Unique morphology and properties study of polyacrylate obtained via frontal photopolymerization

Remarkable radial temperature distribution at the advancing reaction front was determined in frontal photopolymerization (FPP). Through SEM observation of the cross-fracture section and vertical fracture section for the heat-insulated FPP polymer rod, a cylindrical multi-layer structure was first observed in the FPP product. A sleeve-like model was proposed to describe the unusual morphology on the basis of polymer self-drawing and convection around reaction front. Vertical gradient distribution of molecular weight and its polydispersity in the poly(isobornyl acrylate) rod obtained via FPP was found through GPC determination, which might be developed as an in situ self-fractionation technique for polymer. Thermal analysis of the polymers showed that the FPP tended to produce purer polymer than the traditional thick film photopolymerization.     

Mathematical modeling of thiol-ene frontal polymerization

Frontal polymerization is a method of manufacturing polymers via a self-propagating reaction wave. A mathematical model that describes thiol-ene frontal polymerization is presented and studied, both numerically and analytically. Spatio-temporal profiles of the temperature and concentrations of the reactants as well as the propagation velocity of the wave are determined. Conditions for existence of polymerization fronts are discussed.     

Spinning wave motion in frontal polymerization

We present a mathematical model describing dynamics of spinning waves which propagate during frontal polymerization reaction taking place in a cylindrical reactor tube. The self-organization of spatio-temporal solution of wave equations due to interplay between thermal diffusion and kinetics gives rise to pattern formation. We begin with a fundamental equation of motion of radial coordinate after defining an asymptotic phase for spinmode. The motion is analyzed near a critical (Hopf) point and a perturbation solution is used to obtain patterns for a case of preparation of poly(2-hydroxyethyl methacrylate) (PHEMA) via frontal polymerization reaction. The model uses the distance as seen in photograph taken using scanning electron microscope (SEM) from which motion begins around core of spiral and calculates pitch of spiral which matches closely with experimental observation in micrograph. Also the model predicts qualitatively the ramp wave and spiral wave motion as observed under SEM. Both these results are reported in open literature for the first time to our best knowledge.     

Novel allylic phosphonium salts in free radical accelerated cationic polymerization

In the present study, the synthesis and evaluation of novel allylic phosphonium salts as addition fragmentation agents in combination of conventional (photo-/thermal) free radical source for cationic polymerization are described. The amide based allylic phosphonium salts, namely 2-(N, N-dimethylcaboxy-propenyl) triphenylphosphonium hexafluoroantimonate (DMTPH) and 2-(morpholinocarboxy-propenyl) triphenyl phosphonium hexafluoroantimonate (MTPH) were synthesized and characterized. The thermal and photo-latency of these salts was examined with and without free radical sources in bulk polymerization of cyclohexene oxide (CHO) salts at 70 °C and λ > 290 nm irradiation, respectively. In presence of thermal free radical source, the order of activity was observed as PAT > BPO > AIBN. The order of activity of free radical sources on photopolymerization was found to be benzoin > benzophenone > TMDPO. In addition, photopolymerization of other cationically polymerizable monomers (such as n-butyl vinyl ether, isobutyl vinyl ether, N-vinyl carbazole and glycidyl phenyl ether) was also examined at λ > 290 nm irradiation. It is concluded that the rate of cationic polymerization can be accelerated using novel phosphonium salts with combination of free radical sources, by both thermal and photochemical mode.     

Photoinitiated dispersion polymerization using polyurethane based macrophotoinitiator as stabilizer and photoinitiator

Polyurethane based macrophotoinitiator (PU-PI) had been designed and synthesized, and applied to photoinitiated dispersion polymerization of methyl methacrylate as both photoinitiator and stabilizer, with ethanol/water mixture as reaction medium. Monomer conversion over 90% was achieved within 25 min of UV irradiation at room temperature, and monodisperse PMMA microspheres were obtained. The structure of the microspheres had been analyzed by XPS, showing that about 50% of surface of the microspheres were covered with the stabilizer. PU-PI effectively stabilized the polymeric particles in photoinitiated dispersion polymerization due to the unique stabilization process. The size and size distribution of the microspheres became insensitive to the reaction condition such as stabilizer/initiator concentration, initial monomer concentration and reaction medium. The size of the microspheres obtained changed in the range from 0.88 μm to 1.06 μm at different reaction condition, with polydispersity index as low as 1.011. The research may provide a quick and facile approach to prepare monodisperse microspheres with tailored functional surface.     

