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.     

Emulsion polymerization of vinyl stearate

The polymerization of vinyl stearate in aqueous emulsions with a non-ionic emulsifying agent and potassium peroxydisulfate as initiator has been investigated by use of a dilatometric method to follow the reaction. In general, the reaction kinetics do not follow the pattern established for styrene. Variation of initiator concentration produced latices containing approximately equal numbers of latex particles, even though the rate of reaction was almost directly proportional to the peroxydisulfate concentration. For a given initiator and monomer concentration polymerization occurs very slowly when the monomer is completely solubilized but as the number of micelles is reduced and the number of emulsion droplets increased, the rate increases to an optimum value, whereafter it decreases. A mechanism is proposed by which the sparsely soluble vinyl stearate reacts and redistributes itself into latex particles of a different size range from the micelles and emulsion droplets originally present.     

Emulsion polymerization of vinyl acetate using iodine‐transfer and RAFT radical polymerizations

This study deals with control of the molecular weight and molecular weight distribution of poly(vinyl acetate) by iodine‐transfer radical polymerization and reversible addition‐fragmentation transfer (RAFT) emulsion polymerizations as the first example. Emulsion polymerization using ethyl iodoacetate as the chain transfer agent more closely approximated the theoretical molecular weights than did the free radical polymerization. Although ¹H NMR spectra indicated that the peaks of α‐ and ω‐terminal groups were observed, the molecular weight distributions show a relatively broad range (M_w/M_n = 2.2–4.0). On the other hand, RAFT polymerizations revealed that the dithiocarbamate 7 is an excellent candidate to control the polymer molecular weight (M_n = 9.1 × 10³, M_w/M_n = 1.48), more so than xanthate 1 (M_n = 10.0 × 10³, M_w/M_n = 1.89) under same condition, with accompanied stable emulsions produced. In the M_n versus conversion plot, M_n increased linearly as a function of conversion. We also performed seed‐emulsion polymerization using poly(nonamethylene L ‐tartrate) as the chiral polyester seed to fabricate emulsions with core‐shell structures. The control of polymer molecular weight and emulsion stability, as well as stereoregularity, is also discussed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Emulsion polymerization of vinyl acetate initiated by potassium persulfate-cyclohexanone sodium bisulfite redox pair system

The emulsion polymerization of vinyl acetate using potassium persulfate and cyclohexanone sodium bisulfite as redox pair initiation system was studied. The rate of polymerization, maximum conversion, and the number of polymer particles produced were found to change with redox initiator, monomer and emulsifier concentrations, and temperature variation. The rate of polymerization was found to be dependent on the initiator, the monomer, and the emulsifier concentrations to the 0.88, 0.22, and 0.20 powers, respectively. The K₂S₂O₈–NaHSO₃ redox system was found to decrease maximum conversion and doesn't form a stable emulsion. The apparent arrhenius activation energy (E^a) estimated for the polymerization system was 65.6 kJ/mol. The viscosity average molecular weights for some obtained poly(vinyl acetate) were determined.     

Emulsion polymerization of vinyl acetate using a polymerizable surfactant. III. Mathematical model

A mathematical model was developed to aid in the further understanding of the growth of latex particles in the emulsion polymerization of vinyl acctate using a polymerizable surfactant, sodium dodecyl allyl sulfosuccinate (TREM LF-40). The model incorporates the main features of the system observed experimentally: copolymerization in the aqueous phase, at the particle surface, and chain transfer to TREM LF-40. The reactions at the particle/water interface and, more specifically, the chain transfer to TREM LF-40 leading to a decrease in the average number of radicals per particle, was found to be the most significant mechanism for explaining the difference in kinetic results found for TREM LF-40 and its nonpolymerizable counterpart. The copolymerization of vinyl acetate with TREM LF-40 was also shown to slow the overall polymerization rate. However, the copolymerization alone was not sufficient to account for the decreased polymerization rates observed experimentally. A combination of copolymerization and chain transfer to TREM LF-40 was found to provide a good fit of the experimental results. © 1993 John Wiley & Sons, Inc.     

