Monomolecular Termination Research Articles

Effects of silane precursors on curing behavior of UV-curable hybrid coatings

This study demonstrates the effect of some commonly used silane precursors on curing behavior and photopolymerization kinetics of UV-curable hybrid coatings. Tetraethoxyorthosilane (TEOS), vinyltrimethoxysilane (VTMS), and 3-methacryloxypropyltrimethoxysilane (MPS) were polymerized under acidic conditions by the sol–gel method. The resulting structure of the tested siloxane oligomers was characterized using 29Si-NMR and 1H-NMR spectroscopy. Photo-DSC was used to evaluate photocuring enthalpy. A fully integrated model was applied to investigate propagation rate coefficients and to identify the most effective termination mechanism (monomolecular, bimolecular, or mixed). Results showed that oligomers containing MPS effectively promoted total heat of the polymerization process and consequently affected the maximum rate of polymerization and final double bond conversion. Enthalpy rating for samples containing MPS was higher than that of those containing VTMS, showing that MPS was more compatible with the acrylate systems. In addition, evaluations for photopolymerization kinetic constants proved that non-functional oligomers containing TEOS caused radical trapping and increased activity of the monomolecular termination mechanism.

Kinetic behavior of photo-polymerization of UV-curable resins with carboxylic acid and amino groups

Photo-polymerization behaviors of bisphenol-A epoxy diacrylate (EPA) and six kinds of EPA-derived resins containing different amounts of carboxylic acid, urethane, amide, and imide groups were studied by a photo differential scanning calorimetry. The dark polymerization was performed and pseudo-steady state assumption of growing radicals was made to obtain the kinetic constants for propagation, bimolecular termination, monomolecular termination, and the concentration of growing radicals of different resins as a function of extent of reaction. Compared with EPA, it was found that the rate of polymerization and kinetic constants of the six resins were relatively small because the mobility of reacting species in resins was restricted by carboxylic acid, urethane, amide, and imide groups. Finally, three different photo-initiators were used to initiate the polymerization, and their kinetic behaviors were compared. The effect of tertiary amine group of photo-initiator on the rate of polymerization of resins having carboxylic acid group and the initiator efficiency were discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Exposure Reciprocity Law in Photopolymerization of Multi‐Functional Acrylates and Methacrylates

AbstractThe applicability of the exposure reciprocity law (ERL) to the photopolymerization of acrylates and methacrylates was investigated. It was found that when the oligomer/monomer mass ratio in the systems studied was equal to or greater than 6:4, the ERL applied quite well. The applicability of the ERL to individual (meth)acrylates was judged by comparing the ratio of the lowest DC under one of the three I‐t combinations to the highest DC under another I‐t combination. The ERL is deemed to apply better to a monomer whose ratio is close to unity. It was found that the viscosity and the number of double bonds are two primary attributes. Photopolymerization of those with more than one double bond and with viscosity higher than 100‐200 mPa · s closely obeys the ERL. This is because the two factors promote the realization of the reaction‐diffusion‐controlled process, which facilitates monomolecular termination of free radicals.magnified image

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.

Photoinduced Cross-Linking Polymerization in the Presence of Aromatic Thiols

The aim of this work was to find the effect of two aromatic thiols (one heteroaromatic and one aromatic), efficient co-initiators in two-component photoinitiating systems, on the termination mechanism of a photoinduced polymerization occurring in stationary and nonstationary conditions. This task has been approached by modeling using two termination models: the bimolecular termination model and the mixed termination model. For the first time the influence of thiols was considered separately on the monomolecular and bimolecular termination. The effect of the thiols consisting in the increasing contribution of the second-order termination was assumed to reside in higher values of the bimolecular termination rate coefficient (ktb) in comparing to polymerization in the absence of thiol. On the other hand, the decreasing contribution of the first-order termination (considered as a significant decrease in the mobility and/or reactivity of radical sites) was assumed to be involved in lower values of the monomolecular termination rate coefficient (ktm). It was found that the presence of the thiol increases the contribution of the bimolecular termination and decreases the contribution of the monomolecular termination at the early polymerization stages. Monomolecular termination becomes less important as the contribution of the reaction diffusion in bimolecular termination increases.

