Kinetic gelation predictions of species aggregation in tetrafunctional monomer polymerizations

During the polymerization of tetrafunctional monomers, a heterogeneous distribution of species exists throughout the reaction mixture and dramatically complicates the network structural evolution. This work attempts to quantify this heterogeneity using kinetic gelation simulation predictions of a defining parameter called the heterogeneity index. The heterogeneity index directly measures how heterogeneous the polymerization is with respect to interactions of like species or pairs of species. Examples of the species are polymer segments, monomeric double bonds, pendant double bonds, initiator molecules, radicals, and free volume. By implementing this index, it is clear that the heterogeneity in the polymer network dramatically changes as a function of conversion and polymerization conditions. In particular, microgel formation and monomer pooling were characterized and quantified with the heterogeneity index. In addition to characterizing the structural heterogeneity, the influence of the heterogeneity on the initiator efficiency and trapping of free radicals was also studied and qualitatively compared to experimentally observed behavior. ©1995 John Wiley & Sons, Inc.     

Simulation of Non‐Linear Free‐Radical Polymerization Using a Percolation Kinetic Gelation Model

A new kinetic gelation model that incorporates the kinetics of representative non‐linear free‐radical polymerization is presented. Specifically, free‐radical homopolymerization, polymerization in the presence of a chain‐transfer agent (CTA, CTA‐induced polymerization), and copolymerization of a mixture of the bi‐ and tetrafunctional monomer is used to simulate kinetic effects on polymerization statistics and microstructures. An algorithm for random next‐step selection in a self‐avoiding random walk and efficient mechanisms of a component's mobility are introduced to improve the generality of the predictions by removing commonly occurring deficiencies due to early trapping of radicals. The model has the capability to take into account into several free‐radical polymerization mechanisms such as crosslinking, branching, and transfer reaction, and also to predict the onset of the sol–gel transition, and the effect of chemical composition on the transition point. It is shown that a better understanding of microstructure evolution during polymerization and chemical gelation is attained. Lastly, one important benefit of the model is to simulate very highly packed random chains or microgels within a polymer network.     

Microgel formation in the free radical crosslinking polymerization of ethylene glycol dimethacrylate (EGDMA). II. Simulation

The simulation of free radical crosslinking polymerizations of multifunctional monomer by a percolation model was performed on a two-dimensional lattice with periodical boundaries. The model was formulated in such a way that a variety of monomers, translational diffusion of monomer and polymer molecules, segmental diffusion of polymer, shielding effects, microgel formation, and unequal reactivities of vinylene bonds can be considered. Simulation results of the 2-D model agreed well with those of a 3-D model. The percolation model was able to qualitatively verify several experimental results. © 1995 John Wiley & Sons, Inc.     

Reaction kinetics and microgel particle size characterization of ultraviolet‐curing unsaturated polyester acrylates

The free‐radical reaction kinetics and microgel formation of UV‐curing unsaturated polyester acrylates were studied in terms of the effects of internal maleic and terminal acrylate unsaturations. A triacrylate‐functional monomer, trimethylolpropane triacrylate, was used as the reactive diluent. A time‐resolved Fourier transform infrared technique was used to evaluate the consumption of double bonds and showed that internal (maleic) double bonds were involved in microgel formation at a rate similar to that of the more reactive terminal (acrylic) double bonds. Dynamic light scattering was used to measure the microgel particle size. The introduction of internal unsaturations caused smaller microgels, whereas terminal acrylate unsaturations resulted in larger particle sizes. These results were attributed to the higher tendency of the internal maleic double bonds toward intramolecular cyclization reactions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6544–6557, 2006     

Microstructural evolution in polymerizations of tetrafunctional monomers

To characterize the network structural evolution during polymerizations of multifunctional monomers and predict the relationship between reaction conditions and polymer structure, a balanced approach between experimental characterization and simulation is used. A novel method for characterizing the free volume distribution and network structure throughout the polymerization is presented. This technique involves the use of photochromic probes that require a certain local, critical volume to isomerize. By incorporating probes that require different isomerization volumes, a distribution of local volume in the network is determined. Since this technique allows in situ monitoring of the volume distribution, the effects of conversion, rates of polymerization, and monomer structure on the free volume distribution can be elucidated. Volume distributions are presented for diethylene glycol dimethacrylate. Information from the photochromic technique is further supported by kinetic gelation model predictions and transmission electron microscopy pictures of the evolving polymer structure.     

