Studies in radiation-induced polymerization of vinyl monomers at high dose rates. I. Styrene

The kinetics and degree of polymerization of the radiation-induced polymerization of styrene were investigated at dose rates up to 3 Mrads per sec. It was found that both the polymerization rates and the molecular weights at high dose rates lie well above the values predicted by extrapolation from low dose rate studies where the mechanism is primarily free radical in nature. The evidence obtained indicates that with increasing dose rate there is an increase in the ionic contribution to the polymerization mechanism. At the highest dose rates investigated, 3 Mrads per sec, the ionic mechanism dominates while the free-radical mechanism contributes insignificantly. There was no inhibition of the polymerization of styrene by dissolved air at the higher dose rates. Investigation of the polymerization of styrene saturated with water tends to support the ionic mechanism via free cations. Investigation of equimolar solutions of styrene and methyl methacrylate further support the ionic mechanism, yielding predominately polystyrene and a small amount of 50–50 copolymer. In the intermediate dose rate range, 1000 rads per sec, there is evidence that the high dose rate free-radical kinetic mechanism is the principal mechanism.     

Radiation-induced polymerization of styrene at high dose rates

A kinetic equation was derived for the radiation-induced polymerization of styrene under the assumption that both radical and cationic polymerizations take place concurrently throughout the whole range of the dose rate of radiation and the water content of the styrene. The equation enables one to calculate rates of the total, radical, and cationic polymerization at a given dose rate and water content and agrees satisfactorily with experimental results, which cover dose rates from 4.2 × 10 to 2.1 × 10⁵ rad/sec and water contents from 3.2 × 10^?3 to 3.5 × 10^?2 mol/l. Experimental estimation of the contribution of radical and cationic mechanisms was done by GPC curves of polymers obtained under various conditions. When the contribution of ionic mechanism is expressed in weight percent, it changes from 0% to 100% in the range of the experiment; on the other hand, if it is expressed in mole percent, it is independent of the dose rate and remains constant throughout the whole range of the experiment.     

Casting of organic glass by radiation-induced polymerization of glass-forming monomers at low temperatures. VI. Casting polymerization of methyl methacrylate prepolymer by irradiation

Casting of methyl methacrylate prepolymer by irradiation at relatively low temperatures was studied. It was difficult to completely glassify the methyl methacrylate monomer and prepolymer, and so the casting was carried out at temperatures above its melting point (?48°C). The optical strain formed in this stream remained of the stress type, and optical strain formation was reduced with increasing prepolymer concentration and decreasing temperature.     

Study of radiation-induced graft polymerization of vinyl monomers to cellulose by infrared spectroscopy. II. Cellulose–polystyrene copolymers

Infrared spectral analysis has been applied for the study of cotton-polystyrene copolymers prepared by using the preirradiation method. The degree of grafting was determined from the absorption at 700 cm^?1, which is characteristic of the polystyrene spectrum, with an accuracy of ± 1.5%.     

Radiation-induced polymerization of methyl methacrylate and alkyl acrylates at high dose rate

Polymerization of methyl methacrylate and methyl, ethyl, and n-butyl acrylates was carried out in a wide range of dose rate, 10–10⁶ rad/s by γ-ray and electron beam irradiation. With methyl methacrylate and n-butyl acrylate, and steady-state kinetics in radical polymerization was maintained in an entire dose rate range at about the initial stage of the polymerization. With methyl and ethyl acrylates, the rate of polymerization increased much less markedly than expected from the square root law and the molecular weight decreased much less gradually with dose rate. In all these monomers it was found that autoacceleration of the rate of polymerization due to gel effect becomes vague at high dose rate. Two-peaked molecular weight distribution was observed for the polymers obtained at high dose rate.     