Tentative study on kinetics of bulk polymerization of methyl methacrylate in presence of montmorillonite

AbstractThe kinetics of the bulk polymerization of methyl methacrylate (MMA) in the presence of montmorillonite (MMT) were studied. The effect of MMT on the radical polymerization of MMA was researched by determining the polymerization rate dilatometrically. It was assumed that there were both bimolecular and monomolecular termination processes involved in the termination of the radicals in the polymerization. It was found that a lower benzoyl peroxide (BPO) concentration promotes a higher fraction of monomolecular mode in chain termination. The results show that there is an optimal ratio of MMT to initiator that increases the bulk polymerization rate of MMA. The X‐ray results show that the layer structure of the formed PMMA–MMT composites was also affected by the BPO concentration. With lower initiator concentration, less pronounced layer structure will be observed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3690–3695, 2003

Termination mechanism during the photo-induced radical cross-linking polymerization in the presence and absence of oxygen

The objective of this study was to characterize changes in the termination mechanism and in the rate coefficients during a light-induced polymerization occurring under continuous illumination and during the after-effect (after the irradiation has been discontinued) in the presence or in the absence of oxygen. The illumination was stopped at various double bond conversions. The polymerization of a model monomer-(diethylene glycol)dimethacrylate was analyzed using three termination models: monomolecular (I), bimolecular (II) and mixed (III) and the following parameters were estimated: kmt and kp[P˙]0 (Model I), kbt[P˙]0 and kp[P˙]0 (Model II) and kmt, kbt[P˙]0 and kp[P˙]0 (Model III), where kmt is the monomolecular termination rate coefficient, kbt is the bimolecular termination rate coefficient, kp is propagation rate coefficient and [P˙]0 denotes macroradical concentration at the beginning of the dark reaction. The parameters of the models were averaged over increasingly large dark conversion. Multistage statistical analysis of the results showed that over the conversion range studied, the bimolecular reaction was the main way of termination during continuous illumination, but during the after-effect termination mechanism changed with time of dark reaction from the bimolecular to the mixed one indicating an increasing importance of radical trapping. The obtained results suggest that oxygen accelerates the bimolecular termination, but pseudo first-order reaction with oxygen also contributes to the overall termination process. Moreover, the postpolymerization is more sensitive to negative influence of oxygen than the polymerization under continuous irradiation.

Rate coefficients as a function of time during the after-effect of a photoinduced crosslinking polymerization

The polymerization of a dimethacrylate monomer occurring in the dark after the initiating light has been switched off (so-called after-effect) was modeled by assuming of two different termination mechanisms: monomolecular(Model 1) and bimolecular (Model 2). Time dependent monomolecular termination (k t m ) and bimolecular termination (k t b ) rate coefficients and a time independent propagation rate coefficient kp are used in the models. Both k t m and k t b are polynomial functions of time. A very important feature of the newly developed models is that free terms of polynomials correspond to values of k t m (Model 1) or kp[P'] 0 (Model 2) at the moment of stopping the irradiation (t d a r k = 0; [P'] 0 constitute macroradical concentrations at td a r k = 0). Those values can be considered as describing the process during continuous irradiation. Calculations were performed for eight photoinduced polymerization processes, in which initiation had been interrupted at various degrees of double bond conversions (denoted as starting conversions). It was found that the conversion dependence of k t b lP'] 0 and k t m parameters extrapolated to t d a r k = 0 appearing in two various termination models was similar.

Variations of Rate Coefficients and Termination Mechanism During the After-Effects of a Light- Induced Polymerization of a Dimethacrylate Monomer