Peculiarities of establishment of steady-state frontal polymerization of vinyl monomers

Frontal radical polymerization of vinyl monomers in non-stationary mode is investigated theoretically. It is shown that the formation and the time of establishment of steady-state polymerization heat autowaves considerably depend on the initial temperature, T_i. When T_i is less than the adiabatic heating temperature (T_a), the excess heat and relatively high conversion foster front formation before the non-stationary one. Whereas, for T_i > T_a, steady-state frontal polymerization regime is established, when the heat flow from the outer source is considerably less than the heat evolving due to the chemical reaction.     

Study of cationic ring‐opening photopolymerizations using optical pyrometry

Studies of the photoinitiated cationic ring‐opening polymerizations of epoxide and oxetane monomers were conducted using optical pyrometry. Using this technique, the temperature of these photopolymerizations was monitored as a function of time. The effects of photoinitiator type and monomer structure on the rates of photopolymerization were investigated. Optical pyrometry was also used to investigate the acceleration of the photopolymerizations of various epoxide and oxetane monomers. Certain mixtures of monomers displayed synergistic effects that markedly increased their overall rates of polymerizations. In all cases in which acceleration of polymerization rate was noted, it could be attributed to an increase in the speed of ring opening of the initially formed protonated cyclic ether. The effects of relative humidity on the rate of cationic ring‐opening photopolymerizations of cyclic ether monomers were also investigated. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3303–3319, 2004     

Photoinitiated free radical polymerization of methylmethacrylate by using of quinoxalines in the presence of aldehydes

A series of quinoxalines at different concentrations were used in the presence of benzaldehyde (BA) and phthaldialdehyde (PDA) to initiate the polymerization of methylmethacrylate (MMA) formulation containing 2-ethyl-2-(hydroxymethyl)-1,3-propanediol-trimethylmethacrylate (TMPTMA) as a multifunctional acrylate. Highest conversion percentage of polymerization of MMA were obtained in the presence of benzaldehyde when diphenylquinoxaline, phenanthroquinoxaline and acenaphthoquinoxaline were used as an initiator. In the case of benzaldehyde, benzoyl radical were possibly responsible for the initiation of polymerization where quinoxalines do not initiate polymerization of acrylates in the presence of oxygen.     

激光光散射法研究紫外光引发聚合

采用激光光散射法跟踪,发现紫外光引发聚合体系中有关助剂及紫外光强度对体系的光固化的诱导期和聚合反应速度具有明显的影响,而过程中使用硫醇类光固化促进剂具有最佳效果.同时发现,体系中表观聚合速度与紫外光强的1.2次方成正比.     

Investigation of redox initiators for free radical frontal polymerization

BACKGROUND: The reaction temperature for frontal polymerization (FP) initiated by redox initiators can be greatly decreased compared with FP initiated by peroxide initiator and disulfide initiator. We report the synthesis of poly(hydroxyethyl acrylate)s via free radical FP using benzoyl peroxide (BPO)/N,N‐dimethylaniline (DMA) and ammonium persulfate (APS)/N,N,N′,N′‐tetramethylethylenediamine (TMEDA) couples as redox initiators at ambient pressure. RESULTS: The results show that unlike the phenomenon of bubbles and ‘fingers’ when using BPO alone, a self‐sustaining and stable front can be obtained when the [DMA]/[BPO] ratio is higher than 1 (mol/mol). A slight increase of the DMA (or TMEDA) reductant concentration causes a marked decrease of front temperature to 53 °C (or 61 °C). CONCLUSION: We investigated the effects of the ratio of the oxidant to the reductant and the initiator and monomer concentrations on certain parameters of FP: formation of bubbles, front velocity and front temperature. This opens the way to the potential development of FP using more appropriate monomers with low boiling points. Copyright © 2009 Society of Chemical Industry     

