γ-Ray-induced copolymerization of ethylene and vinyl chloride in liquid carbon dioxide

The γ-ray-induced copolymerization of ethylene and vinyl chloride with the use of liquid carbon dioxide as a solvent was studied under a total pressure of 400 kg/cm², with a dose rate of 2.5 × 10⁴ rad/hr at 30°C. A rubberlike, sticky polymer is obtained when the molar concentration of vinyl chloride is less than 30% in the monomer mixture, and the polymer is a white powder at higher concentrations of vinyl chloride. Infrared, x-ray, and differential thermal analyses confirm that the polymerization products are noncrystalline, true random copolymers. The rate of copolymerization decreases markedly when a small amount of vinyl chloride is added to ethylene monomer. In the range of vinyl chloride concentration higher than 5%, however, the rate and the molecular weight of copolymer increase with increasing concentration of vinyl chloride. It has been concluded from kinetic considerations based on these results that the rate of initiation increases proportionally with the concentration of vinyl chloride. Further, the growing chain radicals are shown to be deactivated by the cross-termination reaction between the radicals with terminal unit of ethylene and vinyl chloride, and no transfer reaction occurs.     

Model for Kinetics of Vinyl Chloride Polymerization

This paper gives a critical review of recent models for the polymerization of vinyl chloride. In solution and bulk polymerization the effect of eventual degradative chain transfer to monomer, addition of chain transfer agents, and precipitation of polymer is discussed. A model for emulsion polymerization is described which includes particle formation and kinetics of polymerization where especially desorption and reabsorption of radicals in the particles are included.     

SYNTHESIS AND CHARACTERIZATION OF HYPERBRANCHED VINYL POLYMERS FROM RAFT POLYMERIZATION OF AN ASYMMETRIC DIVINYL MONOMER

Hyperbranched vinyl polymers were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization of a styrenic asymmetric divinyl monomer.This was achieved by using cumyl dithiobenzoate or S-dodecyl-S′- (α,α′-dimethyl-α″-acetic acid)trithiocarbonate as the chain transfer agent,1,1′-azobis(cyclohexanecarbonitrile) or thermal initiation as a source of radicals.Cross-linking was inhibited by a rapid RAFT-based equilibrium between active propagation chains and dormant species,and thus a h...     

Mechanism of vinyl chloride polymerization

An overall mechanistic scheme for the suspension polymerization of vinyl chloride is presented. The process can be resolved into five discrete stages, each of which presents a unique environment for the interaction of the systems parameters. It is shown that the surface area of the polymer formed during the reaction is not a major factor in autoacceleration and that the increase of kinetic chain length with conversion is due to a radical dilution effect. The latter is a direct result of the difference in rates between polymerization and radical formation, the former being greater. The increase of the initial polymerization rate and the reduction of autoacceleration brought about by chain transfer agents can be explained by the lower diffusion rate and greater bulkiness of the chain transfer agent radical relative to that of the monomer radical. The chaintransfer agent CBr₄ is preferentially absorbed by PVC from solution in vinyl chloride. With lauryl peroxide as initiator it is shown that the “hot spot” is the result of a build-up of initiator in the monomer caused by its exclusion from the polymer phase. Vinyl chloride was found to dissolve 0.03% PVC at ambient temperature and to have no effect on the decomposition rate of lauryl peroxide.     

Studies on poly(vinyl chloride). III. The role of the precipitated polymer in the kinetics of polymerization of vinyl chloride

In polymerization of vinyl chloride monomer, free radicals precipitate on or within aggregates of partially swollen dead polymer. Polymerization on the solid polymer is characterized by autoaccelerating rates due to a progressive reduction in termination rate. This reduction in termination rate is due to the fact that as the reaction progresses and more polymer accumulates there is a decrease in probability that chain transfer of polymer radicals to monomer will generate a mobile radical, which can readily terminate an occluded or stuck free radical. From the appearance of the particles of solid polymer in the system, it has been concluded that free radicals precipitate both on polymer particle surface and inside the open structure of polymer particles.     