This work was aimed at studying variations in the termination mechanism occurring during the after-effects of a light-induced polymerization of a dimethacrylate monomer after the irradiation had been discontinued. The experimental method was based on differential scanning calorimetry. The initiation was stopped at various moments of the reaction corresponding to different degrees of double-bond conversion (starting conversions). Three termination models: monomolecular, bimolecular, and mixed were used to calculate the ratio of the bimolecular termination and propagation rate coefficients ktb/kp and/or the monomolecular termination rate coefficient ktm. The models were determined over short time intervals (conversion increments) of the dark reaction giving different values of rate coefficients for each time interval (interval approximation method). Two-stage statistical analysis was used to find the model that best reproduced the experimental data obtained for each conversion increment. This enabled variations in the termination mechanism during the after-effects to be followed. It was found that the termination mechanism changed with the time of the dark reaction from the bimolecular reaction to the mixed reaction when the light was cut off at low and medium double-bond conversions. At higher starting conversions a monomolecular termination mechanism dominated from the beginning of the dark reaction. The mixed termination model was the only model to describe correctly the variations of rate coefficients in the dark, i. e., the increase in ktm and the decreasein the ktb/kp ratio.

Modelling of the butadiene and isoprene polymerization processes with a binary neodymium-based catalyst

In the present work we study the mechanism of the polymerization processes of isoprene and butadiene with the NdCl3·3TBP–TIBA catalyst system (TBP=tributylphosphate, TIBA=triisobutylaluminium) using two mathematical models: one which takes into account a monomolecular termination and the other which considers that no termination reactions occur during the polymerization process. Both for isoprene and butadiene, by comparing with the experimental data, the best results were achieved from the equations derived in the frame of the polymerization model considering the absence of the termination reactions. The product between the rate constant of the propagation reaction and the active centres concentration depends mainly on the [Al]/[Nd] molar ratio used which is an argument for the participation of TIBA in the formation of the active centres.

Kinetics of redox polymerizations of acrylic acid in inverse dispersion and in aqueous solution

Redox polymerizations of acrylic acid in inverse dispersion and in aqueous solution (with surfactant) were conducted by using sodium metabisulphite/potassium bromate initiators. The monomer conversions were determined by using high-performance liquid chromatography, and the polymer particles in the final lattices were examined using a scanning electron microscope with freeze-fracture equipment. Experimental rate expressions implied that complex reactions are involved in the redox polymerizations. A chemical reaction scheme was proposed, and kinetic models were developed for the redox polymerization in aqueous solution. Comparison between the experimental rate expressions and the kinetic models suggested a combination of bimolecular and monomolecular termination modes, a chain transfer function of the surfactant, and an oxidizing role of the oxygen molecules. The differences in the experimental rate expressions between the redox polymerization in inverse dispersion and that in aqueous solution are in good agreement with the kinetic model predictions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 313–324, 1999

Monomolecular and bimolecular termination in the polymerization of di(meth)acrylates

AbstractThe termination mechanism in the polymerization of a series of analogous di(meth)‐acrylates differing only by the type of the heteroatom (S or O) in the ester group was investigated. The experimental method was based on differential scanning calorimetry measurements of photochemically initiated polymerization. The ratio of the bimolecular termination and propagation rate constants k/kp and/or the monomolecular termination rate constant k were determined according to three termination models (monomolecular, bimolecular, and mixed). Statistical techniques were used for choosing between the models to find out by which mechanism (monomolecular, bimolecular or mixed) the termination occurs. The results obtained prove than monomolecular reaction is an important termination way in the polymerization of multifunctional monomers and must be taken into account when kinetic constants are determined, even at low degree of conversion. The sulfur‐containing monomers are characterized by much lower k values than the analogous sulfur‐free monomers.

ESR study of the molecular dynamic of peroxyradicals in the post‐irradiation oxidation of isotactic polypropylene

AbstractBy adopting the modified Bloch equations, the analysis of the motion of macroperoxyradicals during the post‐irradiation oxidation of isotactic polypropylene at room temperature has been performed. Rotations about the chain axis and cubic jumps are the motion of prominent importance in the early stages of reaction involving mostly radicals trapped at defects or in amorphous phase; on the other hand rotations about the CO bonds at decreasing rates are dominant in the latter stages. Clear evidence of the progressive immobilization of peroxyradicals, which may be reckoned with monomolecular termination mechanism and of the trapping of peroxyradicals in the crystalline phase, have been obtained.

