Influence of monomer structure on chemo- and stereoselectivity of 1,3-diene polymerization

MAO/CpTiCl₃ is an active catalyst for the polymerization of various types of 1,3-dienes. Butadiene, (E) - and (Z) −1,3-pentadiene, (E) −2-methyl-1,3-pentadiene and 2,3-dimethylbutadiene yield, at room temperature, polymers with a cis-1,4 or a mixed cis/1,2 structure. 4-Methyl-1,3-pentadiene and (E,E) −2,4-hexadiene give, respectively, a 1,2 syndiotactic and a trans-1,4/1,2 polymer. MAO/CpTiCl₂·2THF and MAO/(CpTiCl₂)_n are less active than the CpTiCl₃ catalyst, but give the same type of polymers. A change of stereospecificity with temperature was observed in the polymerization of (Z)-1,3-pentadiene: a cis-1,4 isotactic polymer was obtained at +20°C, and a crystalline 1,2 syndiotactic polymer at −20°C. This effect was attributed to a different mode of coordination of the monomer, which is cis-η⁴ at +20°C and may be trans-η² at −20°C. Results obtained with catalysts from CpTi(OBu)₃ and Ti(OBu)₄ are reported for comparison. An interpretation is given of the formation of cis-1,4 isotactic poly(2-methylpentadiene) and of 1,2 syndiotactic poly(4-methylpentadiene), as well as of syndiotactic polystyrene.     

Cationic polymerization of dienes VIII: Is the elimination of cross-linking by a bulky electron donor a general behavior in the presence of aluminium trichloride?

Previous works on the polymerization of 1,3-pentadiene initiated by aluminium trichloride in non polar solvent at room temperature in the presence of bulky electron donor (ED) as tri-p-tolylamine have highlighted a stabilization of the polymerizing actives centres by ED, which allowed a reduction of some side reactions and the formation of more precisely defined polypentadienes than ever by cationic polymerization in non polar medium. The aim of this research was to investigate the role of bulky EDs such as tri-p-tolylamine and similar compounds in polar medium in order to obtain if possible a complete control of the polymerization of isoprene and 1,3-pentadiene. The beneficial effect of tri-p-tolylamine was shown in the case of isoprene polymerization at room temperature, with an important reduction of the cross-linked fraction for long reaction times and strong reduction of termination reactions. At −30 °C in the presence of tri-p-tolylamine, polypentadienes more controlled than in non polar solvent could be obtained, with a nearly complete elimination of the cross-linked fraction, while keeping the microstructure approximately constant.     

Polymers and telomers of perfluoro-1,4-pentadiene

Perfluoro-1,4-pentadiene was prepared by pyrolysis of sodium 3,5,6-trichloroperfluorohexanoate and subsequent dehalogenation. The monomer was polymerized under a number of conditions of temperature and pressure using various periods and various dose rates of γ-ray initiation. The compound was found to undergo doublebond migration producing perfluoro-1,3-pentadiene. The polymer samples, some of them rubbery, others grainy, are assumed to be copolymers of the 1,4-pentadiene and the 1,3 pentadiene, the 1,4-diene polymerizing according to a cyclic mechanism and the 1,3-diene undergoing the 1,4-addition that is characteristic of butadienes under certain conditions. Infrared and nuclear magnetic resonance studies tend to support these assumptions. Under polymerization conditions the monomer also produced four distinct dimers and traces of two monomeric substances.     

On the termination reaction in the anionic polymerization of methyl methacrylate in polar solvents—I. Kinetic studies

The kinetics of the termination reaction in the anionic polymerization of methyl methacrylate, using monofunctional initiators with sodium as the counterion in the presence of excess NaB(C₆H₅)₄ have been studied. For monitoring the living-end concentration, a labelling technique was used. Tetrahydrofuran (THF) and tetrahydropyran (THP) were used as solvents. The rate constants for the propagation reaction in THF are smaller than those in THP by a factor of two. The kinetic investigation of the termination shows that only a fraction of chains becomes terminated. This fraction, as well as the rate of termination, is dependent on the initial monomer concentration. None of the reaction mechanisms discussed in the literature (i.e. termination by ester groups of the polymer or of the monomer) is able to explain these results. A new mechanism for the termination is proposed and verified; it is based on the assumption that a deactivating species is formed in the initial step of the polymerization. This species reacts with the living polymers during polymerization, leading to termination in a second-order reaction. The Arrhenius plot of the termination rate constant is linear (activation energy E_u = 48 kJ/mol; frequency exponent A = 13) in both solvents. The initial concentration of the deactivating species shows a linear dependence on the initial concentration of both the monomer and the living ends.     

