Gel formation in free-radical crosslinking copolymerization

Polymer gels with varying amounts of crosslinker and solvent were prepared by solution free-radical crosslinking copolymerization of methyl methacrylate/ethylene glycol dimethacrylate (MMA/EGDM) and styrene/p-divinyl benzene (S/p-DVB) comonomer systems. The structural characteristics of the gels were examined using equilibrium swelling in toluene and gel fraction measurements. Experimental results were compared with the predictions of a kinetic model developed recently for free-radical crosslinking copolymerizations. Experimental data on S/EGDM networks reported by Hild, Okasha, and Rempp were also used to test this model. It was found that the model correctly predicts the development of the gel properties in free-radical crosslinking copolymerization. © 1995 John Wiley & Sons, Inc.     

Determination of the mode of termination in free-radical copolymerization

In this study the apparent rate constant model was used to determine the mechanism of termination and other rate constants for free radical copolymerizations that are chemically controlled with no penultimate effect. The data required were the number average molecular weight and the conversion obtained at various initial initiator concentrations and monomer compositions, and two rate constants of propagation of the homopolymerizations of its two monomers. The bulk free radical copolymerization of methyl methacrylate (MMA) and styrene(St) at 60°C was studied. Three published literature values of the propagation rate constants k₁₁ and k₂₂. 750, 150; 515, 145; 705, 145, were used for the determination. The unit of the rate constants used was liter/mole/s. The modes of the termination reaction obtained, depending on k₁₁, k₂₂ used, are given as follows:

Suppression of inhomogeneities in hydrogels formed by free-radical crosslinking copolymerization

Network microstructures of poly(acrylamide) (PAAm) and poly(N,N-dimethylacrylamide) (PDMA) hydrogels were compared by static light scattering and elasticity measurements. The hydrogels were prepared by free-radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N-dimethylacrylamide (DMA) with N,N′-methylenebis(acrylamide) as a crosslinker. During the formation of PAAm gels, the reaction time dependence of the scattered light intensity exhibits a maximum at a critical reaction time, while in case of PDMA gels, both a maximum and a minimum were observed, corresponding to the chain overlap threshold and the gel point, respectively. This difference in the time-course between the two gelling systems is due to the late onset of gelation in the DMA system with respect to the critical overlap concentration. Compared to the AAm system, no significant scattered light intensity rise was observed during the crosslinking polymerization of DMA. It was shown that, regardless of the crosslinker ratio and of the initial monomer concentration, PDMA gel is much more homogeneous than the corresponding PAAm gel due to the shift of the gelation threshold to the semidilute regime of the reaction system. The results suggest that the spatial gel inhomogeneity can be controlled by varying the gel point with respect to the critical overlap concentration during the preparation of gels by free-radical mechanism.     

Semibatch operation and primary cyclization effects in homogeneous free-radical crosslinking copolymerizations

A theoretical analysis is presented on the use of semibatch reactors for producing non-linear copolymers through the homogeneous free-radical polymerization of mono- and divinyl-monomers. The kinetic scheme distinguishes two kinds of pendant double bonds and five kinds of polymer radicals, as it also takes into account the primary cyclization of terminal divinyl units. In spite of the fairly large number of groups and reactions, the generating function of vector chain length distribution of polymer species can still be accurately computed by a numerical implementation of the method of characteristics and thus average molecular weights before and after gelation can be reliably obtained. The influence of feed policies of the monomers, initiator and transfer agent on gelation, average chain lengths and weight fraction of sol is discussed, as well as their sensitivity to some kinetic parameters.     

Effects of termonomers on crosslinking rate and crosslinking structure of ethylene propylene terpolymers

Study was made concerning both the rate of crosslinking and the structure of crosslinks of terpolymers that have dicyclopentadiene, 1, 4–hexadiene, methylene norbornene, and ethylidene norbornene for termonomers so as to look into the role of termonomers during crosslinking of ethylene propylene terpolymers. In the study, activation energy and frequency factor, both apparent were determined by the rate constant of crosslinking and differences in crosslinks were analyzed following both chemical and physical methods. Infrared analysis into the volume of consumption of double bond was adopted to examine the manner of crosslinking. In respect to the magnitude of the rate constant, ethylidene norbornene came in first being followed by 1,4-hexadiene, methylene norbornene, and dicyclopentadiene. The structure of termoners affected the apparent activation energy and the apparent frequency factor was subject to the influences of said structure and others including the variation in the degree of unsaturation. During the early stage of crosslinking, the formation of the crosslink of polysulfide-type was paramount to all others and the speed of conversion therefrom either to disulfide–type or to monosulfide varied with the kind of terpolymers and was in proportion to the rate constant of crosslinking.     