Emulsion polymerization of vinyl acetate using a polymerizable surfactant. I. Kinetic studies

A polymerizable surfactant, sodium dodecyl allyl sulfosuccinate (TREM LF-40; Henkel) and its nonpolymerizable counterpart were used in comparative studies of the emulsion polymerization of vinyl acetate. The conversion-time behavior differed for the two surfactants; the TREM LF-40 showed a decrease in the polymerization rate with increasing concentration while its hydrogenated derivative showed the opposite behavior, the rate increasing with increasing surfactant. Particle size analysis revealed a decreasing particle size with increasing surfactant concentration for both series of reactions. An explanation for the seemingly ambiguous results obtained for the polymerizable surfactant was sought by examining the reactivity of its vinyl group in copolymerization with vinyl acetate and its allylic group in a chain transfer reaction. The results suggest that both the copolymerization and chain transfer reactions can lead to the observed reduction in polymerization rate with increasing TREM LF-40 concentration. © 1992 John Wiley & Sons, Inc.     

Photoinitiated RAFT polymerization of vinyl acetate

A photoinitiation process was investigated to develop a rapid and well‐controlled RAFT polymerization method applied to vinyl acetate (VAc) using methyl (ethoxycarbonothioyl)sulfanyl acetate (MESA) and bis(2,4,6‐trimethylbenzoyl)phenylphosphine oxide as the RAFT agent and photoinitiator, respectively. MESA was selected as the photochemically inert RAFT agent to minimize photolysis of the thiocarbonylthio groups during polymerization. Poly(vinyl acetate) with a prespecified well‐controlled molecular weight (MW) and a narrow MW distribution was successfully synthesized. The polymerization reaction proceeded as a living polymerization and was remarkably rapid compared with approaches that use thermally initiated processes with a very short induction period. A detailed kinetic study of the mechanism underlying the polymerization reaction, however, revealed that the chain ends containing xanthate moieties were not perfectly stable upon UV‐irradiation, and they generated radicals via homolytic cleavage. This reaction appeared to proceed by a combination of a degenerative transfer RAFT mechanism and a dissociation‐combination mechanism. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Emulsion polymerization and copolymerization of vinyl benzoate

Emulsion polymerization of vinyl benzoate and its copolymerization with vinyl acetate or styrene are described. The effect of the potassium persulfate initiator, and the sodium lauryl sulfate emulsifier concentration on the rate of vinyl benzote homopolymerization and the molecular weight of the polymers was determined. In copolymerization with vinyl benzoate, both comonomers, vinyl acetate and styrene, decrease the initial polymerization rate. With increasing amounts of styrene in the comonomer mixture the polymerization rate increases but with vinyl acetate an opposite effect is observed. Reactivity ratios of copolymerizations were determined. For the vinyl benzoate [M₁]-styrene [M₂] comonomer system a r₁ = 0.03 and a r₂ = 29.58 and for vinyl benzoate [M₁]-vinyl acetate [M₂], a r₁ = 1.93 and a r₂ = 0.20 was obtained. From the vinyl benzoate-styrene reactivity ratios the Qe parameters were calculated.     

Nitroxide-mediated photo-living radical polymerization of vinyl acetate

The photoradical polymerization of vinyl acetate was performed using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate (BAI). The MTEMPO/BAI system using 2,2’-azobis(isobutyronitrile) or 2,2’-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator did not succeed in controlling the molecular weight and produced polymers that showed a bimodal gel permeation chromatography with the broad molecular weight distribution. On the other hand, the polymerization using 1-(cyano-1-methylethoxy)-4-methoxy-2,2,6,6-tetramethylpiperidine and BAI proceeded by the living mechanism based on linear increases in the first order time–conversion and conversion–molecular weight plots. The molecular weight distribution also increased with the increasing conversion due to cloudiness of the solution as the polymerization proceeded. It was found that the polymerization had a photolatency because the propagation stopped by interruption of the irradiation and was restarted by further irradiation.     