Chemoselectivity in 4-methyl-1,3-pentadiene polymerization in the presence of homogeneous Ti-based catalysts

The homogeneous catalyst CpTiCl₃-MAO is able to produce a random copolymer of 4-methyl-1,3-pentadiene with ethylene. ¹³C NMR analysis of the copolymers shows that after insertion of ethylene units, the next 4-ymethyl-1,3-pentadiene unit can be inserted in either 1,2 or 1,4 arrangement. The high chemoselectivity observed in the 1,2-syndiotactic homopolymerization of 4-methyl-1,3-pentadiene with respect to the lower one observed in the copolymerization with ethylene is attributed to a back-biting coordination to the Ti of the active species of the penultimate monomer unit of the growing chain.     

Polymerization of 1,P-Epoxides Initiated by Tetraalkyl Aluminates. I. Polymerization of Ethylene Oxide in the Presence of Sodium Tetrabutyl Aluminate

The polymerization of ethylene oxide (EO) initiated by NaA1Bu₄ is shown to proceed upon initial complex formation between monomer and initiator. In polymerization in toluene a high order of the kinetic equation with respect to initiator was found, indicating that chain propagation proceeds on dimers and trimers of the active center. An induction time of polymerization in THF is observed. It is necessary to reach a specific concentration of the NaAIBu₄.EO complexes which take part in the polymerization process. The wide molecular weight distribution, the high effectivity coefficient (initiation efficiency), and the polymerization rate increase with polymer yield are evidence of a polycentric polymerization mechanism.     

Polymerization of acrylamide and methacrylamide photoinitiated by azidopentamminecobalt(III) chloride

The kinetics of polymerization of acrylamide and methacrylamide, photoinitiated by azidopentamminecobalt(III) chloride in homogeneous aqueous acid medium was studied systematically. Monochromatic wavelengths 365, 405, and 435 mμ were employed for irradiation. Polymerization proceeded without any induction period, and the reaction was followed by measurements of rate of monomer disappearance (bromometrically), rate of complex disappearance (spectrophotometrically), and the chain lengths of the polymer formed (viscometrically). The dependences of the rate of polymerization on variables like light intensity, light absorption fraction by the complex, wavelength, monomer concentration, hydrogen ion concentration, nature of the acid used (HClO₄, HNO₃, and H₂SO₄), etc., were studied. The rate of polymerization of acrylamide depended on the unit power of monomer concentration and on the square root of light absorption fraction k_ε and light intensity I. The rate of methacrylamide polymerization was proportional to the unit power of monomer concentration and fractional powers of 0.25 and 0.30 of k_ε and I, respectively. A kinetic reaction scheme is proposed and discussed in the light of the experimental results, and it has been concluded that (1) the primary photochemical act is an electron transfer reaction from the azide ion to Co(III) in the complex, (2) initiation of polymerization is by azide radical, (3) termination is by mutual destruction of polymer radicals.     

Polymerization of acrylonitrile with VCl4–Al(C2H5)3 catalyst system in presence of acetonitrile

The polymerization of acrylonitrile with the homogeneous catalyst system of VCl₄–AlEt₃ in acetonitrile at 40°C has been investigated. The rate of polymerization is found to be first-order with respect to monomer and inversely proportional to the catalyst concentration. The overall activation energy for this catalyst system is 10.97 kcal/mole. The inverse proportionality of rate of polymerization with the catalyst concentration is attributed to the permanent complex formation between the catalyst complex and acrylonitrile, and a reaction scheme is proposed.     

Equilibrated radical polymerization of acrylic propionic anhydride as a mono-ene counterpart of acrylic anhydride

Radical polymerization of acrylic propionic anhydride (APA) as a mono-ene counterpart of acrylic anhydride (AA) was investigated in order to obtain supporting evidence for a previously proposed mechanism based on polymerization equilibrium for the cyclopolymerization of AA. The deviation from first-order dependence of the rate of polymerization R_p on monomer concentration was enhanced by a decrease in monomer concentration. The Arrhenius plot of lnR_p vs. 1/T was clearly curved in a highly dilute system. These results suggest increased significance of the depropagation reaction equilibrated with propagation under polymerization conditions favorable to five-membered ring formation in cyclopolymerization of AA. Under these conditions, the disproportionation reaction of APA became important and the liberated AA was incorporated into the polymer chain through copolymerization with APA, resulting in poly(APA-co-AA) having five-membered anhydride units.     

Effect of the nature of a solvent on pseudoliving free-radical polymerization of styrene mediated by TEMPO

The kinetics and mechanism of the pseudoliving free-radical polymerization of styrene are studied for the first time under the conditions of reversible inhibition by TEMPO nitroxides. An abnormal decline in the reduced rate of polymerization, which is inconsistent with a decrease in the concentration of the monomer, and an increase in the steady-state concentration of the free nitroxide are discovered. The main quantitative characteristics of the pseudoliving process are determined, namely, the rate constants of reinitiation and reversible recombination, and the constant of pseudoliving equilibrium between dormant and growing chains. It is shown that the polarity of a medium and the concentration of the monomer determine the character of polymerization in a solution: With an increase in the polarity of the solvent, the pseudoliving equilibrium constant increases, the reduced rate of styrene polymerization decreases, and the molecularmass-distribution of the polymer formed at initial conversions narrows. The smaller the concentration of styrene in the reaction system, the more pronounced the above differences associated with the solvent nature.     

