyst concentration accelerates the initiation rate and thus decreases the polydispersities. It is also shown that for a slow initiation system, delaying monomer addition helps to reduce the polydispersities. Radical termination and transfers not only slow down the monomer conversion rates but also broaden polymer molecular weight distributions. Under the limiting conditions of fast propagation and termination and slow initiation, the model predicts the conventional free radical polymerization behaviors.     

Propagation Kinetics of Free-Radical Methacrylic Acid Polymerization in Aqueous Solution. The Effect of Concentration and Degree of Ionization

Propagation rate coefficients, k_p, of free-radical methacrylic acid (MAA) polymerization in aqueous solution are presented and discussed. The data has been obtained via the pulsed laser polymerization – size-exclusion chromatography (PLP-SEC) technique within extended ranges of both monomer concentration, from dilute solution up to bulk MAA polymerization, and of degree of ionic dissociation, from non-ionized to fully ionized MAA. A significant decrease of k_p, by about one order of magnitude, has been observed upon increasing monomer concentration in the polymerization of non-ionized MAA. Approximately the same decrease of k_p occurs upon varying the degree of MAA ionization, α, at low MAA concentration from α = 0 to α = 1. With partially ionized MAA, the decrease of k_p upon increasing MAA concentration is distinctly weaker. For fully ionized MAA, the propagation rate coefficient even increases toward higher MAA concentration. The changes of k_p measured as a function of monomer concentration and degree of ionization may be consistently interpreted via transition state theory. The effects on k_p are essentially changes of the Arrhenius pre-exponential factor, which reflects internal rotational mobility of the transition state (TS) structure for propagation. Friction of internal rotation of the TS structure is induced by ionic and/or hydrogen-bonded intermolecular interaction of the activated state with the molecular environment.     

Ring‐Opening Polymerization of Propylene Oxide by Double Metal Complex in Micro‐Reactor

The ring‐opening polymerization of propylene oxide catalyzed by double metal complex (DMC) is carried out in continuous micro‐reactor (C‐MR). It is found that the monomer conversion at the C‐MR outlet is usually 100% within 2 min of average residence time, which means that the polymerization rate in the C‐MR is faster than that in a traditional semi‐continuous tank reactor. However, the induction period still exists in the polymerization in C‐MR, but can be shortened by increasing the reaction temperature or the micro‐reactor length. The mechanism of monomer coordination and ring opening on DMC during the induction period is confirmed by the ¹H NMR analysis of the samples obtained under very short average residence time. The molecular weight distribution (MWD) of product from C‐MR is generally narrow, which indicates that the process still maintain the characteristics of the “living” polymerization. That is, there is a very high rate ratio of chain transfer to chain propagation provided by the DMC catalyst. However, with the same average residence time, the MWD of product from the longer C‐MR is broader, which can be attributed to the increase of the chain propagation rate caused by rise of pressure.     

再活化链式聚合

基于传统的链式聚合和逐步聚合二种高分子链增长过程,提出了再活化链式聚合。按此聚合机理,高分子的链增长是通过将一个非活性或睡眠状态的链(Mm)重新活化为活性种(Mm*),活性种再和一个单体(M)反应,生成一个较大分子量的休眠产物(Mm 1)来实现的。再活化链式聚合主要例子包括苯胺和或许其它芳香族单体的氧化聚合,活性自由基聚合,以及核酸和蛋白质合成中的生物聚合。     

Polymerization of N-carboxy anhydrides by organotin catalysts

Organotin compounds were found to lead to polymerization of N-carboxy anhydrides. The polymerization was studied in detail using γ-benzyl N-carboxyl-t-glutamate anhydride (BGA). Compounds such as tributyltin methoxide, bis(tributyltin)oxide, and N-tributyltin imidazole polymerized BGA while others like dibutyltin dichloride, which are Lewis acids, failed. Polymerization of BGA in dioxane at various monomer to dibutyltin dimethoxide ratios showed a first order reaction to monomer. The plot of In M₀/M₁ vs time showed two stage kinetics, the second one being faster. The pseudo first order rate constants were smaller than those for primary amine initiated polymerizations and much smaller than that for polymerization initiated by sodium methoxide. The molecular weights were independent of the monomer to initiator ratio both in dioxane and in DMF. In the reaction of an equimolar amount of tributyltin methoxide with NCA, the methyl ester of the amino acid was formed.The mechanism suggested is that of addition of the organotin compound to the NCA forming an organotin carbamate which decarboxylates, leaving an active -N-Sn-group which adds to another NCA molecule. This process is repeated in every step of the propagation.     

