Free-radical Crosslinking Copolymerization Research Articles

Critical Properties in Free-radical Crosslinking Polymerization

Molecular weight developments in free-radical crosslinking copolymerization of vinyl and divinyl monomers were studied. A kinetic model developed recently was used for this purpose. The model predictions were compared to the experimental data reported previously. Agreement of the kinetic theory with experiments is satisfactory. The inappropriateness of the classical exponent \gamma is pointed out. This is due to the conversion dependent kinetics of free-radical polymerization reactions.

Free-radical crosslinking copolymerization: Effect of cyclization on diffusion-controlled termination at low conversion

The pregelation period of free-radical methyl methacrylate/ethylene glycol dimethacrylate (MMA/EGDM) and acrylamide/ N,N′ -methylenebisacrylamide (AAm/BAAm) copolymerization systems was studied by means of the dilatometric technique. The amount of cyclization in MMA/EGDM copolymers was estimated using proton nuclear magnetic resonance and analytical titration methods. The results suggest that, in MMA/EGDM copolymerization and at a monomer concentration of 220 gl −1 , nearly half of the pendent double bonds are consumed by cyclization. It was found that the average termination rate constant of macroradicals at zero conversion, k t 0 is dependent on the crosslinker concentration. k t 0 increases first but then decreases continuously as the amount of crosslinker increases. Moreover, termination rate constants become less sensitive to conversion in the presence of small amounts of crosslinker. All of these phenomena can be explained in terms of cyclization, as it decreases the size of macroradical coils by producing intramolecular links.

Fluorescence technique for studying the sol-gel transition in the free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate

The steady-state fluorescence technique was used to study the free-radical crosslinking copolymerization of methyl methacry- late and ethylene glycol dimethacrylate in the absence and presence of toluene at 75°C. A sol-gel phase transition was observed and monitored by an excited aromatic molecule during the gelation of the above system. Bond percolation theory was employed to quantify the results. The critical exponent and gel point were found to be around 0.45 and 0.36 both in bulk and solution polymerization.

Kinetics of gelation in free radical crosslinking copolymerization

A general equation is derived for the critical conversion at the gel point in free radical crosslinking polymerizations by omitting the equal reactivity assumption from the Stockmayer criterion of gelation. Calculations show that the monomer depletion, the volume contraction during polymerization and the initiator depletion have to be taken into account when the pre-gelation period is sufficiently large. The nucleation and growth of the gel molecule are described using kinetic differential rate equations. If the assumptions of equal vinyl group reactivity and random crosslinking are introduced, the resulting equations are equal to those of the statistical theories.

Gel properties in free radical crosslinking copolymerization: A kinetic approach

AbstractA kinetic model is presented for prediction of the gel properties in free‐radical monovinyldivinyl monomer copolymerization. It is an extension of the kinetic model proposed recently for the sol species. The model involves the moment equations of the primary chains in the gel, and predicts the gel growth and structure as a function of the reaction time. Formulae for the chain length averages of the primary chains in the sol and in the gel were derived. The equations for calculating the number‐ and weight‐average primary chain lengths are identical to the equations derived by statistical methods.

Computer simulation of network formation in free-radical crosslinking copolymerization

Abstract A new Monte Carlo simulation model for the kinetics of network formation in free-radical copolymerization of vinyl and divinyl monomers is used to investigate various important property changes during polymerization, such as the whole molecular weight distribution profile, the distribution of crosslink points among various polymer molecules, weight fraction of gel and sol as well as the spatial distribution of the crosslinked polymer chains. This simulation model does not assume a finite reaction system as an approximation for the infinite system, but employs a sampling technique in which polymers are sampled from an infinite number of polymer molecules in the reaction system. The structure of each crosslinked polymer is simulated based on the crosslinking density distribution of the primary polymer molecules that results from a kinetically controlled network formation. The proposed simulation method is quite general and can be applied to various complex polymerization reactions that involve crosslinking and branching in a kinetically controlled system irrespective of reactor types used.