Use of “living” radical polymerizations to study the structural evolution and properties of highly crosslinked polymer networks

Crosslinked polymer networks are used in a wide variety of applications. To use these materials effectively, a fundamental understanding of their structural evolution and the relationship between material properties and structure is essential. In this article, a novel technique employing “iniferters,” i.e., living radical polymerizations, to photopolymerize these networks is utilized to study the property and structural evolution of these highly desirable materials. Living radical polymerizations are used in this work since this technique avoids the problem of carbon radical trapping encountered while using conventional initiators. Dynamic mechanical measurements are performed on highly crosslinked methacrylate networks to glean information regarding their structural heterogeneity. By performing these measurements on homopolymerized samples at various stages of the reaction and on copolymerized samples of multifunctional methacrylates, the mechanical properties are characterized as a function of double bond conversion and comonomer composition. From such analyses, with respect to both temperature and frequency, quantitative conclusions regarding the structure of the networks are drawn. This effort is aimed at exploiting the living radical polymerizations initiated by p-xylylene bis(N,N-diethyl dithiocarbamate) (XDT), to study the mechanical property evolution and structural heterogeneity of crosslinked polymers which is nearly impossible otherwise. Polymers examined in this study include networks formed by homopolymerization of diethylene glycol dimethacrylate (DEGDMA) and polyethylene glycol 600 dimethacrylate (PEG600DMA) as well as copolymers of DEGDMA and PEG600DMA. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2297–2307, 1997     

Initiation and termination mechanisms in kinetic gelation simulations

A kinetic gelation simulation is presented which includes a distinct initiator species decaying exponentially with time such that the rate of initiation varies as in an actual polymerization. The effects of the initiator quantity and the initiator decay constant on the polymerization are shown along with the effect of varying the probability that two radicals on adjacent sites will terminate. Varying the initiation rate and termination probability are examined in order to determine their influence on the trapping of radicals, the relative reactivity of pendant functional groups, and the overall structure of the polymer. In general it appears that the incorporation of an initiator enables the simulation to more adequartely describe the polymerization reaction, particularly time-dependent phenomena such as rate of reaction and radical concentrations.     

Preparation of Acrylic Microgels by Modified Microemulsion Polymerization and Phase Inversion

Semi‐transparent reactive microgel in nanosize has been prepared by modified microemulsion polymerization using a common emulsifier, crosslinking agent and functional monomer. The microgels are translucent reactive nanoparticles, with the size of 40–100 nm, consisting of inner‐crosslinked polymer up to 40%. FT‐IR proved the functional groups, such as epoxy and hydroxy, are on the surface of the microgel nanoparticles. Rheological detection demonstrated the apparent pseudoplasticity of the non‐aqueous microgel dispersion prepared by the phase transfering from the O/W microlatexes.     

Off‐lattice long‐range percolation modelling of a network polymerisation

A system involving homopolymerisation of a 3‐functional monomer has been studied by using an off‐lattice long‐range percolation model. The elementary reaction steps were controlled by substitution effects and by local concentrations of units. The latter were controlled by setting a constant value to a range‐of‐reaction parameter called the capture radius. The critical conversion at gelation in the system turned to the classical one as the value of capture radius increased.     