Polymerizability of norbornene in ring-opening metathesis polymerization initiated by Mo-based complexes

Concerning the study on the relation between structural parameters and reactivity in ring-opening metathesis polymerization of cyclo-olefins, the “living” polymerization of norbornene initiated by Schrock's-type complexes was considered as a reference and studied from the kinetic point of view. First kinetic orders with respect to both monomer and active species allow the values of absolute rate constants of propagation to be determined. The thermodynamic parameters obtained from kinetic experiments performed at different temperatures seem to indicate that monomer coordination and metallacycle formation are rate-determining steps in the process studied. © 1994 John Wiley & Sons, Inc.     

Polymerization of Vinyl Chloride at Reduced Monomer Accessibility. I. Polymerization at Subsaturation Pressure with Suspension PVC as a Seed

Vinyl chloride was polymerized at 40–96% of saturation pressure in water suspended systems at 55°C with suspension PVC as a seed. The monomer was continuously charged as vapor from a storage vessel kept at lower temperature. Characterization included molecular weight distribution (MWD) and degree of long-chain branching (LCB) by GPC and viscometry, thermal dehydrochlorination, microscopy, and technical standard tests. A granular seed was necessary to obtain granular polymer and reasonable polymerization rate. In properly seeded systems with monomer-soluble initiator, crust formation is very low. With monomer-soluble initiator the polymerization reactions are restricted to the seed polymer, comprise its total structure, reducing its porosity. At pressure near saturation, molecular weight increased with conversion, and autoacceleration occurred. The changes in molecular weight are presumably due to the lack of a liquid monomer phase, which reduces the mobility of the radicals leading to decreased termination. At decreasing polymerization pressure the mobility is further reduced due to de-swelling of the gel. At low pressure the polymerization rate decreased, the amount of low molecular weight material increased, and considerable long-chain branching occurred as a result of monomer depletion. With water-soluble initiator (at pressure near saturation), new tiny particles were formed in the water phase at the boundary to the seed particles. The seed structure remained unchanged. The polymer formed showed broad MWD and high LCB. No marked reduction in polymerization rate was observed. The dehydrochlorinatlon measurements indicate that the end groups and not the branch points are responsible for the decrease in thermal stability noticed.     

Cationic Polymerization of Cyclic Sulfides

Abstract

The mechanism of the cationic polymerization of several thietanes and of propylene sulfide under the influence of triethyloxonium tetrafluoroborate in methylene chloride is described. The thietane polymerizations stop at limited conversions because of a termination reaction occurring between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms of the polymer chain. The maximum conversions obtained under identical conditions differ markedly for the different monomers. Ratios of rate constants of propagation (k_p) to rate constants of termination (k_t) have been calculated. The differences in k _p/k_t. values for the different monomers are explained in terms of differences in basicity and differences in steric hindrance of the monomers compared to the corresponding polymers. In the case of propylene sulfide it is proposed that the main termination reaction is the formation of 12-membered ring sulfonium salts by an intramolecular reaction of the third sulfur of the growing polymer chain with the reactive chain end (three-membered ring sulfonium salt). This terminated polymer is able to reinitiate the polymerization, for example, by reaction of a monomer molecule at the exocyclic carbon atom of the sulfonium salt function. The cyclic tetramer of propylene sulfide is formed in this reaction. After complete polymerization, formation of cyclic tetramer continues, probably via a backbiting mechanism. In methylene chloride as solvent, the absolute value of the rate constant of propagation for 3,3-dimethylthietane changes with changing concentration of initiator and by adding different amounts of indifferent electrolyte to the reaction mixture. From these changes, and assuming that the value of the dissociation constant of the growing chain-ends is close to values of dissociation constants of low molecular weight sulfonium salts, separate rate constants for propagation via free ions and ion-pairs were calculated. The propagation constant of free ions is about 70 times higher than that of ion pairs in methylene chloride at 20°C. Free ions and ion pairs are nearly equally reactive in nitrobenzene.     