End‐group fidelity of copper(0)‐meditated radical polymerization at high monomer conversion: an ESI‐MS investigation

Copper(0)‐mediated radical polymerization (single electron transfer‐living radical polymerization) is an efficient polymerization technique that allows control over the polymerization of acrylates, vinyl chloride and other monomers, yielding bromide terminated polymer. In this contribution, we investigate the evolution of the end‐group fidelity at very high conversion both in the presence and in the absence of initially added copper (II) bromide (CuBr₂). High resolution electrospray‐ionization mass spectroscopy (ESI‐MS) allows determination of the precise chemical structure of the dead polymers formed during the polymerization to very high monomer conversion, including post polymerization conditions. Two different regimes can be identified via ESI‐MS analysis. During the polymerization, dead polymer results mainly from termination via disproportionation, whereas at very high conversion (or in the absence of monomer, that is, post‐polymerization), dead polymers are predominantly generated by chain transfer reactions (presumably to ligand). The addition of CuBr₂ significantly reduces the extent of termination by both chain transfer and disproportionation, at very high monomer conversion and under post‐polymerization conditions, offering a convenient approach to maintaining high end‐group fidelity in Cu(0)‐mediated radical polymerization. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

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。     

Polymer particle formation in suspension polymerization of vinyl chloride and vinyl acetate

Equipment has been designed and assembled in such a way that direct microscopic observation of polymer particle formation in suspension polymerization of vinyl chloride and vinyl acetate is possible. The apparent mode of transformation from monomer droplets into polymer particles has thus been studied under two sets of conditions: (1) with agitation and (2) without agitation. In both cases, as the initial vinyl acetate/vinyl chloride ratio was raised, the apparent change in the shape and transparency of particles occurring during the course of polymerization became less evident. In vinyl chloride homopolymerization and vinyl acetate–vinyl chloride copolymerization with relatively high vinyl chloride concentrations, the polymer particles burst during the course of polymerization. Some factors which affect the change in the size of particles are also discussed.     

RAFT-mediated emulsion polymerization of vinyl acetate: a challenge towards producing high molecular weight poly(vinyl acetate)

The preparation of poly(vinyl acetate) with well-controlled structure has received a great deal of interest in recent years because of a large number of developments in living radical polymerization techniques. Among these techniques, the use of reversible addition–fragmentation chain transfer (RAFT)-mediated polymerization has been employed for the controlled polymerization of vinyl acetate due to the high susceptibility of this monomer towards chain transfer reactions. Here, a novel water-soluble N,N-dialkyl dithiocarbamate RAFT agent has been prepared and employed in the emulsion polymerization of vinyl acetate. The kinetic results reveal that the polymerization nucleation mechanism changes from homogeneous to micellar and RAFT-generated radicals can change the kinetic behavior from conventional emulsion polymerization to living radical polymerization. At higher concentrations of the modified RAFT agent, as a result of an aqueous phase reaction between RAFT and sulfate radicals, relatively more hydrophobic radicals are generated, which favors entry and propagation into micelles swollen with monomer. This observation was determined from the investigation of the polymerization rate and measurements of the average particle diameter and the number of particles per liter of the aqueous phase. Molecular weight analysis also demonstrated the participation of the RAFT agent in the polymerization in such a way as to restrict chain transfer reactions. This was determined by examining the evolution of polymer chain length and attaining higher molecular weights, even up to 50?% greater than the samples obtained from the conventional emulsion polymerization of vinyl acetate in the absence of the synthesized modified RAFT agent.     