On the mechanism of formation of isotactic and syndiotactic polydiolefins

The mode of formation of isotactic and syndiotactic polymers from 1,3-dienes is examined in the light of the most recent results. An interpretation is given for the formation of trans-1,4 isotactic polymers from CH₂=CH-CH=CHR (R = Me, Et, Pr, etc.) type monomers with heterogeneous VCl₃-based catalysts. Evidence is reported showing that stereoregular 1,2 or cis-1,4 polymers derive from a growing polymer chain anti-η³-bonded to the transition metal and a cis-η⁴ coordinated monomer. The influence on stereoselectivity of the substituents at the central carbon atoms of the monomer is discussed. The peculiar behavior of (Z)-1,3-pentadiene and 4-methyl-1,3-pentadiene, which give 1,2 polymers with catalysts that give 1,4 polymers from other monomers, is attributable to the fact that they can coordinate trans-η², in addition to cis-η⁴.     

Sterically hindered active centres in cationic polymerization: Tertiary aromatic amines in the polymerization of 1,3-pentadiene initiated by aluminum trichloride in non-polar solvent

Previous work on the polymerization of 1,3-pentadiene initiated by aluminum trichloride (AlCl₃) in non-polar solvent at 20 °C in the presence of triphenylamine (NPh₃) has highlighted a stabilization of the active centres and a solubilization of AlCl₃ induced by NPh₃ [Delfour M, Bennevault-Celton V, Anh Nguyen H, Cheradame H, Macedo A. Macromol Chem Phys 2004;205:2312-26]. This paper reports the effect of 9-phenylcarbazole, the structure of which is close to that of NPh₃. The results showed a strong decrease of the insoluble fraction (IF), however this electron donor (ED) was a transfer agent alike NPh₃ (alkylation of aromatic rings by the growing polymer chains). To reduce transfer reaction, the influence of two para-substituted triphenylamines—tribromophenylamine (N(PhBr)₃) and tri-p-tolylamine (N(PhCH₃)₃)—on the polymerization of 1,3-pentadiene was studied in the same conditions. Although the presence of Br substituents has no effect on IF, it induced a decrease in the termination reactions with the counter-ion and the transfer reactions. Concerning methyl groups, it has been shown a strong decrease of IF and a negligible amount of alkylation reaction. Moreover the quantity of grafted molecules was reduced and tri-p-tolylamine reduced both the reactions of cyclization and isomerization. Thanks to the stabilization of the active centres by N(PhCH₃)₃ and probably the solubilization of aluminum trichloride, it was possible to obtain a more precisely defined polypentadiene than ever by cationic polymerization in a non-polar medium.     

Silver and palladium nanoparticles-containing products of the low-temperature wave conversion of an energetic material: Catalytic activity in piperylene hydrogenation

The hydrogenation of 1,3-pentadiene into pentenes over the commercial 0.5% Pd/Al₂O₃ catalyst and over a new catalyst containing 1.0% Pd and 3.7% Ag (μ-catalyst) has been investigated. The new catalyst has been prepared via the flameless wave conversion of cyclotrimethylenetrinitramine in a porous composite. The catalytic properties of the new composite in the hydrogenation reaction depend on the hydrogen/1,3-pentadiene ratio and on the catalyst activation temperature. The reaction conditions for selective 1,3-pentadiene hydrogenation have been optimized. The pentenes yield as a function of temperature passes through a maximum at any H₂/C₅H₈ ratio between 1 and 2. The 2-pentene/1-pentene ratio in the reaction products increases as the temperature is raised.     

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate

Particle formation and coagulation in the seeded semibatch emulsion polymerization of butyl acrylate were studied under monomer‐starved conditions. To investigate the importance of the kinetics of the water phase in the nucleation process, the monomer feed rate was used as a variable to alter the monomer concentration in the aqueous phase. The emulsifier concentration in the feed was employed to alter the particle stability. Particle formation and coagulation were discussed in terms of critical surface coverage ratios. Particle coagulation occurred if the particle surface coverage dropped below θ_cr1 = 0.25 ± 0.05. The secondary nucleation occurred above a critical surface coverage of θ_cr2 = 0.55 ± 0.05. The number of particles remained approximately constant if the particle surface coverage was within θ_cr1 = 0.25 < θ < θ_cr2 = 0.55. This surface coverage band is equivalent to the surface tension band of 42.50 ± 5.0 dyne/cm that is required to avoid particle formation and coagulation in the course of polymerization. The kinetics of the water phase was shown to play an important role during homogeneous and micellar nucleations. For any fixed emulsifier concentration in the feed and above θ_cr2, the number of secondary particles increased with monomer concentration in the aqueous phase. Moreover, the presence of micelles in the reaction vessel is not the only perquisite for micellar nucleation to occur, a sufficient amount of monomer should be present in the aqueous phase to enhance the radical capture by partially monomer‐swollen micelles. The rate of polymerization increased with the surfactant concentration in the aqueous phase. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3612–3630, 2000     