Photomediated controlled radical polymerization

Photomediated controlled radical polymerization is a versatile method to prepare, under mild conditions, various well-defined polymers with complex architecture, such as block and graft copolymers, sequence-controlled polymers, or hybrid materials via surface-initiated polymerization. It also provides opportunity to manipulate the reaction through spatiotemporal control. This review presents a comprehensive account of the fundamentals and applications of various photomediated CRP techniques, including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT), nitroxide mediated polymerization (NMP) and other procedures. In addition, mechanistic aspects of other photomediated methods are discussed.     

Modeling the emulsion polymerization of amino-functionalized latex particles

A mathematical model for a semicontinuous seeded cationic emulsion polymerization was developed. The model includes the most distinctive features of the copolymerization of a cationic hydrophilic monomer with a hydrophobic one, including polymerization of the hydrophilic monomer in the outer shell of polymer particles and in the aqueous phase, and the possibility of having radical concentration profiles in the polymer particles.The reactions were carried out by means of a semicontinuous seeded cationic emulsion polymerization under starved conditions for styrene, which was the main monomer employed.The model predicts the evolution of the fractional overall conversions, the thickness of the outer shell, the total surface charge density and the partial conversions for the semicontinuous seeded cationic emulsion polymerization of styrene and aminoethyl methacrylate hydrochloride. Furthermore, the model can distinguish between the surface charge density provided by the cationic monomer than that given by the cationic initiator. Therefore, this model can predict the best conditions to obtain well-defined latexes with specific amounts of surface amino and amidine groups useful for immunoassays.     

前端聚合的研究进展

前端聚合因具有简易、节能、反应速度快和独特的产物形貌等优点而受到越来越广泛的关注。本论文将前端聚合按照聚合方法分为3类：热前端聚合,光前端聚合和等温前端聚合,讨论了前端聚合的组成,综述了前端聚合的应用,前端聚合在功能材料、有机无机纳米复合材料和工业生产上将会得到广泛的应用。     

Kinetic study of photoinitiated frontal polymerization

The frontal polymerization of a monomer exposed continuously to UV radiation in the presence of a photobleachable initiator has been studied on the basis of the rate equations. The time dependence of photoinitiator concentration profile and that of monomer conversion within the irradiated sample have been calculated, assuming steady‐state conditions. It is thus possible to visualize how fast the polymerization profile is moving within the sample and how effectively the polymerization of a monomer layer located at a given distance from the surface proceeds upon exposure to UV radiation. © 2001 Society of Chemical Industry     

Photochemically initiated free radical promoted living cationic polymerization of isobutyl vinyl ether

A new photoinitiating system for living cationic polymerization of vinyl ethers such as isobutyl vinyl ether (IBVE) has been reported. The photoinitiating system comprises free radical photoinitiators such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA), benzophenone or thioxanthone, together with an onium salt, such as diphenyliodonium chloride and zinc bromide. In the first step, photochemically generated free radicals are oxidized to the corresponding carbocations which subsequently react with vinyl ether monomer to yield an adduct. In the presence of zinc salt, this adduct initiates living cationic polymerization of IBVE.     