Kinetic modelling of network formation and properties in free-radical crosslinking copolymerization

A kinetic model is presented for the post-gelation period of free-radical monovinyl-divinyl monomer copolymerization reactions. The model involves the moment equations of both the primary and the branched molecules in the sol, and predicts the vinyl-group conversions, the number of crosslinks and the chain-length averages as a function of the reaction time. Formulae for the weight fraction and cycle-rank density of the gel and its equilibrium degree of swelling are derived. The predictions are found to be in good agreement with experimental data on the copolymerization of styrene (S) with m-divinylbenzene (DVB) obtained by Hild and Okasha. The reaction time for incipient phase separation during S-DVB copolymerization in the presence of a solvent and the threshold concentration of the DVB for the formation of heterogeneous structures are also calculated.

Free-radical crosslinking copolymerization of styrene/unsaturated polyester resins: 1. Phase separation and microgel formation

An important feature of free-radical crosslinking copolymerization of styrene (ST)/unsaturated polyester (UPE) resins is the formation of a heterogeneous structure through strong intramolecular reactions and phase separation. Such structure formation affects not only the cure behaviour and rheological changes of the resin but also the physical properties of final products. In this study, the phase separation process was investigated by optical microscopy and time-resolved light scattering goniometry. Partially reacted samples of ST/UPE resin before macrogelation were also prepared by stopping the polymerization at preset times. The size and composition of primary polymers were determined using dynamic light scattering goniometry and Fourier transform infra-red spectroscopy.

Free-radical crosslinking copolymerization of styrene/unsaturated polyester resins: 3. Kinetics-gelation mechanism

Based on the results of integrated analysis given in parts 1 and 2, a kinetics-gelation mechanism of styrene (ST)/unsaturated polyester (UPE) resins is proposed. The curing process can be divided into five stages: induction, microgel formation, transition with or without phase separation, macrogelation and post-gelation. A summary of the experimental results at different stages of ST/UPE resins with ST/UPE molar ratios of 2 and 4 at 40°C is given. The kinetics-gelation mechanism based on the concept of microstructure formation is then discussed.

Free-radical crosslinking copolymerization of styrene/unsaturated polyester resins: 2. Electron spin resonance study

Electron spin resonance (e.s.r.) spectroscopy provided useful information regarding the nature of propagating radicals and the radical concentration during the free-radical copolymerization of styrene (ST)/unsaturated polyester (UPE) resins. The hyperfine structure of the spectrum showed that there were three types of radicals in the ST/UPE copolymerization at low temperatures. Changes of these radical species were determined by the e.s.r. spectrum simulation method. The behaviour of trapped radicals formed during polymerization was also examined by temperature scanning.

Kinetics of network formation in free-radical crosslinking copolymerization

A new kinetic theory for network formation in free-radical copolymerization of vinyl and divinyl monomers is used to investigate various important property changes in the postgelation period, such as the weight fraction of gel, the molecular weight distribution, the cross-linking density within sol and gel fractions, the partition of the unreacted pendant double bonds between sol and gel, etc. This theory is based on the cross-linking density distribution among primary polymer molecules, and the history of the generated network structure is fully accounted for. It is shown clearly that the deviation from the Mory-Stockmayer theory becomes significant as the variance of the cross-linking density distribution increases

Molecular weight distribution in free-radical crosslinking copolymerization

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMolecular weight distribution in free-radical crosslinking copolymerizationHidetaka TobitaCite this: Macromolecules 1993, 26, 4, 836–841Publication Date (Print):February 1, 1993Publication History Published online1 May 2002Published inissue 1 February 1993https://doi.org/10.1021/ma00056a039RIGHTS & PERMISSIONSArticle Views512Altmetric-Citations75LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Special Features of Network Formation by Chain Crosslinking Copolymerization

Special features of free-radical crosslinking copolymerization and the structure of the resulting products have been reviewed. Characteristic is the effect of spatial correlations on the apparent reactivity of pendant double bonds. These correlation make the apparent reactivity in the course of copolymerization to increase (cyclization) and decrease (steric hindrances). At intermediate and higher concentrations of the crosslinker, compact structures are formed which are internally crosslinked. Only pendant double bonds in the peripheral layer are able to take part in polymerization reactions whereas the internal ones cannot react. The state of theoretical simulations of this structure growth is discussed with a special emphasis on the development of the kinetic (coagulation) network formation theories so that the above mentioned features may be taken into account. Also, the important role of the presence of a diluent during polymerization id discussed. It can result in a change of network chain conformations necessary for networks exhibiting volume phase transitions. Alternatively, it can induce liquid-gel phase separations resulting in inhomogeneous networks having a variety of morphologies.

