Polymerization initiated by the redox system hydrogensulfite/oxygen, 1. Polymerization of acrylic acid

The polymerization of acrylic acid in water by the redox initiation system hydrogensulfite/oxygen was found to proceed fast to give oligomers in high yield, in spite of introducing a large amount of oxygen known as a strong radical inhibitor. In the polymerization system, the concentration of oxygen was kept at a very low level on account of the rapid reaction of hydrogensulfite with oxygen. At high concentration of hydrogensulfite, both hydrogensulfite oxidation rate (R_ox) and polymerization rate (R_p) vary with the flow rate of air. R_p is proportional to the square root of R_ox, suggesting that the termination is a bimolecular reaction, a radical intermediate formed from hydrogensulfite oxidation being the initiator of this polymerization system. Hydrogen was found to behave as a chain transfer agent, and its chain transfer constant is 0,15 at pH 6,9 and 25°C.     

Chain transfer polymerization of methyl methacrylate initiated by organolanthanide complexes

A novel chain transfer polymerization mediated by Cp*₂ Sm(III) species and organic acids is described. The chain transfer polymerization involves the reaction of organic acids such as thiols or ketones with an active bond between samarium(III) and the enolate at a living chain end of poly(methyl methacrylate) (PMMA). This chain transfer reaction resulted in termination of the living chain end and the regeneration of the active initiator which would consist of (C₅Me₅)₂Sm(III) and deprotonated organic acids. The chain transfer polymerization were confirmed by turnover numbers (TON). tert‐Butyl thiol exhibited the chain transfer reactivity effectively to control the molecular weight of PMMA without decreasing of the polymer yield and the stereoregularity. As a result of this chain transfer polymerization, thermal and optical properties of the PMMA obtained were improved by the control of chain end groups or by reducing a large amount of the samarium initiator.     

Molar mass control of poly(N-isopropylacrylamide) and poly(acrylic acid) in aqueous polymerizations initiated by redox initiators based on persulfates

Poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) were synthesized by aqueous radical polymerization initiated by the redox couple ammonium persulfate/sodium metabisulfite. PNIPAM was also synthesized by using a similar redox system: ammonium persulfate/2-aminoethanethiol hydrochloride (AET), the reducing agent being in this case a well known chain transfer agent. The final products were characterized by size exclusion chromatography. The influence on the weight-average molar mass (M̄_w) of several factors, such as monomer concentration and ionic strength of the reaction solution, were investigated. It was found that the concentration of ammonium persulfate does not practically affect the M̄_w of the products for both polymers investigated and independently of the reducing agent used. On the contrary, for constant monomer concentration, M̄_w is inversely proportional to the concentration of the reducing agent, either sodium metabisulfite or AET. This behaviour, combined with similar observations concerning the synthesis of polyacrylamide and other redox initiator systems containing metabisulfite as reducing agent, proves the special role of this reducing agent in the aqueous radical polymerizations.     

Online Monitoring of Chain Transfer in Free‐Radical Polymerization

A recently introduced, automated method for online monitoring of polymerization reactions was used to study free‐radical transfer reactions. The persulfate initiated polymerization of acrylamide (AAm) in water was chosen as the test system. Chain transfer properties of ethanol (EtOH) and propanol (PrOH) were investigated. Different methods of computing the transfer constant are compared, including those based on the slope and intercept behavior of the monitored cumulative weight‐average molecular mass as a function of conversion, M̄_w (f), the reduced viscosity, and corresponding size exclusion chromatography analysis of the reaction end products. To a close approximation, the chain transfer agents were found to obey the form expected when ideal free‐radical polymerization takes place and radical transfer from propagating radicals to the chain transfer agent (CTA) is slower than from the CTA to monomer, that is, the polymer molar mass decreases with increasing chain transfer agent, but there is no appreciable effect on the kinetics of monomer conversion. The AAm kinetics were characterized in terms of the ratio k_p² /k_t, where k_p and k_t are the propagation and termination rate constants, respectively.     