Rate Constant of Initiation Reaction in Cationic Polymerization of Vinyl Monomers. I. Polymerization of Styrene Derivatives Catalyzed by Triphenylmethyl Stannic Pentachloride

Abstract

To investigate the method of measurement of the initiation rate constant in cationic polymerization, styrene and α-methylstyrene are polymerized by triphenylmethyl stannic pentachloride (Ph₃CSnCl₅). Adding these monomers to a solution of Ph₃CSnCl₅, the strong absorption of triphenylmethyl cation at 400 to 450 mμ disappears. The rate of disappearance of the absorption at 430 mμ is proportional to the concentration of Ph₃CSnCl₅ and monomer. It is confirmed that this disappearance of the absorption is due to the conversion of triphenyl cation to styryl or α-methylstyryl cation. Therefore, the rate of consumption of triphenylmethyl cation corresponds to that of the addition of triphenylmethyl cation to the olefinic double bond in a monomer, that is, the rate of the initiation reaction. Considering the dissociation constant of Ph₃CSnCl₅, the initiation rate constants of styrene and α-methylstyrene in ethylene chloride solution at 30°C are 11.6 × 10^?2 and 2.85 liters/mole-min, respectively. These values seem to be much smaller than the propagation rate constant of each monomer. However, the effect of polymerization conditions, for example, the kind of a monomer and the polarity of a solvent, on the initiation rate constant is the same as in the propagation reaction. This fact suggests the similarity of a reaction mechanism in both elementary reactions.     

Termination Mechanism in the Anionic Polymerization of Methyl Methacrylate

Recent studies have revealed that at temperatures around 200°K in tetrahydrofuran solvent, poly(methy1 methacrylate) ion pairs are long-lived and very reactive. At higher temperatures however termination of the ion pairs occurs, as evidenced by the broadening of the molecular weight distribution of the resultant polymer and by the incomplete polymerization of the monomer. Three mechanisms have been proposed to describe these termination reactions; an inter molecular reaction with the monomer ester function, an intramolecular cyclization of the anion, or reaction with the polymer ester function. In the absence of monomer only the last two mechanisms can be operative. A series of experiments was undertaken in which the molecular weight distribution broadening with temperature increase was measured under typical polymerization conditions or in the absence of monomer. The effect of each of the three counterions Li, Na, and K was also monitored. The results obtained are discussed in terms of these three possible termination mechanisms. Termination rate constants calculated from the molecular weight distribution are also presented.     

Polymerization of 1,P-Epoxides Initiated by Tetraalkyl Aluminates. I. Polymerization of Ethylene Oxide in the Presence of Sodium Tetrabutyl Aluminate

The polymerization of ethylene oxide (EO) initiated by NaA1Bu₄ is shown to proceed upon initial complex formation between monomer and initiator. In polymerization in toluene a high order of the kinetic equation with respect to initiator was found, indicating that chain propagation proceeds on dimers and trimers of the active center. An induction time of polymerization in THF is observed. It is necessary to reach a specific concentration of the NaAIBu₄.EO complexes which take part in the polymerization process. The wide molecular weight distribution, the high effectivity coefficient (initiation efficiency), and the polymerization rate increase with polymer yield are evidence of a polycentric polymerization mechanism.     

Activation Mechanisms of Trialkylaluminum in Alkali Metal Alkoxides or Tetraalkylammonium Salts / Propylene Oxide Controlled Anionic Polymerization

The anionic polymerization of propylene oxide (PO) initiated by alkali metal alkoxides is in non polar solvents a very slow and non controlled reaction process. Transfer reaction to monomer is predominant, allowing only the preparation of low molar masses PPO. The influence of the addition of trialkylaluminium to either an alkali metal alkoxide or a tetraalkylammonium salt used as initiator for PO polymerization in hydrocarbon media was investigated. A strong enhancement of the polymerization rate accompanied by a drastic decrease of the transfer reactions is observed, allowing the synthesis of PPO with well controlled molar masses. At constant monomer and alkali metal alkoxide concentrations, the polymerization rate increases with increasing trialkylaluminium concentration. Results indicate that the trialkylaluminium derivative is involved in the formation of two distinct complexes, one with the alkali metal alkoxide or the tetraalkylammonium salt and another one with the PO monomer which is strongly activated towards nucleophilic active species. Significant differences between the alkali metal and tetraalkylammonium based initiators are observed. In particular much less trialkylaluminum activator is needed with the ammonium salt to get the same rate of propagation and controlled polymerization.     