Particle structure of suspension poly(vinyl chloride) and its origin in the polymerization process

The initial stage of the suspension polymerization of poly(vinyl chloride) (PVC) is characterized by the formation of colloidally stable micron-sized grains of PVC inside the polymerizing ca. 150 μm vinyl chloride droplets. The fate of these micron-sized PVC grains depends upon the agitation conditions. If no agitation is employed, they serve as growth centers for further polymerization to give a final particle possessing a uniform internal bead morphology. In agitated systems, these grains coagulate early in the conversion to give a more irregular structure in the interior of the PVC particle. The formation of these stable growth centers appears to be unique to PVC. The polymerization of acrylonitrile, also insoluble in its monomer, is characterized by rapid agglomeration of the precipitated polymer throughout the polymerization. In PVC, the colloidal stability of the polymerizing grains is demonstrated to be electrical in nature. A pericellular membrane or skin formed by polymerization in both the water and vinyl phase completely surrounds the polymerizing droplet after about (1–2)% conversion. This skin is responsible for the charge retention of the PVC grains inside the polymerizing monomer droplets.     

Polymerization of vinyl chloride at reduced monomer accessibility. III. Polymerization kinetics and molecular structure using PVC latex as seed

Vinyl chloride was polymerized at 53–97% of the saturation pressure in a water-suspended system at 55°C with an emulsion PVC latex as 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 chromatography and viscometry and by thermal dehydrochlorination. To avoid diffusion control intense agitation was necessary. At a certain conversion, aggregation of primary particles resulted in restricted polymerization rate. Before aggregation, formation of new particles did not occur as the number of particles was high enough to ensure capture of all oligoradicals. The kinetic equation accepted for ordinary emulsion polymerization of vinyl chloride was qualitatively found to be valid after the pressure drop as well. Decreased termination rate may result in increased polymerization rate at reduced monomer concentration, i.e., a gel effect, especially at low particle numbers and high polymer contents. The molecular weight decreased with decreasing monomer concentration. This is in accordance with the new mechanism suggested for chain transfer to monomer starting with occasional head-to-head additions.     

Calculation and Applications of VCM Distribution in Vapor/Water/Solid Phases during VCM Polymerization

The distribution of vinyl chloride monomer (VCM) in vapor, water, swollen polymer, and free monomer phases as a function of conversion of VCM can be calculated from the related partition coefficients. It was found that the amount of monomer in the vapor and water phases is particularly significant, being 20% (at 60°C) of that in the polymer phase at the peak exotherm. Neglecting the VCM dissolved in water and that in the head space of the reactor would seriously overestimate the polymerization rate and overdesign the required cooling capacity of the reactor. From the distribution the relation between conversion (x) vs pressure (P) after the pressure starts to drop can be developed and used to determine conversion at termination by pressure measurement. The results of ×vs P from our partition coefficient approach are consistent with those derived from Flory-Huggin's equation. Also the knowledge of VCM distribution at termination of the polymerization will assist VCM accountability and stripper design.     

Particle nucleation and monomer partitioning in styrene O/W microemulsion polymerization

Particle nucleation in the polymerization of styrene microemulsions was found to take place throughout the polymerization as indicated by measurements of the particle number as a function of conversion. A mechanism based on the nucleation in the microemulsion droplets was proposed to explain the experimental findings although homogeneous nucleation and coagulation during polymerization were not completely ruled out. A thermodynamic model was developed to simulate the partitioning of monomer in the different phases during polymerization. The model predicts that the oil cores of the microemulsion droplets were depleted early in the polymerization (4% conversion). Due to the high monomer/polymer swelling ratio of the polymer particles, most of the monomer resides in the polymer particles during polymerization. The termination of chain growth inside the polymer particles was attributed to the chain transfer reaction to monomer. The low n? (less than 0.5) of the microemulsion system was attributed to the fast exit of monomeric radicals.     

Particle morphology in the early stages of radiation-induced heterophase polymerization of vinyl chloride