Polymerization kinetics of HEMA/DEGDMA: using changes in initiation and chain transfer rates to explore the effects of chain-length-dependent termination

The effect of kinetic chain length and chain transfer on the polymerization kinetics and network structure in polymerizations of loosely crosslinked 2-hydroxyethyl methacrylate/di(ethylene glycol) dimethacrylate mixtures was explored. Polymerization behavior of the monomer mixture in the presence and absence of a chain transfer agent was monitored at various initiation rates and chain transfer agent concentration levels. Dependence of the polymerization rate on the initiation rate was found to deviate from the classical square-root relationship because of chain-length-dependent termination. This effect was further confirmed by addition of a chain transfer agent. The presence of a chain transfer agent led to the formation of shorter kinetic chains, which enhanced termination and slowed the polymerization. Investigation of the polymerization kinetics after cessation of irradiation yielded kt/kp[M] values for both systems. Prior to the onset of reaction diffusion-controlled termination, the system that included a chain transfer agent exhibited much higher kt/kp[M] values than the polymerization system without added chain transfer agent. In addition, the onset of reaction diffusion-controlled termination was delayed to higher conversions in the system containing chain transfer agent. The impact of a chain transfer agent on the polymerization behavior and kinetics demonstrates that the chain-length-dependent termination phenomenon is indeed important and must be considered in kinetic modeling of loosely crosslinked systems.     

Studies on polymerizations initiated by syncatalytic systems based on aluminium organic compounds, 7. Reaction mechanisms

The polymerizations of isobutene and its copolymerizations with isoprene by syncatalytic systems consisting of an aluminium compound R₂AlX (with R = alkyl and X = alkyl or halogen) and of a halogen or an interhalogen compound are initiated by the addition of a positive halogen ion to the monomer. Chlorine is the best co-initiator. Whereas in this syncatalyzed polymerization of isobutene the initiation is relatively slow and monomer transfer and termination are unimportant, in the polymerization of isobutene by AlCl₃ or EtAlCl₂ the initiation is very fast and there is at least one efficient termination. Our interpretation of these results includes a discussion of the different initiations and chain-breaking reactions. In the syncatalyzed polymerizations the initiation rate is governed by the relatively slow interaction of initiator and co-initiator. Since the termination reactions are relatively unimportant, second and subsequent portions of monomer polymerize completely and faster than the first. The polymerizations initiated by AlCl₃ or EtAlCl₂ stop at incomplete conversion because of a termination which is chlorination of the carbenium ions by the anions; the AlCl₃ thus formed is complexed by the unreacted monomer.     

Hydroxytelechelic polybutadiene, 13. Microstructure,hydroxyl functionality and mechanisms of the radical polymerization of butadiene by H2O2

The mechanism of the radical polymerization of butadiene with H₂O₂ has been clarified by indepth examination of very low M?_n hydroxytelechelic polybutadiene (HTPB) synthesized with high concentration of H₂O₂. For the well-known R45M®-HTPB^1,2, only primary alcohol functions were found, and most of them originate from initiation steps (termination mechanisms only proceed by coupling of macroradicals); in low M?_n HTPB (HTPB-C), the presence of a secondary alcohol function suggests termination reactions by coupling between macroradicals and primary hydroxyl radicals. Being more branched, HTPB-C has a larger hydroxyl content, and also a larger amount of epoxidized 1,4-units. Finally, some lower alcohols, utilized for homogenizing H₂O₂ with butadiene, are also transfer agents giving rise to ether groups.     