Living and apparently living carbocationic polymerizations

Some years ago, the occurrence of living carbocationic polymerization had not generally been expected to be possible, since it is well known that most carbocationic species are quite unstable and have very short lifetimes, and since transfer to monomer had been shown to be important, particularly near room temperature. However, during the last years, many reports of living carbocationic polymerizations have been made. They were based on the observation of various features usually linked to living polymerizations, such as a linear increase of mol. wts. with conversion, sometimes even after several monomer additions, which was attributed to the absence of termination and transfer. In some cases, narrow mol. wt. distributions were also obtained. There seems to be now a general agreement that in these polymerizations a reversible termination occurs, making eventually further growth possible on all macromolecules. Another general feature of those apparently living systems is that the ratio of propagation rate and initiation rate is not too high, so that the concentration of macromolecules is approximately equal to that of the initiator. But the experimental data do not necessarily imply, as this has been generally assumed, that transfer is absent and that the nature of active sites is completely different from those in more classical systems. It is shown that the values of transfer constants already measured in these last ones are compatible with the results obtained in the apparently living systems. A perfectly linear relationship between number-average degree of polymerization (D̄P̄_n) and polymer yield may be observed even up to mol. wts. of about 2.10⁴ with transfer constants k_trM/k_p as high as 5.10⁻⁴ in apparently living systems. Termination and transfer might be, however, reduced in some cases by various means that are examined, such as the presence of polar additives, a lowering of temperature and the presence of excess monomer. The distinction between systems obeying all the main criteria for living polymerization and those which are only apparently living is discussed.     

Unexpected Kinetics in the Polymerization of Ethene by Cp$\rm{ {_{2}^{{\ast}}}}$ZrCl2/MAO

A sterically hindered metallocene catalyst, Cp ZrCl₂ activated with methylaluminoxane (MAO), is found to polymerize ethene at temperatures up to 60° with a good propagation rate constant but low number of active sites, and with negligible β‐hydride elimination or β‐hydride transfer to monomer. Moreover, transmetalation to Al is found to be effectively irreversible for alkyl groups larger than Me. With the major mechanisms for chain transfer and termination suppressed, one might expect a living polymerization. The bulk polymerization of ethene was indeed found to be quasi‐living even when performed at well above room temperature, and furthermore provided rate constants which agreed remarkably well with those from the mass‐spectrometric study.     

Polymerization of ethylene with supported zirconium organic compounds

The polymerization of ethylene was studied with supported catalysts prepared by the reaction of Zr(benzyl)₄ and Al₂O₃. The influence of the preparation of the catalysts and the reaction conditions on the overall rate of the polymerization was established. The rate exhibits a maximum which depends on the thermal pretreatment of the support and the quantity of the fixed Zr(benzyl)₄. The concentration of the active centres of the catalysts was determined by quenching the polymerization with labelled butanol. The catalysts possess active centres which have different rates of propagation for ethylene. The decrease of the polymerization rate is caused by chemical reaction. The higher the concentration of free hydroxyl groups on the catalyst, the higher the rate of deactivation. Hydrogen in the monomer feed produces a decrease or an increase of the polymerization rate depending on the preparation of the catalysts.     

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.     

Polymerization of allyl esters of unsaturated acids. I. Cyclopolymerization of methyl allyl maleate

Radical polymerization of methyl allyl maleate is kinetically discussed in terms of cyclopolymerization using 2,2′-azobisisobutyronitrile (AIBN) as an initiator and benzene as a solvent at 60°C. The ratios of the rate constants of the unimolecular cyclization reaction to those of the bimolecular propagation reaction of the uncyclized allyl and vinyl radicals, K_A and K_V, are estimated to be 9.7 and 1.35 mole/liter by fitting the kinetic equations obtained here to the dependence of the degree of cyclization on monomer concentration, respectively; the large difference between K_A and K_V is also discussed in detail. On the basis of these results the formation mode and the sequence distribution of the structural units of the polymer produced are discussed in detail; thus, for the polymer obtained in the bulk polymerization, about 90% of the cyclic structures can be formed via the intramolecular attack of uncyclized allyl radical on maleic double bond and the probability of succession of the cyclic structural units in diad sequence is exemplified as 0.27.     

Polymerization of vinyl chloride at reduced monomer accessibility. II. Polymerization at subsaturation pressure with emulsion PVC as seed

Vinyl chloride was polymerized at 59–92% of saturation pressure in a water-suspended system at 45–65°C with an emulsion poly(vinyl chloride) (PVC) latex as a seed. A water-soluble initiator was used in various concentrations. The monomer was continuously charged as vapor from a storage vessel kept at lower temperature. Characterization included determination of molecular-weight distribution and degree of long-chain branching by gel permeation chromatography (GPC) and viscometry, thermal dehydrochlorination, and microscopy. The polymerization rate decreases with decreasing pressure but is reasonable even at the lowest pressure. The molecular weight decreases with decreasing pressure and increasing initiator concentration and also with increasing polymerization temperature, if the initiator concentrations are chosen to give a constant initiator radical concentration. The degree of long-chain branching increases with increasing initiator concentration and decreasing monomer pressure but is unaffected by the polymerization temperature, if the initiator radical concentration is kept constant. The thermal stability decreases with decreasing M _n, while the degree of long-chain branching has only a minor influence. The most important factor in the system influencing the molecular parameter is the monomer accessibility.