Polymerization Rate Coefficients Research Articles

Modification of poly-HEMA with nonreactive POSS derivatives by in situ photopolymerization

Photo-induced polymerization of 2-hydroxyethyl methacrylate (HEMA) in the presence of various amounts of nonreactive polyhedral oligomeric silsesquioxanes (POSS) functionalized with glycidyl, fluoroalkyl or hydroxyl groups was investigated. HEMA/POSS systems were characterized before, during and after the photocuring, with the special emphasis on the photopolymerization kinetics (measured by isothermal differential scanning calorimetry). It was found that the introduction of tested POSS derivatives into HEMA strongly affects the photopolymerization kinetics (enhancement of the gel effect, increase in the polymerization rate and conversion), mainly due to the increase in the viscosity of the initial formulation which leads to a reduction in the termination rate coefficient. However, interactions HEMA–POSS cause also a slight increase in the propagation rate coefficient. The behavior of the polymerization rate coefficients during the reaction suggests that POSS cages may mitigate the inhibitory effect of viscosity on the diffusion of macroradicals by exerting a slip effect. The materials produced are microcomposites due to the partial phase separation occurring during the curing process. Small amounts of added POSS modifiers cause plasticization of the material; at higher loads, POSS domains behave like nanofiller aggregates that increase the glass temperature. The nonreactive POSS have very little effect on thermal decomposition of the poly-HEMA matrix, which can result in a degree from the phase separation; the latter is also the main cause of the deterioration of the mechanical properties of composites compared to a pure polymer matrix.

Anchor Effect in Polymerization Kinetics: Case of Monofunctionalized POSS

The effect of the anchoring group on the detailed polymerization kinetics was investigated using monomethacryloxy-heptaisobutyl POSS (1M-POSS). This compound was copolymerized with lauryl methacrylate (LM) as the base monomer, at various molar ratios. The process was initiated photochemically. The polymerization kinetics were followed by photo-DSC and photorheology while the polymers were characterized by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). For comparison, a methacrylate containing the branched siloxy-silane group (TSM) was also studied. It was found that the modifiers with a bulky substituent have a dual effect on the termination process: (i) At low concentrations, they increase the molecular mobility by increasing the free volume fraction, which leads to an acceleration of the termination and slows the polymerization; while (ii) at higher concentrations, they retard molecular motions due to the “anchor effect” that suppresses the termination, leading to acceleration of the polymerization. The anchor effect can also be considered from a different point of view: The possibility of anchoring a monomer with a long substituent (LM) around the POSS cage, which can further enhance propagation. These conclusions were derived based on kinetic results, determination of polymerization rate coefficients, and copolymer analysis.

Photoinitiated polymerization in ionic liquids and its application

AbstractIonic liquids (ILs) are low‐melting organic salts often liquid at room temperature, whose unique properties are the reason of increasing interest for their applications as solvents, reaction media and functional additives. The exceptional properties of ILs have proved to be particularly useful in polymer science giving the potential to produce polymeric materials with improved properties or to immobilize ILs in polymer matrices while keeping their special characteristics. One of the possibilities is polymerization in ILs which can also affect positively polymerization reactions. An especially attractive technique is photopolymerization due to the ease of process control, short reaction time and ambient working temperature. This review gives a literature survey of developments in photopolymerization processes carried out in ILs as well as applications of these processes. It covers both the photopolymerization in ILs as well as photopolymerization of IL monomers. The first part presents a short overview of physicochemical and photochemical properties of ILs; it includes also photochemical reactions and photoinitiation of polymerization in ILs. The second part covers both the basic research (kinetics of photopolymerization including polymerization rate coefficients and polymerization of IL monomers) as well as applications of UV‐induced polymerization in ILs. © 2016 Society of Chemical Industry

Silica/aluminum oxide hybrid as a filler for photocurable composites

Abstract In this study silica/alumina hybrid (COK 84) was tested as filler for photocurable composites dedicated for protective coatings and was compared to fumed nanosilica (Aerosil 200). The model monomer was polyethylene glycol diacrylate. The effect of the filler type and content on the photopolymerization kinetics was followed; the kinetic studies were accompanied by measurements of thermal and mechanical characteristics of the composites produced. Polymerization rates and polymerization rate coefficients (propagation k p and termination k t b ) were determined as functions of double bond conversion. At low filler contents the acceleration of the polymerization was observed; higher loadings had detrimental effect both on the polymerization rate as well as on final conversion, the latter effect was stronger in the case of COK 84. The influence of the filler on the curing kinetics was discussed in terms of the behavior of the polymerization rate coefficients which is associated with filler/polymer interactions in the interphase. Tensile testing showed that COK 84 better improves mechanical characteristics of the poly-PEGDA matrix as compared to Aerosil 200, but only in a limited range of content.

Dual Catalyst Model Describing the Effect of Branching on the Molecular Architecture of Polyolefins

Analytical solutions for the impact of branching on the polymer molecular architecture in the presence of two catalysts are developed. One catalyst forms only linear chains while the other produces linear as well as branched species. Emphasis is placed on the average molecular weight and polydispersity index of the populations associated with each catalyst rather than the overall quantities. By defining the cross‐incorporation number, x, which represents the relative value of the β‐hydride elimination and terminal double bond polymerization rate coefficients of each catalyst, we map the effect of branching on the molecular architecture of the polymer in order to understand the underlying kinetics and guide system design.