Heterogeneous polymerization of some methacrylate monomers

It has been found that sodium metabisulphite failed to initiate the polymerization of methyl and butyl acrylates but succeeded in polymerizing methyl, ethyl, and hydroxy propyl methacrylates. All the samples of the poly(hydroxypropyl methacrylates) were crosslinked, even those prepared at low conversions. Gel permeation chromatographic analysis indicated a multinodal molecular weight distribution, suggesting the existence of more than one mechanism occurring in such a heterogeneous system. It has also been proved by electron spin resonance that the polymerization process, using sodium bisulphite as the initiator in such a system, is a radical mechanism. The use of substances such as quartz, glass powder, and kaoline powder enabled the detection of the bisulphite radicals by trapping them. Using ¹³C NMR spectroscopy, the estimated triad fractions from α-methyl peaks and from quaternary carbon peaks of the poly(methyl methacrylates) and poly(ethyl methacrylates) showed that the polymers obtained were all predominantly syndiotactic in structure, as expected. The poly(ethyl methacrylate) samples were found to have a slightly higher syndiotacticity than the poly(methyl methacrylates), also as expected. The greater bulk of the ethyl ester group relative to that of the methyl ester group probably leads to greater steric hindrance in the isotactic and heterotactic triad (i.e., in a mesodyad).     

Preparation of high purity,anionic polymerization grade alkyl methacrylate monomers

Summary A method has been developed which provides very pure alkyl methacrylate monomers for anionic polymerization. This method takes advantage of the chemistry of trialkylaluminum compounds-their reactivity with alcohols and moisture, and their complex formation with methacrylic esters which facilitates titration of impurities. When coupled with previously known polymerization techniques, this purification methodology allows for both the synthesis of narrow distribution poly(alkyl methacrylates) of controlled and predictable molecular weight and for the utilization of a wide variety of methacrylate monomers. The procedure described herein should be equally applicable to group transfer polymerization.     

The plasma polymerization of vinyl monomers. II. A detailed study of the plasma polymerization of styrene

A study has been made on the plasma polymerization of styrene monomer in a cold, low-power, inductively coupled RF plasma. Styrene monomer yielded an insoluble, crosslinked film which was slightly colored. A kinetic study is reported for styrene. The effects of power level, bleed rate of monomer, pressure, and reactor geometry on the rate of polymer formation are reported. A mechanism is postulated for plasma polymerization. It was found that the initiation step was the rate controlling step and that the reaction followed a cationic polymerization scheme. Both crosslinking and discoloration of the polymers occur at the time of polymerization and are not a result of exposure of the reacted polymer to the plasma. The polymerization was shown to take place in the bulk phase as well as on the reaction wall surfaces.     

Controlled release of biofunctional substances by radiation-induced polymerization: 1. Release of potassium chloride by polymerization of various vinyl monomers

The release behaviour of a drug from flat circular capsules obtained by radiation-induced polymerization at low temperatures and with different hydrophilic properties has been studied. The effect of various factors on release property was investigated. The release process could be divided into three parts, an initial quick release stage, stationary state release stage and a retarded release stage. Release behaviour in the stationary state was examined using Noyes-Whitney and Higuchi equations. It was shown that the hydrophilic property of polymer matrix expressed by water content was the most important effect on diffusion and release rate. Rigidity of the polymer may also affect diffusivity. The first quick release step could be attributed to rapid dissolution of drug in the matrix surface due to polymer swelling.     

Methyl methacrylate polymerization in nanoporous confinement

The effect of nanoconfinement on the rate of isothermal polymerization of methyl methacrylate (MMA) polymerization is investigated using differential scanning calorimetry. Controlled pore glass (CPG) with pore diameters of 13, 50, and 111 nm are used for the confinement of the reaction. Both hydrophilic and hydrophobic pore surfaces are studied. The effective reaction rate and the apparent activation energy at low conversions, prior to autoacceleration, are unchanged in hydrophobic pores. On the other hand, in hydrophilic pores, the reaction rate increases by as much as a factor of 8 in the smallest 13 nm hydrophilic pores, and the effective activation energy decreases. For both pore surfaces, the time required to reach autoacceleration decreases with decreasing pore size, with the effect much more pronounced in the hydrophilic pores. The results are consistent with a model of nanoconfined free radical polymerization which accounts for suppressed termination due to a decrease in the diffusivity of nanoconfined chains.     