Seeded polymerization of vinyl acetate using monodisperse poly(vinyl acetate) latex particles

The seeded polymerizations of vinyl acetate, using monodisperse poly(vinyl acetate) latex particles prepared in the absence of emulsifiers with potassium persulfate, have been investigated at 70°C with potassium persulfate as an initiator. New small particles were formed in the system containing a small amount of seed particles, but were not observed in the system containing a large amount of seed particles. The size of the secondary particles increased, and their number decreased, with an increase in the seed particle number. The minimum diameter of PVAc particles, which are stabilized by the sulfate ion groups bound at the end of polymer chains during polymerization, was determined to be 0.12 μm diameter from the limiting total surface area of seed particles which prevented further secondary nucleation. The minimum diameter of the particles increased as the speed of the stirrer increased. The new small particle number calculated using this value agreed well with that formed in the seeded polymerization.     

Emulsion polymerization of vinyl chloride,potassium persulfate decomposition

Decomposition of potassium persulfate in sodium dodecyl sulfate solutions (of concentrations under and above CMC) and in the presence of poly(vinyl chloride) and vinyl chloride was studied. The decomposition rate has a maximum close to the CMC value of the final solutions. On the basis of experimental data a hemolytic mechanism in which a radical derived from dodecyl sulfate participates is proposed.     

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.     

Kinetic analysis of seeded emulsion polymerization of vinyl acetate

Particle number and size data from a series of seeded, emulsifier-free, vinyl acetate emulsion polymerization experiments have been analyzed with the aid of polymerization and particle growth models. A secondary population of particles, with a significantly greater number concentration than the seed, was nucleated in all experiments. The two populations (seed and new) had rather narrow size distributions and large diameters. Hence the reactions were in the area normally associated with Smith–Ewart Case III kinetics. Water-phase termination reactions can be important in this reaction region but radical desorption from such large particles does not significantly influence the kinetics. The results of the analysis were used to evaluate the magnitude of water-phase termination; to estimate radical capture coefficients; and to evaluate competitive particle growth.     

Effect of fullerene on radical polymerization of vinyl acetate

The effect of fullerene (C₆₀) on the radical polymerization of vinyl acetate (VAc) with dimethyl 2,2′‐azobisisobutyrate (MAIB) in benzene was investigated kinetically and by means of ESR. C₆₀ was found to act as an effective inhibitor in the present polymerization. All C₆₀ molecules used were incorporated into poly(VAc) during polymerization. The relationship of induction period and initiation rate reveals that a C₆₀ molecule can trap 15 radicals formed in the polymerization system. The polymerization rate (R_p) after the induction period is given by R_p = k [MAIB]^0.6 [VAc]^2.0 (60 °C), which is similar to that observed in the absence of C₆₀. Stable fullerene radical (C) was observed in the polymerization system by ESR. The C concentration increased with time and was then saturated. The saturation time well corresponds to the induction period observed in the polymerization. About 20% of C₆₀ molecules added could survive as stable C. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2572–2578, 2000     

Radical polymerization of vinyl acetate with primary radical termination

Vinyl acetate was polymerized at high initiation rate with 2,2′-azobis(2,4-dimethyl valeronitrile) as initiator at 50°C. In this polymerization, the power dependence of polymerization rate on the initiation rate is smaller than at lower concentration of monomer. This dependence was kinetically analyzed at each given concentration of monomer. Average degree of polymerization of polymer formed depends on the concentration of initiator. This dependence was explained by considering chain and primary radical terminations and transfer to monomer of polymer radical, and the initiator efficiency (=0.503) was deduced. It was found that the chain termination is inversely proportional to solvent viscosity, but the primary radical termination is not inversely proportional to solvent viscosity. Further, the value of the primary radical termination rate constant (=1.4 × 10⁹l./mole-sec) was estimated.     

Radical polymerization of vinyl acetate in individual and mixed solvents

The kinetics of polymerization of vinyl acetate in individual and mixed solvents was studied. The reaction rate constant and the rate of chain transfer in the mixed solvent were calculated, molecular weights and some adhesive characteristics of poly(vinyl acetate) obtained by the radical polymerization in solution were determined. A comparative analysis of the polymers obtained in the individual and mixed solvents was performed. It is shown that the change in the solvent composition can affect the rate of reaction and the poly-(vinyl acetate) adhesive properties.     

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.