Photopolymerization of methyl methacrylate in bulk and in pyridine diluted systems with pyridine-sulfur dioxide complex alone and in combination with benzoyl peroxide as photoinitiators

It was found that the Py-SO₂ complex (4–140 mmol·1^?1) initiates the photopolymerization of methyl methacrylate (MMA) in bulk at 40°C, the initiator exponent, k/k_t, and the monomer exponent being 0,30, 1,27·10^?2 1·mol^?1·s^?1, and 1,00, respectively. The radical generation is followed by an in-situ initiator-monomer complexation reaction. The kinetics of the photopolymerization of MMA were also studied using the (Py-SO₂)-complex (0,01–0,35 mol·1^?1) in redox combination with benzoyl peroxide (BZ₂O₂) (2 ? 110 mmol·1^?1) as initiator in the presence of variable concentrations of pyridine (1 ? 7 mol·1^?1). The initiation depends on the concentrations of all the reactants in the system, and the reaction order with respect to either component of the redox pair is equal to 0,5 or reasonably less than 0,5, depending on whether or not the initiating component in question is present in the system in excess compared to its counterpart in the redox combination. Variable monomer exponents, largely dependent on [BZ₂O₂], characterize the redox photopolymerization process. The monomer exponent value, unity at [BZ₂O₂] = 0, progressively decreases with increasing [BZ₂O₂] (non-ideal kinetics). The rate enhancing effect of the solvent pyridine is more prominent at higher [BZ₂O₂]. The kinetic nonidealities in each case were further analyzed and interpreted in terms of (a) primary radical termination and (b) degradative initiator transfer. Relative effects of the variation of the level of dilution with pyridine were also analyzed.     

Über die elementarreaktionen der radikalischen polymerisation von benzylmethacrylat

The influence of various solvents on the rate constants of chain growth and chain termination of benzyl methacrylate has been determined. The absolute rate constants were obtained by combination of stationary and nonstationary methods (rotating sector). The initiator efficiency f depends on the viscosity of the monomer solvent system and on the type of monomer. f is nearly independent of the monomer concentration and of the initiation rate. From that one may conclude that a considerable proportion of the twin recombination reactions takes place outside the primary cage formed by the solvent. The rate constant of chain growth k_w is nearly identical to k_w of methyl methacrylate and shows a slight increase with increasing solvent viscosity. This has been found in the case of methyl methacrylate too. The rate constant of chain termination k_a increases less than proportionally to the reciprocal value of the viscosity of the monomer solvent system. A possible correlation to the internal viscosity is discussed.     

Polymerization of methyl methacrylate initiated by manganese(III) acetate

The kinetics of thermal polymerization of methyl methacrylate (MMA) initiated by manganese(III) in sulfuric acid is studied in the temperature range between 40 and 607deg;C. The rate of monomer consumption is found to be dependent on [MMA]², [H⁺]^?1 but independent of [Mn³⁺]. The rate of manganic ion consumption is directly proportional to [MMA], [Mn³⁺] and inversely proportional to [H⁺]. The chain length is directly proportional to [MMA] and inversely proportional to [Mn³⁺] and [H⁺]. A plausible reaction mechanism with initiation by Mn³⁺ OH^? species and termination by the interaction of Mn³⁺ with the polymer radical is proposed.     

Cationic polymerizations initiated by high energy radiation

Ionizing radiations generate both free radicals and ions upon interaction with matter and under appropriate conditions either free radical or ionic polymerizations are observed. Cationic polymerizations are favored by the presence of halogenated solvents and by low reaction temperatures. Selective inhibitors and copolymerization studies make it possible to determine the relative proportions of the cationic and the free radical contributions to the overall process under a variety of experimental conditions (temperature, reaction medium, radiation dose-rate). Pure, carefully dehydrated hydrocarbon monomers undergo cationic polymerization when irradiated at room temperature. This reaction proceeds at a very high rate and is believed to involve free propagating cations. Propagation rate constants are available for the following monomers: styrene (k_p = 3,5·10⁶ at 15°C), α-methylstyrene (k_p = 4.10⁶ at 0°C), cyclopentadiene (k_p = 6.10⁸ at ?78°C), isobutene (k_p = 1,5·10⁸ at 0°C), and isobutyl vinyl ether (k_p = 3.10⁵ at 30°C). The present views on the mechanism of radiation induced cationic chain initiation and chain propagation are briefly discussed.     