Control of network structure in emulsion crosslinking copolymerization

A kinetic model for network structure development during crosslinking copolymerization of vinyl and divinyl monomer is proposed. The model calculations suggest that polymer networks synthesized by free-radical copolymerization are, in general, inhomogeneous at least on a microscopic scale. By application of the same kinetic parameters as those for bulk polymerizations, it was found that the crosslinking density of the polymers formed in the earlier stages of polymerization is very high in emulsion polymerizations and polymer networks tend to be highly heterogeneous. Homogeneous networks cannot be formed even under Flory's simplifying assumptions for vinyl/divinyl copolymerization in emulsion polymerizations. The present kinetic model can be used to find semi-batch policies to control the network structure, and semi-batch policies were used to illustrate the synthesis of homogeneous polymer networks.     

The effect of crosslinking agents on the copolymerization of styrene and n-butyl methacrylate

Summary The effect of low concentrations of multifunctional vinyl monomers acting as crosslinking agents, on the properties of poly(styrene-co-n-butyl methacrylates) with 30% wt of n-butyl methacrylate obtained by solution polymerization at a monomer conversion of approximately 60 wt % has been investigated. Ethylene glycol dimethacrylate, 1,4-butanediol dimethacrylate, 1,1,1-trimethylolpropane trimethacrylate and p-divinyl benzene were used as crosslinking agents. The relationship between the gel content, the relative viscosity increment, the weight-average molecular weight, non-uniformity, glass transition temperature, melting volume index and the concentration of the crosslinking agent was determined and discussed.     

Study of crosslinking copolymerization and its fractal behavior of styrene-divinylbenzene

The crosslinking copolymerization of styrene-divinylbenzene (DVB) with 6% (w/w) DVB was investigated quantitatively by liquid chromatography (LC), viscosity, GPC, and static and dynamic laser light. Through these experiments, reaction order, mechanism, and evolution of molecular weight, size, polydispersity, and branching of the copolymer chains during the gelation were measured. The critical exponents γ, β, δ, τ, and ν_p of the gelation were calculated. Furthermore, fractal behavior of the copolymer was evidenced firmly; the progression of fractal dimensions of these species following the gelation were also obtained using the dependence of intensity scattered on angles by synchrotron small angle X-ray scattering. A methodology for characterization of gelation forming network polymer is provided objectively. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:363–371, 1998     

Rheological modelling of the free-radical crosslinking of PDMS rubber in the presence of TEMPO nitroxide

The aim of the present work is to study the free-radical kinetics of PDMS rubber crosslinking in the presence of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) nitroxide. For this purpose a new method based on the relationship between the kinetics of the macro-radicals coupling [R_cc(t)] was derived from a fundamental kinetic model and the viscoelastic changes of the complex shear modulus (G′(t)_ω and G″(t)_ω). The kinetic model takes into account the initiator (Dicumyl peroxide in the present study) decomposition and the trapped PDMS macro-radicals in the presence of a radical scavenger such as TEMPO. Activation energy E_ac and collision frequency factor A_0c for the bimolecular termination reaction coefficient rate k_cc have been derived from the anisothermal DSC results according to the Kissinger method. Furthermore, it was observed that addition of TEMPO nitroxide can boost the initiator efficiency. The concentration variation of the active PDMS carbon-centred radicals [R_p(t)]_act and the [R_cc(t)] with reaction time were predicted using this kinetic model. On the other hand, the influence of TEMPO concentration in formulation ([N]₀) and effect of temperature on viscoelastic variations are studied. As a main result, the rheological modelling shows that this new method accurately predicts the time variation of complex shear modulus at any temperature and [TEMPO]/[DCP] ratio.     