Preparation and reactivity of metal-containing monomers. Communication 3. Radiation graft polymerization of Co(II), Ni(II), and Cu(II) acrylates on polyethylene

1. Graft polymerization of Co(II), Ni(II), and Cu(II) acrylates was carried out on polyethylene powder subjected to prior irradiation in the air and graft copolymers were obtained with up to 1.4 mass % metal content. 2. The graft polymerization of metal acrylates on polyethylene proceeds 10 times more slowly than in the case of acrylic acid with virtually the complete absence of thermal homopolymerization. The graft rate is independent of the concentration of the metal-containing monomer in the range from 1 to 10 mass %. 3. The graft polymerization of acrylates is initiated by radicals formed as a result of the decomposition of the hydroperoxide groups with subsequent predominantly bimolecular termination of the growing polymer chains in the case of Co(II) and Ni(II) acrylates and monomolecular termination in the case of Cu(II) acrylate. The effective activation energies for the graft polymerization reactions were determined.

Electron beam curing of acrylic oligomers

The electron-beam curing process of acrylic oligomers, with and without γ-Fe 2O 3 pigment filler and blended linear polymer, was investigated in terms of molecular structure and reaction mechanism. The polymerized fraction of trimethylolpropane-triacrylate (TMPTA) oligomers increases with increasingly total dose, and is independent of the dose rate. Since the reaction rate is linearly dependent on the dose rate, the reaction mechanism involves monomolecular termination. The reaction rate does not depend on the number of functional groups of the oligomer at low doses, but above 0.3 Mrad the rate is slower for oligomers of higher functionality. A gel is formed more readily upon curing of a polyfunctional than a monofunctional oligomer, especially at high conversion to polymer; the resulting loss of flexibility of the polymer chains slows the reaction. Decrease of the molecular weight per functional group results in lower conversion; this is also due to the loss of chain flexibility, which is indicated as well by a higher glass-transition temperature. Modification of the acrylate oligomers with urethane results in more effective cross-linking reactions because of the more rigid molecular chains. Addition of γ-Fe 2O 3 pigment reduces the reaction rate very little, but has the effect of providing added structural integrity, as indicated by the decrease of solvent-extractable material and the improvement of anti-abrasion properties. However, the flexibility of the coating and its adhesion to a PET base film are diminished. To increase the flexibility, linear polyvinylchloride and/or polyurethane were added to the acrylic oligomers. Final conversion to polymer was nearly 100 per cent, and a higher elastic modulus and better anti-abrasion properties were realized.

Photopolymérisation de macromères multifonctionnels—I. Étude Cinétique

Highly crosslinked polymers have been obtained by photopolymerization of multiacrylate macromers under intense u.v. or laser irradiation. The marked inhibitory effect of O 2 was quantitatively evaluated by a kinetic study of these ultra-fast reactions by i.r. spectroscopy. The rate of polmerization ( R p) and the amount of residual unsaturation in the cured polymer were shown to depend primarily on the nature of the macromer chain and on the functionality of the monomer used as diluent. Despite the high rate of initiation, polymerization develops effectively, with large quantum yields: θ p = 8300 polymerized units per photon absorbed for irradiations in N 2. The linear relationship between R p and light-intensity strongly suggests a polymerization mechanism based on monomolecular termination.

Copolymerization of acrylic acid to nylon-6 by trapped radicals—II: Liquid phase grafting and properties of copolymers

Initial rates of grafting, R g, have been determined for film of nylon-6, which had been -,,-irradiated in vacuo and subsequently immersed in outgassed, aqueous, acrylic acid at 50'. Equilibrium swellings due to water and to monomer are attained rapidly by nylon-6 and give rise to a high R g. During grafting a constant swelling due to water occurs immediately but the rate of uptake of monomer is less than R g, These effects are discussed on the basis of compatibility. At a fixed equilibrium monomer concentration, [AA], in the film, R g ∝ D β where D is the dose and β = 0.94. At a fixed dose, R g ∝ [AA] α where α = 1.04. The value of β indicates mainly monomolecular termination by radical burial. This is postulated to occur by trapping of growing grafts in crystalline regions and/or transfer from them to such regions. By selectively dyeing and sectioning films of different degrees of grafting (DG), the progress of the grafting front inwards has been shown to follow closely the corresponding grafting curve of DG vs time. Grafting does not alter the moisture regain of nylon. Surface resistance and swelling in water change with DG, the effect being enhanced markedly by converting the grafts to their sodium salts.