Kinetics of the free radical copolymerization of diethyl maleate with methyl methacrylate

The kinetics of the free radical copolymerization of diethyl maleate (M₁) with methyl methacrylate (M₂) was studied in bulk at 60°C with azoisobutyronitrile (AIBN) as initiator. The reactivity ratios of the system were determined by means of ¹H NMR spectroscopy. The obtained values show that diethyl maleate is not only unable to homopolymerize (r₁₂ = 0) but also display a very low tendency to incorporate in the polymer chain (r₂₁ = 370) under the present conditions. Molecular weight measurements showed that a normal, although very slow growth reaction involving diethyl maleate takes place. Therefore, chain transfer could be neglected in the kinetic analysis. The dependence of the initiation rate constant on the monomer feed was analysed by means of the inhibition method. An equation for describing the observed dependence of polymerization rate on feed composition was derived taking into account the influence of solution viscosity on the termination rate constant (diffusion theory).     

Phosphorus functional telomers of vinyl polymers through functional thermal iniferter

The phosphorus-functional iniferter, N,N′-dimethyl-N,N′-bis[2-(N-diethoxyphosphinoyl-N-methylamino)ethyl]thiuram disulfide ( 2 ), was synthesized and characterized. Thermal polymerization of methyl methacrylate (MMA) and styrene in presence of this compound led to the formation of α, ω-phosphorylated PMMA and polystyrene. The kinetics of polymerizations of MMA and styrene using this iniferter was studied at 70°C and the different parameters, related to initiation, chain transfer, and primary radical termination reactions, were evaluated. In the case of styrene the extent of primary radical termination was considerably less and mutual termination reactions were found to be absent. The telomers were analysed for their functionality.     

Studies on polymerizations initiated by syncatalytic systems based on aluminium organic compounds, 6. Comparison with isobutene polymerizations initiated by ethylaluminium dichloride or aluminium trichloride

The results of previous studies carried out on the polymerization of isobutene initiated by diethylaluminium halides plus halogens are compared with results obtained with other syncatalytic systems such as diethylaluminium chloride (or triethylaluminium) plus hydrochloric acid (or tert-butyl chloride), as well as with results obtained with aluminium compounds such as ethylaluminium dichloride or aluminium trichloride which do not require co-initiators to initiate the polymerization of isobutene. The kinetic data indicate clearly that the polymerization of isobutene initiated by syncatalytic systems is characterized by a relatively slow initiation and by the absence of important termination reactions and of transfer with monomer. In contrast to this, the polymerizations initiated by strong Lewis acids such as ethylaluminium dichloride or aluminium trichloride show a much faster initiation and stop at incomplete conversion which indicates that at least one efficient termination reaction is operative. It is also demonstrated that this termination reaction produces at least one substance which acts as a co-initiator for the isobutene polymerization initiated by diethylaluminium halides or triethylaluminium.     

Vinyl polymerization. 186. Polymerization of methyl methacrylate initiated by 9-anthryldiazomethane

A study of the polymerization of methyl methacrylate initiated by 9-anthryldiazomethane (ADM) has been made. The initial rate of polymerization, R_p, followed the equation: This equation may be interpreted by the termination reaction of the growing radical with the primary radical. The rate of decomposition of ADM was measured in bulk or in benzene solution and the following rate equation was obtained:      

Polymerization of acrylamide in aqueous medium initiated with a redox system composed of cysteine and potassium bromate

Aqueous polymerization of acrylamide initiated with a redox initiator system composed by cysteine and potassium bromate was investigated as a model of the redox initiator system composed of poly(vinyl alcohol) having a thiol groups at one end and potassium bromate. The rate of polymerization was proportional to almost the square root of cysteine and potassium bromate concentration, indicating the initiation system is a usual free radical polymerization initiator. On the other hand, the rate was proportional to the 1.8th power of the monomer concentration. The activation energy of the rate of polymerization was about 5 kcal · mol^?1, indicating also a typical redox initiation system. Chain transfer constant of polyacrylamide radical to cysteine was calculated to be 0.4. From these results, it is concluded that cysteine exerts two important roles: (1) as a reductant of the redox initiation and (2) as a powerful chain transfer reagent.     

