Modelling free-radical copolymerization kinetics, 3. Molecular weight calculations for copolymers with crosslinking

A comprehensive model for molecular weight calculations of free-radical crosslinking copolymerizations was developed using the pseudo-kinetic rate constants and the method of moments. The moments of copolymer chain distributions are defined in such a way so that the molecular weight averages of crosslinking copolymers can be calculated using the moments. The present model is based on a general crosslinking copolymerization scheme, accounting for chain transfer to small molecules and polymer, bimolecular termination, and crosslinking reactions. The influence of crosslinking reactions on molecular weight development is discussed. The effects of the reactivity of pendant double bonds on the moments development were further demonstrated using model simulations. The simulations results suggest that the higher-order molecular weight averages are very sensitive to the reactivity of pendant double bonds. It was found that chain transfer to polymer affects the gelation point significantly. The radical fractions must be calculated accounting for chain transfer reactions in addition to propagations in order to properly evaluate pseudo-kinetic rate constants. The present model was used to predict kinetic behavior and molecular weight development of styrene/m-divinylbenzene and styrene/ethylene dimethacrylate free-radical crosslinking copolymerizations in benzene solution at 60°C. It was found that the present model is in excellent agreement with the experimental data published in the literature. Model predictions and experimental data show that the reactivity of pendant double bonds is much lower than that of vinyl and divinyl monomers. The simulation results suggest that the assumption of the same reactivity of functional groups is likely not valid for many free-radical crosslinking copolymerizations. The present model based on a kinetics approach can be used to predict molecular weight development for vinyl/divinyl free-radical crosslinking copolymerizations and to estimate kinetic parameters in the pre-gelation period.     

High-conversion polymerization. II. Influence of chain transfer on the gel effect

Kinetic equations are presented which describe the course of a free-radical polymerization carried to high conversion when chain transfer to monomer or to other low molecular weight agents is important. Calculations are presented which show that the influence of chain transfer to monomer can greatly affect the rate of polymerization and the molecular weight averages when a polymerization is subject to autoacceleration because of the gel effect.     

Simulation model for network formation in free-radical crosslinking copolymerization: Pregelation period

A new simulation model for network formation in free-radical copolymerization of vinyl and divinyl monomers is proposed. This model is based on the crosslinking density distribution of the primary polymer molecules that results from a kinetically controlled network formation. The crosslinking density distribution provides information on how each chain is connected to other chains and therefore, a detailed analysis of the kinetics of network formation becomes possible by application of Monte Carlo simulations. In this method, not only averages but also various distributions, such as molecular weight distribution and distribution of crosslinked units as well as of unreacted pendant double bonds among various polymer molecules, can be calculated. The present theory is a direct solution for the Bethe lattice formed under nonequilibrium conditions, and therefore, it can be used to examine the applicability of the earlier theories of network formation to kinetically controlled systems. The present method is quite general and can be applied to various complex reactions systems that involve crosslinking, branching, cryclization and degradation in a nonequilibrium system.     

Electroinitiated polymerization of vinylic monomers in polar systems. II. Polymerization of ethyl methacrylate in methanol solutions

Ethyl methacrylate is polymerized by electroinitiation in methanol–electrolyte mixtures in which the monomer is soluble whereas the polymer obtained is insoluble. A technique of changing the polarity of the electrodes is used. With this technique a polymer of high molecular weight can be obtained. The relationships between molecular weight and monomer concentration, current densities, and time of the reaction as well as the yield as a function of current density, time of the reaction and initial monomer concentration are given. A free-radical mechanism is proposed for the reaction.     

High-conversion polymerization. III. Kinetic behavior of ethyl methacrylate

The kinetics of free-radical polymerization of ethyl methacrylate (EMA) have been experimentally studied in bulk and in 50% solution in DMF over the temperature range from 60 to 90°C. At low conversion, the kinetic parameters have been determined, including constants for chain transfer to monomer and solvent. These parameters have then been used to model the kinetic behavior at high conversion using previously derived equations. The results indicate that the equations satisfactorily predict the conversion and molecular weight averages and distributions as a function of time from zero to ca. 95% conversion.     

Solution Polymerization of Acrylonitrile Catalyzed by Sodium Triethylthioisopropoxyaluminate: A Polyacrylonitrile with High Structural Regularity