Anionic polymerization of fluorine-containing vinyl monomers

Summary The anionic polymerization of pentafluorophenylmethyl methacrylate (PFPMA) were examined with several anionic initiators. The initiators of relatively low activity such as t-C₄H₉OK, organozinc compounds and Al(C₂H₅)₃ produced poly(PFPMA) in fairly high yields though no polymer was obtained by the initiations of n-C₄H₉Li and C₂H₅MgBr within 7 days. The anionic polymerization reactivity of PFPMA was found to be strongly affected by the pentafluorophenyl group though the e-value of PFPMA is similar to those of methyl methacrylate and benzyl methacrylate.Part 7, Polym. J. 19, 958 (1987)     

Casting of organic glass by radiation-induced polymerization of glass-forming monomers at low temperature. II. Optical strain of remaining stress type

Previously it was found that casting could be carried out efficiently without strain formation by radiation-induced polymerization of glass-forming monomers. Two types of strain were observed in casting: thermal stream type, which was studied previously, and remained stress type. In this report, the effect of various factors on the formation of remaining stress-type strain in radiation-induced casting polymerization was studied. It was found that the molecular weight of prepolymer did not affect strain formation, while prepolymer concentration and viscosity of the system had a serious influence on strain formation. It could be deduced that this type of strain formed as a result of remaining inner stress due to poor relaxation of the shrinking stress. It was realized that less volume shrinkage of glass-forming monomers accompanying casting polymerization reduced the strain formation of this type in radiation-induced casting polymerization at low temperatures.     

Molecular sieves as drying agents for the monomer in the radiation-induced ionic polymerization of liquid vinyl monomers

The factors which influence the drying of liquid vinyl monomers with molecular sieves are discussed. It is found that the best results are obtained by drying the monomer for at least 12 hr at ?15°C on molecular sieves that received no pretreatment at elevated temperatures. Although the prepolymerization that takes place on the molecular sieves during the drying can be quite extensive, the excellent reproducibility that can be achieved by using this method makes it a promising drying technique.     

Graft polymerization of some vinyl monomers onto acrylic rubber. II. Mechanical properties of graft polymers

Some mechanical properties of styrene and acrylonitrile copolymers grafted onto acrylic rubber are investigated. The impact strength of graft polymers depended upon the nature and the concentration of the catalyst, the composition and the intrinsic viscosity of the rubber, and the acrylonitrile content in the rigid matrix. The most desirable result was obtained when benzoyl peroxide as the catalyst, n-butyl acrylate–acrylonitrile copolymer of 7–10% acrylonitrile content, and about 0–5% acrylonitrile in the rigid matrix were used. Dynamic mechanical tests show the increase in efficiency of rubber modification by the grafted chains. The better weathering resistance of these graft polymers, as compared with commercial ABS plastics, was confirmed.     

Study of radiation-induced graft polymerization of vinyl monomers to cellulose by infrared spectroscopy. I. Cellulose–polyacrylonitrile copolymers

Infrared spectral analysis has been applied for the study of cotton–polyacrylonitrile copolymers prepared by using the preirradiation method. The degree of grafting was determined from the absorption at 2249 cm.^?1, which is characteristic of the polyacrylonitrile spectrum, with an accuracy of ± 1.5%.     

Studies on tributyl tin methacrylate II. Temperature dependence of the polymerization of tributyl tin methacrylate

In continuation of the earlier communication, we report here a detailed kinetic analysis of the polymerization of tributyl tin methacrylate initiated by azobisisobutyronitrile at the temperature range of 50–80°C. The initiator exponents are found to vary with temperature and viscosity of the medium reaching a low value of 0.40 at 50°C. This has been explained as due to primary radical termination which is very important in this system particularly at low temperatures. Energy of activation of viscous flow for this monomer is found to be approximately 4.2 kcal/mol which is substantially higher than for methyl methacrylate. The energy of activation for propagation has been determined applying the procedure of Fischer and Schulz and is 4.6 kcal/mol which is quite comparable to that of methyl methacrylate. The faster rate of polymerization of this monomer compared to other methacrylates has been attributed to lower rate of termination because of steric hindrance and viscosity effects. The energy of activation for initiation in this monomer seems to be quite high and comparable to values obtained for other vinyl monomers.     