Investigations of the particle morphology of poly(vinyl chloride) produced under quiescent conditions during radiation-induced bulk polymerization over the temperature range ?30 to 70°C were carried out. The observations were mainly confined to the early stages of polymerization. For polymerization temperatures below about 20°C, the systems remain predominantly homogeneous during the entire polymerization and the polymer particles increase in size linearly with conversion. At higher temperatures the polymer particles rapidly settle and become cemented together. The findings are discussed in the light of the kinetic data on vinyl chloride polymerization, and a process of particle formation and growth, resembling that recently proposed by Fitch for emulsion systems, was formulated. Primary particles are initially formed by the coiling up of single macromolecules or single macroradicals and, subsequently, they increase in size by sweeping up growing free radicals from the liquid monomer phase. The free radicals which escape capture give rise to new primary particles, but their number progressively decreases as the number of the dispersed particles increases. Simultaneously, the polymer particles undergo flocculation which in a short time results in the formation of large agglomerates. As the volume of the resulting agglomerates increases, the flocculation rate decreases and, eventually, becomes so low that the flocculation does not proceed further. At low temperatures the flocculation almost ceases when the agglomerates are still small enough for sedimentation to occur only very slowly. However, this is not the case at higher temperatures. The addition of substances such as alcohols, brings about a reduction in the flocculation rate and, hence, in the size of the agglomerates formed at the end of the flocculation process. In this way, one can also obtain at high temperatures agglomerates of small sizes which remain dispersed for a long time.     

Emulsion polymerization of vinyl acetate. II

The emulsion polymerization of vinyl acetate was investigated at low ionic strengths and has quite unusual kinetics. The rate of polymerization is dependent on the initiator concentration to the first power and independent of soap concentration. In seeded polymerizations, the rate of polymerization depends on initiator to the 0.8 power, particle concentration to the 0.2 power, and monomer volume to 0.35 power. In all cases the rate of polymerization is almost independent of monomer concentration in the particles until 85–90% conversion. These results were rationalized by the following mechanism: (a) polymerization initiates in the aqueous phase because of the solubility of the monomer and is stabilized there by adsorption of ionic soap on the growing polymer molecule; (b) the growing polymer is swept up by a particle at a degree of polymerization (under our conditions) of about 50–200. Growth continues in the particle. This sweep-up is activation-controlled as both particle and polymer are charged. (c) Chain transfer to the acetyl group of monomer gives a new small radical which cyclizes to the water-soluble butyrolactonyl radical, and reinitiates polymerization in the aqueous phase; (d) the main termination step is reaction of an uncharged butyrolactonyl radical with a growing aqueous polymer radical. A secondary reaction at low ionic strength is sweep-up of an aqueous radical by a particle containing a radical. At high ionic strength, this is the major termination step. The unusual kinetic steps are justified by data from the literature. They are combined with the usual mechanisms operating for vinyl acetate polymerization and kinetic equations are derived and integrated. The integral equations were compared with the experimental data and shown to match it almost completely over the whole range of experimental variables.     

Living cationic polymerization of vinyl ethers with a urethane group

To study the possibility of living cationic polymerization of vinyl ethers with a urethane group, 4‐vinyloxybutyl n‐butylcarbamate ( 1 ) and 4‐vinyloxybutyl phenylcarbamate ( 2 ) were polymerized with the hydrogen chloride/zinc chloride initiating system in methylene chloride solvent at ?30 °C ([monomer]₀ = 0.30 M, [HCl]₀/[ZnCl₂]₀ = 5.0/2.0 mM). The polymerization of 1 was very slow and gave only low‐molecular‐weight polymers with a number‐average molecular weight (M_n) of about 2000 even at 100% monomer conversion. The structural analysis of the products showed occurrence of chain‐transfer reactions because of the urethane group of monomer 1 . In contrast, the polymerization of vinyl ether 2 proceeded much faster than 1 and led to high‐molecular‐weight polymers with narrow molecular weight distributions (MWDs ≤ ～1.2) in quantitative yield. The M_n's of the product polymers increased in direct proportion to monomer conversion and continued to increase linearly after sequential addition of a fresh monomer feed to the almost completely polymerized reaction mixture, whereas the MWDs of the polymers remained narrow. These results indicated the formation of living polymer from vinyl ether 2 . The difference of living nature between monomers 1 and 2 was attributable to the difference of the electron‐withdrawing power of the carbamate substituents, namely, n‐butyl for 1 versus phenyl for 2 , of the monomers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2960–2972, 2004     