Microgel Capsules Tailored by Droplet‐Based Microfluidics

Microgel capsules are micrometer‐sized particles that consist of a cross‐linked, solvent‐swollen polymer network complexed with additives. These particles have various applications, such as drug delivery, catalysis, and analytics. To optimize the performance of microgel capsules, it is crucial to control their size, shape, and content of encapsulated additives with high precision. There are two classes of microgel‐capsule structures. One class comprises bulk microcapsules that consist of a polymer network spanning the entire particle and entrapping the additive within its meshes. The other class comprises core–shell structures; in this case, the microgel polymer network just forms the shell of the particles, whereas their interior is hollow and hosts the encapsulated payload. Both types of structures can be produced with exquisite control by droplet‐based microfluidic templating followed by subsequent droplet gelation. This article highlights some early and recent achievements in the use of this technique to tailor soft microgel capsules; it also discusses applications of these particles. A special focus is on the encapsulation of living cells, which are very sensitive and complex but also very useful additives for immobilization within microgel particles.     

RAFT polymerization of a novel allene‐derived asymmetrical divinyl monomer: A facile strategy to alkene‐functionalized hyperbranched vinyl polymers with high degrees of branching

Hyperbranched vinyl polymers with high degrees of branching (DBs) up to 0.43 functionalized with numerous pendent allene groups have been successfully prepared via reversible addition fragmentation chain transfer polymerization of a state‐of‐art allene‐derived asymmetrical divinyl monomer, allenemethyl methacrylate (AMMA). The gelation did not occur until high monomer conversions (above 90%), as a result of the optimized reactivity difference between the two vinyl groups in AMMA. The branched structure was confirmed by a combination of a triple‐detection size exclusion chromatography (light scattering, refractive index, and viscosity detectors) and detailed ¹H NMR analyses. A two‐step mechanism is proposed for the evolution of branching according to the dependence of molecular weight and DB on monomer conversion. Controlled radical polymerization proceeds until moderate conversions, mainly producing linear polymers. Subsequent initiation and propagation on the polymerizable allene side chains as well as the coupling of macromolecular chains generate numerous branches at moderate‐to‐high monomer conversions, dramatically increasing the molecular weight of the polymer. AMMA was also explored as a new branching agent to construct poly(methyl methacrylate)‐type hyperbranched polymers by its copolymerization with methyl methacrylate. The DB can be effectively tuned by the amount of AMMA, showing a linear increase trend. The pendent allene groups in the side chains of the copolymers were further functionalized by epoxidation and thiol‐ene chemistry in satisfactory yields. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2959–2969     

A molecular dynamics simulation study on polymer networks of end-linked flexible or rigid chains

The differences in formation and structural properties of polymer networks consisting of end-linked flexible or rigid chains were studied by molecular dynamics simulation. Networks were formed from monodisperse, linear, short, flexible or rigid chains with functional end groups and a stoichiometric ratio of trifunctional cross-linkers. The rigid chains had a rodlike shape defined by an angle potential, while the flexible chains had no angle potential. In order to understand the influence of chain rigidity, all parameters of precursor chains (length, reactivity, bond potential, nonbonding potential) were the same, with the exception of the angle potential. The system density rho, corresponding to the concentration of monomer in solvent, was varied from 0.01 to 0.11. Different network structures resulting from the different processes of network formation were observed. Simulations showed that the flexible chains created an inhomogeneous network on a large scale via microgel cluster formation, in agreement with experimental observations, whereas the rigid chains rapidly created a homogeneous network in the entire system volume without first generating microgel clusters, with the additional difference that they gave rise to mutually interpenetrating networks at the local scale.     