Dispersion polymerization of styrene in ethanol: Monomer partitioning behavior and locus of polymerization

A thermodynamic model has been proposed for the simulation of monomer partitioning behavior in the dispersion polymerization of styrene in ethanol. The monomer concentration in the polymer particles is very low (20 vol% at 5% conversion) and decreases further as the polymerization proceeds. It is independent of stabilizer concentration but is strongly dependent on initial monomer concentration. The partitio n coefficient ([M_p]/[M_c]) of styrene increases from 0.8 to 1.1 with incresing conversion. There are two polymerization loci in dispersion polymerization, namely the continuous and polymer phases. Competition between solution and heterogeneous polymerization has been observed in this system. The rate of dispersion polymerization is dependent on initial monomer concentration but is independent of initiator concentration at higher conversions. The molecular weight of the polymers produced by this process increases with increasing conversion and decreases with increasing initiator concentration.     

Cationic polymerization of 1,3-pent adiene in the presence of arenes

The cationic polymerizations of 1,3-pentadiene initiated by AlCl3 in n-hexane at 30℃ have been carried out in the presence of various arenes,i.e.,benzene,toluene,p-xylene,o-xylene,m-xylene and mesitylene.The presence of all these arenes have reduced in different degrees the formation of crossliuked products.Namely,the crosslinking reaction,a major side-reaction during the cationic polymerization of 1,3-pentadiene,has been suppressed by adding the aromatic compounds.The results showed that a chain transfer to arene took place and this transfer process hindered the generation of the crosslinked polymer.IR and 1H NMR spectra have confirmed the existence of the corresponding aryl groups in the resulting polymers.     

UV光引发的丙烯酰胺反相乳液聚合

报道了不透明丙烯酰胺反相乳液体系的UV光引发聚合新方法 .使用普通中压汞灯并辅以适当搅拌 ,UV光引发丙烯酰胺 水 煤油 Span80 +OP 10反相乳液聚合可在 2 0min左右完成 ,所得聚合物分子量达千万 ;聚合过程中不存在恒速期 ,扫描电镜未观察到聚合前后乳胶粒径有数量级的变化 ,表明聚合反应以单体液滴成核为主 .此外 ,考察了光引发剂类型及浓度、单体浓度、乳化剂用量、反应温度等对聚合反应的影响 ,结果表明不同光引发剂的引发活性为Irgacure 2 95 9>(ITX +EDAB) >BDK ,引发剂浓度增加 ,反应速度先增加而后降低 ,存在一最大值 ;单体浓度增加 ,反应速度加快 ,聚合物分子量提高 ;乳化剂用量增加 ,反应速度加快而分子量变化不明显 ;聚合表观活化能为 13 34kJ mol.     

Kinetics of dispersion polymerization: Effect of medium composition

The effect of the medium composition (monomer and solvent) on the kinetics of dispersion polymerization of methyl methacrylate (MMA) was studied via reaction calorimetry. It was found that increasing the monomer concentration increased the reaction rate; the exponent of the dependency of the initial reaction rate on the MMA concentration was found to be 0.93. Narrow particle size distributions were achieved at the lower monomer concentrations (0.24–0.81 mol/L) and a minimum size (2.45 μm) was found at an intermediate concentration (0.44 mol/L). The average molecular weight of the PMMA increased and the molecular weight distribution broadened with increasing monomer concentration. During a dispersion polymerization, the MMA concentration was found to decrease linearly with conversion in both phases, whereas the ratio of concentrations in the particles and continuous phase ([M]_p/[M]_c) remained constant (0.47) with partitioning favoring the continuous phase. The average number of free radicals per particle in MMA dispersion polymerization was estimated to be high from the nucleation stage onward (>5000). The increasing rate during the first ～ 40% conversion was primarily caused by the increasing volume of the polymer particle phase. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3638–3647, 2008     

Synthesis of cationic poly-1,3-pentadiene under the action of Gustavson complex

Polymerization of 1,3-pentadiene was performed using the Gustavson complex (AlCl₂.xylene.0.5HCl) at a nearly full conversion of the monomer. The possibility of control over molecular characteristics of the polymer was demonstrated. Recommendations for application of the poly-1,3-pentadiene synthesized are given.