Control of network structure in free-radical crosslinking copolymerization

A kinetic model which describes the network structure development during free-radical crosslinking copolymerization is proposed. The model was successfully applied to various batch copolymerization systems such as methyl methacrylate/methylene glycol dimethacrylate, styrene p- divinylbenzene , styrene/ethylene glycol dimethacrylate, and acrylamide N,N′- methylene-bis-acrylamide . The model calculations suggest that polymer networks synthesized by free-radical copolymerization are, in general, inhomogeneous at least on a microscopic scale. This model can be used to control the network structure, and a semi-batch policy to produce homogeneous polymer networks is proposed.

Free radical crosslinking copolymerization in the post‐gel state, 3. Swelling and mechanical properties of polystyrene networks

AbstractThe free radical crosslinking copolymerization of styrene with a bi‐unsaturated comonomer (divinylbenzene, ethylene dimethacrylate), present in small proportions, yields three‐dimensional polymer networks. The gelation time tc is generally longer than expected if all double bonds originating from the bi‐unsaturated monomer (including the pendent double bonds) were isoreactive. This delay in gelation has been attributed to the formation of intramolecular cyclizations, or to the lower reactivity of the pendent double bonds with respect to growing radicals. This paper, devoted to an investigation of the network forming reaction after gelation, presents some additional evidences in favor of the latter assumption, resulting from the variation of the fraction of extractable polymer, of the swelling degree and of the modulus of the swollen networks as a function of the overall degree of conversion. Furthermore, a direct proof of the role of the pendent double bonds is given by the “two‐step” crosslinking experiments carried out.

Kinetic investigations of the free radical crosslinking copolymerization in the pre‐gel state, 2. Styrene/ethylene dimethacrylate and styrene/diisopropenylbenzene systems

AbstractA kinetic investigation of the free radical crosslinking copolymerization was carried out using the following monomer systems: styrene/ethylene dimethacrylate, styrene/m‐diisopropenyl‐benzene and styrene/p‐diisopropenylbenzene. The conversion curves of each individual monomer and the instantaneous composition of the copolymer formed were determined from vapor phase chromatography measurements. The rate of consumption of each monomer follows first order kinetics except for the system styrene/p‐diisopropenylbenzene in which strong deviations can be observed. The reactivity ratio of styrene, determined by VPC, is compared with the value obtained by the Kelen‐Tüdős linearization method. As for the system styrene/divinylbenzene, the fundamental role of the pendant bonds in the process has been established. Their ability to react with incoming radicals to form crosslinks determines the formation of interchain linkages and thus the gelation of the system.

Kinetic investigation of the free radical crosslinking copolymerization in the pre‐gel state, 1. Styrene/m‐ and p‐divinylbenzene systems

AbstractThe kinetic investigation of the free radical crosslinking copolymerization of styrene with m‐or p‐divinylbenzene was performed at 60°C in benzene solution with 2,&2prime;‐azoisobutyronitrile as initiator. The mole ratio of divinylbenzene to styrene was steadily kept small. At various stages of the process the concentrations of the remaining monomers were measured by vapor phase chromatography which allowed to calculate both the conversion degrees and the instantaneous composition of the copolymers formed. The radical reactivity ratio of styrene, determined kinetically, was found to be in good agreement with the value obtained by the Kelen‐Tüdös linearization method. The branched polymers formed prior to gelation were studied by light scattering and by size exclusion chromatography. The number of pendant double bonds was determined by analytical titration. From all these results, conclusions were drawn concerning the mechanism of network formation, and more specifically on the reactivity of the pendant double bonds that are formed whenever a divinylbenzene unit is incorporated. This latter factor determines the rate of the formation of the branch points and crosslinks.