Influence of aggregate formation in the copolymerization of ethyl α‐hydroxymethylacrylate with methyl methacrylate

Ethyl α‐hydroxymethylacrylate was homopolymerized and copolymerized with methyl methacrylate in chlorobenzene and 1,4‐dioxane solutions at 50 °C with 1.5 × 10^?2 mol/L 2,2′‐azobisisobutyronitrile as an initiator and a global monomer concentration of 3.0 mol/L. The experiments showed differences in the calculated values of the monomer reactivity ratios in both solvents. The kinetic behavior was analyzed in terms of the implicit penultimate effect and the radical reactivity ratios. All the parameters were examined with respect to the aggregation ability of the ethyl α‐hydroxymethylacrylate monomer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4187–4195, 2005     

Photoinitiated polymerization of methacrylic monomers in a polystyrene matrix: Kinetic,mechanistic, and structural aspects

The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6‐hexanediol dimethacrylate (HDDMA) and 2‐ethylhexyl methacrylate (EHMA)] in a polystyrene (PS) matrix were studied. The aggregation state, vitreous or rubbery, of the monomer/matrix system and the intermolecular strength of attraction in the monomer/matrix and growing macroradical/matrix systems are the principal factors influencing the kinetics and mechanism. For the PS/HDDMA system, where a relatively high intermolecular force of attraction between monomer and matrix and between growing macroradical and matrix occurs, a reaction‐diffusion mechanism takes place at low monomer concentrations (<30–40%) from the beginning of the polymerization. For the PS/EHMA system, which presents low intermolecular attraction between monomer and matrix and between growing macroradical and matrix, the reaction‐diffusion termination is not clear, and a combination of reaction‐diffusion and diffusion‐controlled mechanisms explains better the polymerization for monomer concentrations below 30–40%. For both systems, for which a change from a vitreous state to a rubbery state occurs when the monomer concentration changes from 10 to 20%, the intrinsic reactivity and k_p/k_t^1/2 ratio (where k_p is the propagation kinetic constant and k_t is the termination kinetic constant) increase as a result of a greater mobility of the monomer in the matrix (a greater k_p value). The PS matrix participates in the polymerization process through the formation of benzylic radical, which is bonded to some extent by radical–radical coupling with the growing methacrylic radica, producing grafting on the PS matrix. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2049–2057, 2001     

Regenerable‐Catalyst‐Aided,Opened to Air and Sunlight‐Driven “CuAAC&ATRP” Concurrent Reaction for Sequence‐Controlled Copolymer

An ideal stimuli‐responsive controlled/living radical polymerization should have the ability to manipulate the reaction through spatiotemporal “on/off” controls, achieving the polymerization under fully open conditions and allowing for precise control over macromolecular architecture with defined molecular weights and monomer sequence. In this contribution, the photo (sunlight)‐induced electron transfer atom transfer radical‐polymerization (PET‐ATRP) can be realized to be reversibly activated and deactivated under fully open conditions utilizing one‐component copper(II) thioxanthone carboxylate as multifunctional photocatalyst and oxygen scavenger. The polymerization behaviors are investigated, presenting controlled features with first‐order kinetics and linear relationships between molecular weights and monomer conversions. More importantly, “CuAAC&ATRP” concurrent reaction combining PET‐ATRP, photodriven deoxygenation, and photoactivated CuAAC click reaction is successfully employed to synthesize the sequence‐defined multiblock functional copolymers, in which the iterative monomer additions can be easily manipulated under fully open conditions.     

Novel Reactive Polymers Containing Hemiacetal Ester and Vinyl Moieties: Synthesis and Selective Polymerization of 1‐Methoxyallyl Methacrylate Derived from Methacrylic Acid and Methoxyallene

A novel functional monomer, 1‐methoxyallyl methacrylate (MOAMA), was prepared and its polymerization behavior was investigated. The radical polymerization of MOAMA led to a polymeric network due to the participation of the allyl group in the reaction. Contrarily, anionic polymerization proceeded in a living fashion to yield linear poly(MOAMA) without gelation. Since both the network‐type and linear polymers possess hemiacetal ester and vinyl moieties, the two‐fold polymerization behavior of MOAMA represents a new pathway towards reactive polymers.     