Effect of crosslinking method on collagen fiber-fibroblast interactions

Collagen, the major structural protein of the extracellular matrix in animals, is a versatile biomaterial used in various tissue engineering applications. Cross-linking influences the mechanical properties, resorption kinetics, and biocompatibility of collagen-based biomaterials. In this study, we evaluated the effects of crosslinking on collagen fiber-fibroblast interactions in vitro. Collagen fibers were left untreated or crosslinked by ultraviolet (UV) irradiation, dehydrothermal (DHT) treatment (3 or 5 days), or hexamethylenediisocyanate (HMDIC) exposure. The initial attachment, proliferation (through 8 days), and morphology of human dermal fibroblasts were evaluated on control and crosslinked bundles of 200 collagen fibers in vitro. Initial attachment (number of fibroblasts at day 0) was increased on UV and DHT5-treated collagen fiber bundles. Fibroblast proliferation was similar for control, UV crosslinked, and DHT crosslinked fibers. In contrast, fibroblast attachment was significantly decreased and proliferation was delayed on HMDIC crosslinked fibers. These results, coupled with our previous studies, suggest that UV crosslinking of collagen fibers provides a combination of biocompatibility, mechanical properties, and strength retention suitable for various tissue engineering applications. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1493–1498, 1997     

Monomer chain cross-transfer constants and cross termination factor evaluations on styrene-acrylonitrile copolymerization

Using low conversion bulk copolymerization experimental data and the apparent rate constant model, the monomer chain cross-transfer rate constants and the cross-termination coefficient were evaluated. It was found that the dependence of the cross-termination coefficient on composition is a function of initiator efficiency and that monomer chain cross-transfer rate constant are relevant for the theoretical estimation of the molecular weights and molecular weight distributions.     

Pendant vinyl group reactivity during the free-radical copolymerization of methyl methacrylate and ethylene glycol dimethacrylate

Summary Methyl methacrylate-ethylene glycol dimethacrylate (MMA-EGDM) copolymerization has been investigated in toluene at a monomer concentration of 22 w/v %. The kinetic models developed recently along with the experimental conversion curve and gel point data provided the calculation of the average reactivity of pendant vinyl groups. It was found that the pendant vinyl reactivity for intermolecular links is much less than the monomeric vinyl reactivity and it decreases further as the EGDM concentration increases. At 5–15 mol % EGDM, the average pendant reactivity is 1–2 orders of magnitude lower than the monomeric vinyl reactivity. The reduced pendant reactivity is mainly responsible for the shift of the gel point towards higher conversions.     

Effect of solvent on the termination step for the radical copolymerization of methyl methacrylate and styrene

The kinetics of the radical copolymerization of methyl methacrylate and styrene in benzene, chlorobenzene and benzonitrile has been investigated gravimetrically. The copolymerization termination step is analysed on the basis of the chemically controlled model, the diffusion-controlled model and the combined physical and chemical model, taking into account the corresponding reactivity ratios and the homopolymerization parameters in the three solvents used. The values obtained for the termination step parameters suggest that the termination step might be controlled by the viscosity of the reaction medium, but the sequence distribution and the stereochemical configuration of the polymer chains must also be considered.     

On-line multi-variable predictive control of molar mass and particle size distributions in free-radical emulsion copolymerization

A multi-variable model predictive control (MPC) was formulated to solve control problems associated with a combination of regulation and targeting desired set-points. We investigated the simultaneous control of key polymer properties: the particle size (PSD) and molecular weight distribution (MWD) by manipulating the flow rates of the monomers (styrene, MMA), surfactant, initiator and the temperature of the reactor. A multi-input-multi-output (MIMO) formulation was constructed for the constrained optimal control problem to maximize the width of the PSD (with M_n at a constant set-point), and to maximize the average molar mass. The strategy developed within a gPROMS-API-DCS environment allowed real-time implementation of model-based control of the process. The optimal control problem was implemented via an interface to a dynamic optimization code. Major improvements in process operation and polymer property control resulted on the implementation of our multi-variable MPC algorithm. The manipulation of the four flow rates and the temperature increased the degree of freedom in the system and achieved tighter PSD and MWD control. The on-line performance of MPC for MWD and PSD control was found to be satisfactory.     