Polymerization of Oxetanes with Organoaluminium Catalysts. I. Kinetics of the Polymerization of 3-Methyl-3-chloromethyloxetane with the i-Bu3Al–H2O Catalyst System

The kinetics of the polymerization of 3-methyl-3-chloromethyloxetane with the i-Bu3Al–H2O catalyst system and the molecular-weight characteristics of the resulting polymer have been investigated. The main feature of this process is the ability for self-regulation of the size of the macromolecules with the formation of a polymer of high molecular weight (∼106) and narrow molecular-weight distribution (Mw/Mn=1.30±0.05) which do not depend on conversion and initial monomer and catalyst concentrations. A kinetic scheme for the polymerization involves a slow initiation stage, rapid propagation via the preliminary stage of monomer complexation with active centres, spontaneous monomolecular termination as a result of intramolecular interaction of the active centre with a fragment of its own polymer chain and the inhibition stage due to the deactivation of the unconsumed catalyst by the polymer. The mathematical description of this scheme enabled us to calculate the rate constants for elementary reactions of the process at 20°C: the propagation-rate constant is (1.9±0.3)×103 min−1; the initiation-rate constant is (6.9±0.3)×104 l mol−1 min−1 and the termination-rate constant is (0.16±0.02) min−1.

A quantitative description of the low temperature radical post-polymerization in the system butyl methacrylate-zinc chloride

A study has been made of the low temperature radical post-polymerization by a “living chains” mechanism in the system butyl methacrylate (BMA)ZnCl 2 and in BMA without additives. A theoretical analysis of kinetic regularities of the polymerization process has been carried out for the system BMAZnCl 2 (mole fraction of ZnCl 2 = 0·37). According to the kinetic model presented in the paper, the process may be subdivided into two stages. In the first stage of the process, i.e. from the onset of defrosting to the establishment of a constant concentration of propagation radicals in the system, account was taken of the lower diffusion controlled bimolecular termination rate constant, when the length of the radicals is increased. It is proposed that during the second stage of the process, when bimolecular termination of radicals does not occur at all, a major role is played by monomolecular termination (“disappearance” of radicals) whereby some of the radicals are “lost” at high degrees of conversion. In terms of the model, good agreement is obtainable between theoretically calculated and experimentally determined molecular weights and polydispersity coefficients P w/ P ̄ n for polybutylmethacrylate obtained in the polymerization process. Theoretical proof of the widening of MWD of the polymer observed at high degrees of conversion has been obtained. On the basis of the proposed model one can explain a possible “survival” of varying amounts of propagation radicals at the start of the second stage of the process, the amount depending on the rate of heating of the system, as was found on a previous occasion in a study of the post polymerization of pure (wtihout any additives) butyl methacrylate.

The kinetics of 3-methyl-, 3-chloromethyl-oxetane polymerization on the catalyst system Al(iso-C 4H 9) 3-H 2O and the molecular weight characteristics of polymethylchloromethyloxetane

The kinetics of 3-methyl-, 3-chloromethyl-oxetane polymerization have been investigated on the catalyst system Al(iso-C 4H 9) 3−H 2O⩾2·0 as well as the molecular weight characteristics of the resulting polymer. It is shown that the molecular weight and polydispersity of the polymer do not depend on the degree of polymerization or on the initial monomer and catalyst concentrations. The molecular weight reaches high values (∼6–8 × 10 −5) in the earliest stages of polymerization, and does mot vary up to the very end of the process. The molecular weight distribution is relatively narrow; the Mw / Mn ratio is equal to ·29 ± 0·01, and likewise does not vary with conversion. The experimental values of Mn) exceed the calculated ones by a factor of 600–700, which points to a very low degree of catalyst utilization in initiation steps. The polymer formed inhibits the process as a result of interaction with catalyst. The kinetic scheme of polymerization proposed in the light of the data obtained involves four elementary stages: slow initiation, rapid propagation, spontaneous monomolecular termination and the inhibiting stage.