Simultaneous cationic polymerization and esterification of epoxy/anhydride system in the presence of polyoxometalate catalyst

Polyoxometalate exhibits high catalytic performance for the simultaneous cationic polymerization and esterification of epoxy resin when anhydride is introduced as a co-hardener. The selective catalysis effect of polyoxometalate and the reaction mechanism was studied by differential scanning calorimetry (DSC), mid-infrared spectroscopy (MIR), near-infrared spectroscopy (NIR) and generalized two-dimensional correlation analysis. The cationic polymerization is the dominating reaction in neat epoxy systems. Increasing the amount of polyoxometalate and the polarity of the diluents fastens the curing rate of epoxy resin. Esterification was found to be the preferred reaction once anhydride was employed. When polyoxometalate was blocked by amine to form salt, it performs as an excellent catalyst for esterification in epoxy-anhydride systems. The epoxy materials catalyzed by polyoxometalate show quite good performance compared with ordinary epoxy resins. Moreover, thermal degradation analysis (TGA) shows that polyoxometalate could significantly decrease the thermal degradation temperatures of cured epoxy resins.     

Cationic polymerization of a cycloaliphatic diepoxide with latent initiators in the presence of structurally different diols

The influence of different polyether based diols on the photochemically and thermally induced cationic polymerization of a cycloaliphatic diepoxide is examined. If the polymeric diol is a 1,2-diol based polyether the polymerization is retarded compared to the pure diepoxide, whereas the polymerization is not influenced or even enhanced if the diol is a polyether based on a 1,4-diol or a low molecular weight 1,2-diol. The different behavior is explained by the complexation of protons in a crown ether like structure in the presence of the polyethers based on 1,2-diols. This leads to an effective reduction of the acid concentration available to initiate the polymerization. The model was verified by the addition of a small amount of 12-crown-4. In the presence of the crown ether the polymerization is also strongly retarded.     

Externally stimulated initiator systems for cationic polymerization

At present, there is growing interest in polymerizations initiated by external stimulation, such as photoirradiation, heating or high energy irradiation. Since many industrially important monomers can only be polymerized by a cationic mechanism, much effort has been made to develop systems that produce initiating cations upon external stimulation. In this article, the latest developments in externally stimulated systems for cationic polymerization are reviewed. The goal is, furthermore, to show differences as well as parallels between systems for different modes of external stimulation.     

Cationic polymerization of 2,4,6,8-tetramethylcyclotetrasiloxane processed by tuning the pH of the miniemulsion

The preparation of 2,4,6,8-tetramethylcyclotetrasiloxane () miniemulsions stabilized by commercial surfactants, i.e. sodium dodecylbenzenesulfonate (NaDBSA) and sodium laurate/lauric acid mixture, was studied and is first reported. Then, the pH was tuned in order to process polymerization through interfacial protonic initiation. With lauric acid, no polymerization was observed, even at high temperature or large acid contents. Such effect is ascribed to the poor dissociation and reactivity of high pKa's carboxylic acids. With NaDBSA, acidification by HCl was enough to initiate the polymerization, but the pH of the continuous phase was shown to have a strong effect on the polymerization process. Optimal conditions (at pH 5) gave chains with average number molar masses around 16,000 g mol⁻¹ and polydispersity index close to 1.6, for a total conversion in polymer of about 60% after 2 h reaction. Lower pH (4.2) quickly led to a cross-linked network whereas at higher pH (6.1), conversions were too slow (around 10% after 350 min).     

Self-healing epoxy based on cationic chain polymerization

A two-component healing agent consisting of epoxy- and ((C₂H₅)₂O·BF₃)-loaded microcapsules was synthesized and applied to fabricate self-healing epoxy composites. Curing of epoxy healing agent catalyzed by (C₂H₅)₂O·BF₃ belongs to cationic chain polymerization, which is characterized by fast reaction at ambient temperature and low catalyst concentration. The experimental results showed that cracks in the composites containing the above healing system can be quickly re-bonded with satisfied healing efficiency. In the case of 5 wt% epoxy- and 1 wt% (C₂H₅)₂O·BF₃-loaded capsules, for example, a ∼80% recovery of impact strength was detected within 30 min at 20 °C. Because the healing capsules took effect at low content, mechanical properties of the matrix were largely retained. It is believed that the present healing system provides a possible solution to preparation of self-healing polymeric materials with practical applicability.