The in situ polymerization of vinyl monomers in polyester yarns

The effects of a pretreatment of polyester (PET) yarns with a strongly interacting solvent such as dimethylformamide (DMF) on vinyl monomer incorporation were investigated. When the DMF pretreatment is carried out at high temperatures (above 120°C), the swollen PET structure is stabilized by solvent-induced secondary crystallization. This substrate is highly suitable for the incorporation of vinyl monomers. In situ polymerization of vinyl monomers in DMF-treated PET was investigated using chemical and γ-irradiation polymerization techniques, both in the presence and in the absence of excess monomer outside the PET fibers. When polymerization was carried out in a system in which a constant supply of free radicals was available from the outside of the PET fibers, lower initiator concentrations and smaller γ-irradiation doses were necessary. These results are attributed to a low efficiency of the initiator inside the PET fiber due to mobility restrictions. Water uptake and moisture regain of PET yarns containing poly(hydroxyethyl methacrylate) and poly(acrylic acid) were also investigated. When most of the vinyl polymer was inside the PET fiber, water absorption was limited. The changes in mechanical properties of the PET yarns resulting from the DMF pretreatment were partially reversed by in situ polymerization of vinyl monomers.     

Free radical polymerization of methyl methacrylate at high temperatures

Free-radical polymerization of methyl methacrylate in a tubular reactor has been conducted at above-Tg temperatures. A salient feature of these experiments is the very efficient control of reactor temperature by vapor-liquid equilibrium of the polymerizing mixture via monomer evaporation. The system pressure thus provides a powerful control variable, restricting the temperature in the entire reactor by changing the monomer evaporation rate. In the range of our experimental conditions, the temperature and pressure in the reactor follow the Antoine equation closely. High temperature runs also reduce the length requirement of the reactor. However, molecular weight averages of the products are not impressive, unless slow-burning initiators are used. Modeling of above-Tg reactions has been attempted at two-levels of sophistication. A plug-flow model gives predictions in good agreement with our experimental temperatures and conversion data. The predicted molecular weights are also consistent with the experimentally observed values. However, the more elaborate rheokinctic model suggests that the superficial agreement between model and experiment is due to initiator burn-out, which limits the final conversion to within 40 percent. The liquid layer next to the reactor wall can never be so viscous as to form a stagnant deposit, due to this conversion limitation. The velocity profiles are thus not very much distorted, and a plug-flow model is adequate. With a slow-burning initiator and a sufficiently long reactor, skewing of velocity profile and reactor channeling will eventually emerge. Hence, the rheokinetic model must be evoked to model the system under such conditions.     

Synthesis and polymerization of new multifunctional pyrrolidinone methacrylate monomers

Methacrylate monomers have been widely used in medical and dental applications such as bone cements, dental fillings, bioadhesives, and hydrogels. One major problem of these monomers resides in their low rates of polymerization leading to leaching problems that cause irritation of the surrounding tissues and even cell death. Here we describe the synthesis and polymerization of new mono-methacrylates containing multifunctional pyrrolidinone moieties. Such monomers possess stronger inherent hydrogen bonding potential susceptible to increase their reactivity and polymerization rates. These monomers were shown to homopolymerize rapidly leading to crosslinked polymers. The corresponding rates of polymerization were found to be comparable to difunctional methacrylates such as hexanediol dimethacrylate. Intermolecular hydrogen bonding interactions involving the pyrrolidinone unit leads these monomers to behave as “pseudo” difunctional monomers upon homopolymerization.