Chain transfer to polymer in emulsion polymerization

Chain transfer to polymer in emulsion polymerizations of acrylate monomers and vinyl acetate has been studied using ¹³C NMR spectroscopy to elucidate the chemistry by which chain transfer occurs and to quantify the mol% branches resulting from the reaction. In emulsion polymerizations of n-butyl acrylate, ethyl acrylate and methyl acrylate, chain transfer to polymer proceeds via abstraction of hydrogen atoms from backbone tertiary C-H bonds and typically gives rise to 2-4 mol% branches in the polymers obtained at complete conversion, the level of branching increasing with reaction temperature. For these acrylates, there is no evidence for a significant difference between the extent of chain transfer to polymer. In emulsion polymerizations of vinyl acetate, chain transfer to polymer proceeds mainly via H-abstraction from methyl side-groups, though there is a small contribution from abstraction at backbone tertiary C-H bonds. The levels of branching that result are substantially lower than in acrylate emulsion polymerizations, typically being in the range 0.6-0.8 mol% in the polymers obtained at complete conversion. The level of branching increases with temperature and as the degree of monomer starving (and hence instantaneous conversion) increases. Emulsion copolymerization of vinyl acetate with a small amount (5-20 wt%) of n-butyl acrylate gives rise to a significant increase in the level of branching (to values around 1.3-1.6 mol%), which results predominantly from H-abstraction of backbone tertiary C-H bonds in n-butyl acrylate repeat units by propagating radicals with vinyl acetate end units.     

Living cationic polymerization of isobutyl vinyl ether by the carboxyl group of the carbon black/zinc chloride initiating system

The effect of zinc chloride (ZnCl₂) on the cationic polymerization of isobutyl vinyl ether (IBVE) initiated by carboxyl groups on a carbon black surface was investigated. Although the polymerization of IBVE was initiated by carboxyl groups on the surface, the rate of polymerization was small and the molecular weight distribution (MWD) of poly IBVE was very broad. The rate of the polymerization was found to be drastically increased, and 100% monomer conversion was achieved in a short time by the addition of ZnCl₂. The number-average molecular weights (M_n) of the polyIBVE were directly proportional to monomer conversion in the polymerization initiated by the carbon black/ZnCl₂ system. By addition of the monomer at the end of the first-stage polymerization, the added monomer was smoothly polymerized at the same rate as in the first stage. The M_n of the polymer was in excellent agreement with the calculated value, assuming the polyIBVE chain forms per unit carboxyl group on the surface and MWD was narrow (M_w/M_n = 1.2 ～ 1.3). Based on the results, it is concluded that carbon black/ZnCl₂ system has an ability to initiate the living cationic polymerization of IBVE. Furthermore, it was found that polyIBVE was grafted onto the carbon black surface after the quenching of the living polymer with methanol. © 1995 John Wiley & Sons, Inc.     

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.     

ESR studies of vinyl polymerization. II. Initial reactions of vinyl esters with redox systems in aqueous media

ESR measurements of transient radicals during redox polymerization of various vinyl esters in aqueous solutions have been made by using the rapid-mixing flow method. The initiation was by means of hydroxyl and amino radicals from the systems titanous chloride-hydrogen peroxide and titanous chloride-hydroxylamine, respectively. The well resolved hyperfine structures obtained at monomer concentrations of about 0.05 mole/1. are unambiguously assigned to the monomer radicals formed by addition of initiator radicals to monomers. At higher monomer concentrations, additional weak signals attributed to the growing polymer radicals were observed. The effect of reaction conditions on the signal intensity has been studied in particular for vinyl acetate. The coupling constants of monomer radicals from various vinyl esters (acetate, propionate, butyrate, crotonate, and isopropenyl acetate) were obtained and the spin densities calculated. From the ESR spectra, the monomer radicals have a conformation with the substituent R (R = HO or NH₂) of R? CH₂? CH(OCOR′) locked in a position above or below the radical plane. This is tentatively interpreted as due to formation of intramolecular hydrogen bonds to ring structures or complexes with titanium ions. In addition, hydrogen abstraction reactions of some model compounds for poly(vinyl acetate) have been briefly studied in relation to chain transfer and grafting reactions.