A new catalyst for the polymerization of acrylonitrile has been found by reacting sodium aluminum tetraethyl with a stoichiometric amount of oxygen, alcohols, or mercaptans, etc. The catalyst prepared by reacting NaAlEt₄ with RSH remains active at temperatures as low as ?78°C., polymerizing acrylonitrile into high molecular weight polymers with high conversions. At ?78°C., the propagating chain remains active over a period of days, and the chain length increases with time or conversion. At high conversions the molecular weight of the polymer is directly proportional to the concentration of the monomer, the catalyst concentration being kept constant. The efficiency of the catalyst expressed as the ratio of the number of the polymer molecules produced to the number of the sulfur atoms used is in excess of 80%. The weight-average molecular weight of the polymer measured by light scattering is roughly equal to the number-average molecular weight determined by sulfur analysis assuming only onesulfur atom in each polymer molecule. Although the data given here are only qualitative in nature, it is nevertheless evident that this system bears great resemblances to anionic polymerizations resulting in “living polymers.” The polymer obtained with the NaAlEt₃S(i-Pr) catalyst at ?78°C. differs from free-radical polyacrylonitrile in exhibiting substantially lower solubility, higher melting point, and higher rate of crystallization than that for the free-radical polymer. The polymer is also free from structural mpurities; it does not cause fluorescence in dilute solutions and has no absorption peak at 265 mμ; both these effects are produced by impurities in free-radical polyacrylonitrile. It is concluded that the polymer reported here is more regular in structure than free-radical polyacrylonitrile.     

自由基聚合的调控及其在聚合物分子设计中的应用

可控自由基聚合可以对聚合物的分子量、分子量分布和分子结构进行有效控制,因此对高分子合成具有重要意义。本文对稳定自由基调控的自由基聚合的机理、动力学及其在合成特殊结构聚合物中的应用作一专题报道。     

Critical properties for gelation in free-radical crosslinking copolymerization

A kinetic model was used to predict the molecular weight developments and the critical properties in free-radical crosslinking copolymerization. The predictions of the model were compared to the experimental data reported previously. Agreement of the kinetic model with experiments is satisfactory for both low and high crosslinker contents. The model parameters indicate increasing extent of shielding of pendant vinyl groups as the reaction proceeds due to the increasing number of multiple crosslinkages. The calculation results indicate that the real critical exponents can only be observed in the region ε < 10⁻²–10⁻³ where experimental studies are very difficult. Outside of this region, the apparent critical exponent γ describing the divergence of the weight-average molecular weight was found to deviate from the classical value due to the conversion dependent kinetics of free-radical crosslinking copolymerization.     

Molecular weight distribution formed through chain-length-dependent crosslinking reactions

The molecular weight distribution formed through chain-length-dependent crosslinking reactions in free-radical vinyl/divinyl copolymerization is investigated by using Monte Carlo simulations. When the crosslinking reaction rates for larger polymer molecules are reduced, the high molecular weight tails cannot extend smoothly, resulting in distorted, sometimes bimodal distribution profiles, which exhibit qualitative similarity with those reported experimentally. Although the reduced crosslinking reaction rates between larger polymer molecules may not affect the average crosslinking density levels significantly, they can delay the developments of the weight-average molecular weight significantly. Because the wastage of pendant double bonds through cyclization would result in a similar tendency, one cannot distinguish these two types of nonidealities through the measurements of the pendant double bond consumption and the average molecular weight development.     

In Situ Incorporation of Doxorubicin in Copolymer Particles During Suspension Polymerization

Summary: This work reports the kinetic behavior and the molecular properties of obtained copolymers during vinyl acetate and methyl methacrylate suspension copolymerizations in presence of doxorubicin. The obtained spherical particles are intended for use as chemoembolization agents. It is shown that the presence of doxorubicin in the reaction medium promotes significant changes in the copolymerization kinetics and final molecular weight averages of the polymer product.     

Molecular weight distributions in radiation-induced polymerization. V. A simultaneous kinetic and molecular weight distribution analysis of the liquid-state polymerization of styrene

The γ-ray initiated polymerization of styrene in the liquid state was investigated over the temperature range 0 to ?29°C at constant dose rate. The kinetics and molecular weight distributions were studied for samples prepared by standard techniques and samples subjected to exhaustive drying to remove residual water. In the former case, the rates of reaction were comparable to those for purely free radical polymerization, however, the resulting molecular weight distributions were distinctly bimodal, indicating an additional contribution from the cationic mechanism. On the other hand, the rates of polymerization for rigorously dried samples were 2 to 3 orders of magnitude greater than accepted free-radical values, and the molecular weight distributions were unimodal in nature. The experimental results were compared with theoretical kinetic data and molecular weight distribution data generated from a kinetic scheme taking into consideration polymerization via free-radical, cationic, and radical-cationic species, resulting in the evaluation of a number of quantities of interest. Substitution of determined values for the rate constants and G values results in good agreement between theoretically generated and experimentally determined kinetic data and molecular weight distribution data over the range of experimental conditions studied.     

Gel properties in free radical crosslinking copolymerization: A kinetic approach

A 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.     