用探针粒子示踪微流变法测量明胶的动态不均匀性

利用探针粒子示踪微流变法对明胶溶液等温凝胶化过程的动态不均匀性进行了观测.通过向体系中引入探针粒子,利用广义的Stokes-Einstein关系由探针粒子的位移建立了体系结构的空间分布.进而利用van Hove函数和非高斯因子描述了凝胶化转变前后动态不均匀性的变化,结果表明在凝胶化后体系动态在空间和时间尺度上都是不均匀的,与凝胶化前相比,凝胶化后的快动态有更高的贡献.为了进一步探究这种动态不均匀性在空间和时间上相关的变化,本工作在粒子示踪技术的基础上,实现了四点相关函数和极化率的测量.实验结果表明,凝胶化前体系的快松弛可以原位独立完成,而慢松弛则需要周围的结构单元协同完成;凝胶化后体系的快松弛和慢松弛均需要协同完成.     

Hydrophilic stimuli‐responsive particles for biomedical applications

Hydrophilic and stimuli‐responsive submicronic latex particles based on polyalkyl(meth)acrylamide can be prepared owing to simple radical‐initiated polymerizations in heterogeneous media using a water‐soluble initiator and a crosslinker (methylenebisacrylamide). The paper aims at reviewing the synthesis and properties of functionalized polystyrene‐polyN‐isoprpylacrylamide core‐shell particles or polyN‐isopropylmethacrylamide microgel particles. Particle size of analysis showed that a short nucleation period afforded the synthesis of highly monodispersed latexes. The dramatic change of the colloidal properties (particle size, electrophoretic mobility) was found to reflect the thermal sensitivity of such particles. The hydrophilic nature of the particles below the volume phase transition temperature was found to drastically reduce the physical adsorption of proteins. Some examples of biomedical applications of these stimuli‐responsive particles are briefly reported.     

Study of pH-responsive microgels containing methacrylic acid: effects of particle composition and added calcium

pH-responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the ionic monomer incorporated within the particles. In recent work from our group, it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of pH-responsive microgel dispersions (Saunders, J. M.; Tong, T.; LeMaitre, C.; Freemont, A. J.; Saunders, B. R. Soft Matter 2007, 3, 486). These dispersions change from a fluid to a gel with increasing pH. The present work investigates the pH-dependent properties of dispersions of microgel particles containing MAA (methacrylic acid) and also the effects of added Ca2+. Two microgels are discussed: microgel A is poly(EA/MAA/AM) (EA and AM are ethyl acrylate and allyl methacrylate), and microgel B is poly(EA/MAA/BDDA) (butanediol diacrylate). The pH-dependent particle properties investigated include hydrodynamic diameters and electrophoretic mobilities. The critical coagulation concentrations (CCC) of dilute dispersions and the elastic modulus (G') of concentrated, gelled microgel dispersions were also investigated. In the absence of added Ca2+, the particle swelling and G' were smallest and largest, respectively, for microgel A. The changes in hydrodynamic diameter and mobility with pH were explained in terms of a core-shell swelling mechanism. Added Ca2+ was found to significantly decrease the CCCs, extents of particle swelling, and magnitude of the electrophoretic mobility. This was attributed to the ionic cross-linking of neighboring RCOO- groups by Ca2+. It is suggested that the formation of ionic cross-links is inefficient within the microgel particles because of the presence of covalent cross-links that oppose the large-scale conformational rearrangement of neighboring RCOO- groups. The effect of Ca2+ on the properties of the gelled dispersions is important from the viewpoint of potential application in vivo. Rheological studies of the gelled microgel dispersions showed that added Ca2+ did not have a specific influence on G'. The differences observed in the presence of Ca2+ were attributed to ionic strength effects (screening). The key parameter that controls G' of the gelled microgel dispersions is pH. The results from this work suggest that the elasticity of the gels would be slightly reduced in vivo as a consequence of the high ionic strength present.