Polymerization of vinyl monomers in the presence of the ethylaluminium dichloride-p-chloranil catalytic system

Polymerizations of acrylonitrile, methacrylonitrile, methyl acrylate, and methyl methacrylate initiated by the system ethylaluminium dichloride (EtAlCl₂)-p-chloranil (PClA) were studied. The effect of the catalyst system components on the initiation, propagation and termination steps is discussed in terms of a free radical reaction mechanism. The catalytic system EtAlCl₂-PClA behaved similarly in the copolymerization of MMA with styrene as in the free radical homopolymerization of MMA. Polymerizations of vinyl monomers, not forming relatively stable complexes with EtAlCl₂ (in contrast to acrylic and methacrylic monomers), such as vinyl chloride, styrene and isobutylene, were also studied in the presence of the EtAlCl₂-PClA system.     

Free radical synthesis of α,ω-primary amino functionalized polyisoprene through the functional thermal inferter [bis(N-(2-phthalimidoethyl)piperazine)]thiuram disulfide

The primary amino functional iniferter [bis(N(2-phthalimidoethyl)piperazine)]thiuram disulfide (PEPTD) was synthesied and characterized. Thermal polymerization of isoprene in the presence of the initiator afforded α,ω-functionalized polyisoprene with primary amino end-groups after cleavage of the phthalimido group with butylamine. The kinetics of polymerization of isoprene using this iniferter was studied at 80, 85, 90 and 100°C. Interestingly, isoprene showed a complex kinetic behavior. The different parameters, related to initiation, chain transfer and primary radical termination reactions, as well as the constants for thermal decomposition of the initiator, were determined. From the Arrhenius plot, the activation energy of the overall decomposition constant of PEPTD (2 f k_d) was calculated to be 110 kJ/mol. The functionality of the telechelics was examined by GPC.     

Télomérisation en milieu aqueux, 1. Télomérisation de l'acide acrylique avec l'acide thioglycolique

The telomerization of acrylic acid with thioglycolic acid (mercaptoacetic acid) in aqueous medium, with various initiators (H₂O₂,Na₂S₂O₈,K₈ thermal initiation and 2,2′-azobis (2-amidiniopropane) dihydrochloride) has been achieved. Thus, we were able to point out the limitation of the chain length due to the transfer agent, which is characteristic of a telomerization reaction. Mono-, di-and tri-adducts were isolated and characterized after esterification of the reaction medium with ethanol, then hydrolized. These various adducts were used as standards for size-exclusion chromatography, and the influence of the reaction parameters (initiator, temperature, quality of solvent, amount of telogen was investigated). We also determined the transfer constant of thioglycolic acid (C_T = 0,47). It is worth to note that knowing this value, it is possible to control precisely the number -average degree of polymerization of this type of reaction. This allowed us to prepare telomers with a 50% final weight concentration in the dry extract, as it was recommended for their use in paper industry.     

Benzil/triethylamine combination as photoinitiator for methyl methacrylate polymerization

Photopolymerization of methyl methacrylate (MMA) in bulk and in solution was studied using the combination benzil (BZL)/triethylamine (TEA) as photoinitiator. The initiation was found to occur through photoreduction of the ketone in presence of an amine via exciplex formation. A kinetic study of the polymerization showed that both carbonyl groups of benzil are involved in exciplex interaction, resulting in diradical formation. An analysis of the initiator dependent termination process revealed that primary radical termination is predominant in the particular system compared to the degradative initiator transfer. A kinetic scheme was designed to account for the various wastage reactions occurring in the polymerization system.