Size distribution of polymers during the photoinitiated free-radical copolymerization of methyl methacrylate and ethylene glycol dimethacrylate

Summary Photoinitiated methyl methacrylate-ethylene glycol dimethacrylate (MMA/EGDM) copolymerization has been investigated in toluene at a monomer concentration of 35 w/v %. Diphenyl-2,4,6-trimethylbenzoylphosphine oxide (DPTPO) was used as a photoinitiator at 0.3 wt% concentration. Monomer conversions and the size distribution of the polymer molecules were measured as a function of the reaction time up to the onset of macrogelation. Compared to the photoinitiators benzoin or benzoin derivatives, gelation process proceeds at much higher rates in the presence of DPTPO. The size distribution curves obtained by size-exclusion chromatography (SEC) change from monomodal to bimodal distributions as the polymerization proceeds. Strongly bimodal SEC curves were obtained in the close vicinity of the gel point. This finding confirms the coagulation type gelation mechanism of compact primary particles. It also indicates that the present gelation theories cannot describe the structure dependent kinetics of free-radical crosslinking copolymerization.     

Effect of electrostatic interactions on phase transition in the swollen polymeric network

An earlier relation describing swelling equilibria in polyelectrolyte networks is analysed from the viewpoint of the effect of electrostatic interactions on the existence of phase transition in polyelectrolyte gels. While the increase in the number of charges on the chain initiates the transition, both the increasing concentration of the low-molecular weight electrolyte and the decreasing flexibility of the chain (increasing length of the statistical segment) suppress this phenomenon. Swelling experimental data obtained on polyacrylamide (PAAm) gels may be described semiquantitatively assuming the existence of 1 mol— charges on the PAAm chain.     

酶促木质素与酚共聚反应速率研究

进行了过氧化物酶催化木质素与酚聚合反应及其动力学实验研究。确定了木质素浓度(Y)与吸光度 (X)的关系 ,并得到关联式 :Y =0 .0 916X。通过测定反应剂中酚的浓度 ,得到酚单体消耗速率 ,从一个侧面探索了反应速率。结果表明 ,在反相微乳液系统中 ,酶促反应速率很大。     

A comprehensive study on the kinetics of aqueous free-radical homo- and copolymerization of acrylamide and diallyldimethylammonium chloride by online 1H-NMR spectroscopy

Free-radical homo- and copolymerization of acrylamide (AAm) and diallyldimethylammonium chloride (DADMAC) initiated with potassium persulfate (KPS) were performed in the presence of 0.1 M NaCl solution in D₂O at 50 °C. Online ¹H-NMR kinetic experiments were used to study polymerization kinetics via determination of the individual and overall conversion of the comonomers and compositions of the comonomer mixture and produced copolymer as a function of the reaction time. Reactivity ratios of the AAm and DADMAC were calculated by Mao-Huglin (MH) and extended Kelen-Tudos (KT) methods to be 7.0855?±?1.3963, 0.1216?±?0.0301 and 6.9458?±?2.0113, 0.1201?±?0.0437 respectively. “Lumped” kinetic parameter (k _p k _t ^??0.5 ) was estimated from experimental data. Results showed that k _p k _t ^??0.5 value increases by increasing mole fraction of the AAm in the initial reaction mixture. Drift in the comonomer mixture and copolymer compositions with reaction progress was evaluated experimentally and theoretically. Theoretical values were calculated from Meyer-Lowry equation by using reactivity ratios obtained from MH method. A good fitting between the experimental and theoretical values was observed, indicating accuracy of the reactivity ratios estimated in the present work. It was found from following changes in the copolymer composition with the comonomer conversion that produced copolymer has a statistical structure.     

An alternate approach to the determination of rate parameters in copolymerization

A new method which proposes the use of a steady state CSTR is presented for the determination of rate parameters in a copolymerization system. The approach does not limit the conversion levels in the reactor and allows precise estimation of reactivity ratios. In the presence of gel effect, the suggested approach additionally reveals the cumulative variations of termination rate processes with extent of conversion. Such an a priori method which enables estimation of rate parameters in the high conversion region has not been available in the literature previously. The method proposed is illustrated using copolymerization of styrene–methyl methacrylate as a model system.