A theoretical investigation of the production of branched copolymers in continuous stirred tank reactors

A comprehensive mathematical model for free-radical copolymerization reactions has been developed for a homogeneous continuous stirred tank reactor. The present model is based on a fairly general copolymerization scheme accounting for the formation of linear and branched copolymer chains. Both chain transfer to polymer and terminal double bond reactions are considered in order to predict the long chain branching frequency. Changes in molecular weight, composition and degree of branching occurring during the copolymerization reaction are modelled using the method of moments. To break-down the dependence of the moment equations on higher order moments two different closure methods are considered. The predictive capabilities of the model are examined in relation to the solution copolymerization of methyl methacrylate with vinyl acetate. It is shown that both chain transfer to polymer and terminal double bond reactions significantly contribute to the broadening of the molecular weight and degree of branching distributions. Furthermore, the terminal double bond reaction effects significantly the copolymer number-average molecular weight and the concentration of terminal double bonds.     

Vinyl monomer polymerization mechanism in the presence of trialkylboranes

The polymerization of methyl methacrylate in the presence and in the absence of triethylborane and different inhibitors was carried out. An ethyl radical displacement reaction between the inhibitor radical (produced through the reaction between a radical chain and the inhibitor) and the triethylborane is postulated. This new chain-transfer reaction explains the large excess of inhibitor which must be added in the presence of triethylborane in order to suppress the production of high molecular weight polymer. The results are in agreement with a typical free-radical mechanism.     

Modeling stable free-radical polymerization

A kinetic model has been developed for stable free-radical polymerization (SFRP) processes by using the method of moments. This model predicts monomer conversion, number-average molecular weight, and polydispersity of molecular weight distribution. The effects of the concentrations of initiator, stable radical, and monomer, as well as the rate constants of initiation, propagation, termination, transfer, and the equilibrium constant between active and dormant species, are systematically investigated by using this model. It is shown that the ideal living-radical polymerization having a linear relationship between number-average molecular weight and conversion and a polydispersity close to unity is the result of fast initiation, slow propagation, absence of radical termination, and a high level of dormant species. Increasing stable radical concentration helps to reduce polydispersity but also decreases polymerization rate. Thermal initiation significantly broadens molecular weight distribution. Without the formation of dormant species, the model predicts a conventional free-radical polymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2692–2704, 1999     

Polymerization of Styrene in Supercritical CO_2-swollenPoly(ethylene terephthlate)

Supercriticalfluids(SCFs)havemanyfeatures,suchashighcompressibility,highdiffusivityandlowviscosity,whichmakethemattractiveformanyindustrialprocesses.supercritical(sc)co=isthemostpopularbecauseitisnon-toxic,non-nammable,andeasytoobtainandhasanear-ambientcriticaltemperature.SCCO=canswellglassypolymers'.AdditivesintheSCCO,candiffuseintotheswollenpolymerandtheCO=canberemovedcompletelyondepressurization,leavingmostadditivesinthepolymer.Thussolvent-freeimpregnationofadditivescanbeachieved.Ifadd…     

Characterization and Properties of Macromolecules. XII. Numerical Treatment of Gel Permeation Chromatography Data

Abstract

Gel permeation chromatography provides a continuous recording of polymer concentration as a function of elution time. The latter may be converted by appropriate calibration into molecular weight. The chromatogram may be analyzed numerically to yield the various molecular weight averages. This process involves a number of assumptions and approximations. The magnitude of the errors introduced in the calculated molecular weight averages by the methods described in the literature are reported.     

a,ω-二羧基-甲基丙烯酸丁酯低聚物的合成及MS分析

利用传统自由基聚合法,在四氢呋喃溶液中自由基引发聚合甲基丙烯酸丁酯单体而得到ω-羧基-甲基丙烯酸丁酯低聚物(CTBMA)(分子量在1500左右);利用CTBMA末端酯基的反应特性,在二氧六环/水/KOH混合溶液中皂化CTBMA,使之转化为α,ω-羧基甲基丙烯酸丁酯低聚物(di-CTBMA);研究了溶剂的类别、反应时间等反应条件对皂化产物结构的影响;利用MALDI-TOF-MS及LSIMS对皂化各阶段产物进行了分析监测.实验表明,在适当的皂化条件下,CTBMA皂化时主要为末端酯基转化为羧基,相应得到的产物di-CTMBA具有很好的结构特性,其官能团度(functionality)接近2.     

Homopolymers and copolymers of vinyl ethers of rosin-derived alcohols

Two vinyl ethers of rosin-derived alcohols were homopolymerized with free-radical and cationic initiators. A polymer was obtained from the vinyl ether of the alcohol from Foral in good conversion by free-radical initiators but it was not a high molecular weight product. Cationic initiators were more effective; boron trifluoride etherate proved to be best. All of the homopolymers were obtained as white powders with high crystallinity. Copolymerizations of the vinyl ethers and vinyl chloride have been studied with the use of free-radical initiators. Cationic-initiated copolymers of the vinyl ethers and isobutyl vinyl ether were studied. Those containing less than 20% of the rosin vinyl ethers were rubbery, of high molecular weight, and could be crosslinked with peroxide.