Recent Advances in the Study of High‐Temperature Free Radical Acrylic Solution Copolymerization

AbstractSemibatch starved‐feed solution copolymerization is used to produce acrylic resins for coatings formulations. Mechanisms and rate coefficients for polymerization of methacrylates, acrylates, and styrene (ST) under these high‐temperature conditions are reviewed. An extended set of experimental results at 138 °C is used to refine a model describing the solution copolymerization of ST and methacrylates. The data suggest that both changing initiator efficiency and transfer reactions of oxygen‐centered radicals to polymer affect the development of polymer molecular weight. A penultimate model is used to describe the variation in termination rate coefficient with copolymer composition. Significant progress has been made to develop an extended model capable of representing multicomponent high‐temperature acrylic polymerizations.magnified image

Determination of Intramolecular Chain Transfer and Midchain Radical Propagation Rate Coefficients for Butyl Acrylate by Pulsed Laser Polymerization

A novel method to extract individual free-radical polymerization rate coefficients for butyl acrylate intramolecular chain transfer (backbiting), kbb, and for monomer addition to the resulting midchain radical, , is presented. The approach for measuring kbb does not require knowledge of any other rate coefficient. Only the dispersion parameter of SEC broadening has to be determined by fitting measured MWDs or should be available from separate experiments. The method is based upon analysis of the shift in the position of the inflection point of polymer molecular weight distributions produced by a series of pulsed-laser polymerization (PLP) experiments with varying laser pulse repetition rate. The coefficient kbb is determined from the onset of the sharp decrease of the apparent propagation rate coefficient ( ) with decreasing repetition rate, an approach verified by simulation. With experiments performed between −10 and +30 °C, the estimated values are fitted well by an Arrhenius relation with pre-exponential factor A(kbb) = (4.84 ± 0.29) × 107 s-1 and activation energy Ea(kbb) = (31.7 ± 2.5) kJ·mol-1. At low pulse repetition rates, the experimental values are related to an averaged propagation rate coefficient, , that is dependent on the relative population of chain-end and midchain radicals. Evaluated by comparing simulated and experimental molecular weight distributions, provides an estimate for . The Arrhenius parameters are: A( ) = (1.52 ± 0.14) × 106 L·mol-1·s-1 and Ea( ) = (28.9 ± 3.2) kJ·mol-1.

Estimation of Free Radical Polymerization Rate Coefficients Using Computational Chemistry

AbstractThis study explores the application of computational chemistry to estimate free radical polymerization rate coefficients. The Evans‐Polanyi relationship is combined with computed heats of polymerization to estimate copolymerization reactivity ratios for many vinyl monomer pairs, focusing on acrylates, methacrylates and styrene, with accuracy assessed by comparison to experimental values. The effect of different optimization approaches on the values of thermodynamic properties is explored, and it is concluded that a combination of conventional optimization and relaxed potential energy scans was most effective at identifying global minima. The difference between thermodynamic properties calculated using the harmonic oscillator treatment and a hindered rotor model is evaluated for methyl methacrylate polymerization.

Chain length dependent termination in radical cross-linking polymerization

The work presents an evidence in support of chain length dependent termination during cross-linking polymerization. It is based on the behavior of the ratio of the bimolecular termination coefficient k t b to propagation rate coefficient k p during the after-effect of a photo-induced polymerization. The chain-length dependence was manifested by a decrease of the k t b / k p ratio with the increase in dark reaction time faster than that resulting from the conversion increase. Two monomethacrylate/dimethacrylate and one dimethacrylate/dimethacrylate systems were chosen, which enabled to study the chain-length dependence as a function of cross-link density of the polymer being formed and physical properties of the initial composition. The ratios of the polymerization rate coefficients k t b / k p were calculated for various postpolymerization processes as a function of dark reaction conversion using the mixed termination model (concerning the bimolecular and monomolecular termination occurring parallel). A rapid drop of the k t b / k t ratio during the dark reaction was observed at the beginning of the after-effect and in slightly cross-linked systems suggesting a significant chain length dependence, but when the cross-link density increased with conversion or with cross-linking agent concentration in the feed, the drop of the k t b / k t ratio became much slower indicating that the chain length dependence decreased.

Simultaneous FT‐NIR and ESR analyses to yield propagation rate coefficients for polymerization of methyl methacrylate based monomers

AbstractA hyphenated instrumental approach has been used to obtain reliable values for the propagation rate coefficients as a function of conversion for polymerizations of methyl methacrylate (MMA) and a mixture of MMA and ethyleneglycol dimethacrylate (EGDMA) with a 1:1 concentration of double bonds, from near the onset of the Trommsdorf region into the glass region. ESR spectroscopy was used to measure the radical concentration while FT‐NIR fibre‐optic spectroscopy was employed to measure instantaneously the double‐bond concentration within the temperature‐controlled cavity of the ESR instrument during polymerization. The advantage of this approach to the measurement of the rate coefficient is that it is equally applicable to branching and linear polymerizations. For the polymerization of methyl methacrylate, the values of the rate coefficient at the lowest conversions at which reliable values could be obtained were in agreement with recently reported values obtained by the PLP‐SEC method. For the lowest conversions, the values obtained were 403 l mol−1 s−1 at 306 K for MMA and 520 l mol−1 s−1 at 310 K for a 1:1 mixture of MMA and EGDMA. Copyright © 2003 Society of Chemical Industry

Conversion dependence of free‐radical polymerization rate coefficients for styrene from laser‐induced experiments

Conversion dependence of free‐radical polymerization rate coefficients for styrene from laser‐induced experiments

Conversion dependence of free‐radical polymerization rate coefficients from laser‐induced experiments, 1. Butyl acrylate

Conversion dependence of free‐radical polymerization rate coefficients from laser‐induced experiments, 1. Butyl acrylate

Chain‐length dependence of free‐radical polymerization rate coefficients from laser‐induced experiments

Chain‐length dependence of free‐radical polymerization rate coefficients from laser‐induced experiments