Kinetics of radiation-induced grafting reactions. I. Polyethylene–styrene systems

By means of bromine labeling and ESR, the grafting reactions of styrene onto preirradiated polyethylene have been investigated. Not all the radicals produced by irradiations participate in grafting reactions all together, but they are rendered active bit by bit by the swelling of crystalline parts of polyethylene. The growing rates for polystyryl graft chains at 20°C decrease from 4 monomer units/active site/sec to one-fourth the initial value after 100 min. On the contrary, the average lifetimes increase from <10³ sec to >2.6 × 10³ sec. The number-average molecular weight of graft chains also increases with reaction times and rises to 3.5 × 10⁵ after 90 min at 20°C.     

Kinetics of radiation-induced graft copolymerization of styrene with ethyl acrylate

The radiation-induced graft copolymerization of styrene with ethyl acrylate onto preirradiated polyethylene powder was carried out at 20°C. The grafting yield decreased in the following order: ethyl acrylate ? styrene > styrene–ethyl acrylate mixture. On the other hand, the amount of absorption of liquid monomers in polyethylene powder decreased as follows: styrene > styrene–ethyl acrylate mixture > ethyl acrylate. By kinetic analysis of the grafting yield and amount of absorption of monomers it was elucidated that the value K_p/K_t in an ethyl acrylate system (7.7 × 10^?2) was much larger than those in styrene–ethyl acrylate systems and in a styrene system (ca. 1.0 × 10^?2).     

Monomer chain transfer in the copolymerization of styrene and butyl acrylate

Free radical chain transfer constants have been measured by copolymerizing styrene and butyl acrylate in emulsion at 60°C. Some improvements to this experimental technique are reported and estimates are given of the sensitivity of the calculated values to experimental uncertainties. Monomer chain transfer constants were found to be 1.2 × 10^?4 (styrene) and 2.5 × 10^?4 (butyl acrylate). The sum of the cross-transfer constants is 2.7 × 10^?4. The activation energy for chain transfer to styrene monomer is found to be 24 Kcal/mol in the 44°–60°C range.     

Free-radical polymerization of guanidine acrylate and methacrylate and the conformational behavior of growing chains in aqueous solutions

Monomer salts based on acrylic acids and guanidine—guanidine acrylate and methacrylate—have been synthesized, and the kinetic features of their free-radical polymerization in aqueous solutions have been studied. When polymerization is carried out in organic solvents (methanol, ethanol, or dioxane), the system is heterogeneous over the entire range of monomer concentrations. In aqueous solutions, the reaction systems are homogeneous only at small initial monomer concentrations (less than 1.30 and 0.40 mol/l for guanidine acrylate and methacrylate, respectively; the ammonium persulfate concentration is 5 × 10^?3 mol/l; pH ～ 6.5; 60°C). At higher concentrations, microheterogeneity appears from small conversions (～1%). This phenomenon is associated with the coiling of growing polymer chains owing to associative interactions between guanidine groups occurring in the monomer solution and carboxyl groups of (meth)acrylate polymer units. In aqueous solutions over the entire range of monomer concentrations (0.2–2.5 mol/l), the kinetic orders are the same as in the case of corresponding acrylic acids. The effects of composition of reaction solutions on changes in the initial rate of polymerization and the conformational behavior of the systems under study have been ascertained.     

Synthesis of block copolymers by the transformation of cationic polymerization into reversible atom transfer radical polymerization

Azo-containing polytetrahydrofuran (PTHF) obtained by cationic polymerization was used as a macroinitiator in the reverse atom transfer radical polymerization (RATRP) of styrene and methyl acrylate in conjunction with CuCl₂/2,2′-bipyridine as a catalyst. Diblock PTHF–polystyrene and PTHF–poly(methyl acrylate) were obtained after a two-step process. In the first step of the reaction, stable chlorine-end-capped PTHF was formed with the thermolysis of azo-linked PTHF at 65–70 °C in the presence of the catalyst. Heating the system at temperatures of 100–110 °C started the polymerization of the second monomer, which resulted in the formation of block copolymers. The decomposition behavior of the azo-linked PTHF and the structure of the block copolymers were determined by ¹H NMR and gel permeation chromatography (GPC). Kinetic studies and GPC analyses further confirmed the controlled/living nature of the RATRP initiated by the polymeric radicals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2199–2208, 2002     

Kinetic Study of Radical Polymerization VIII. A Comprehensive Study of Solution Copolymerization of Vinyl Acetate and Methyl Acrylate by 1H‐NMR Spectroscopy

Radical copolymerization reaction of vinyl acetate (VA) and methyl acrylate (MA) was performed in a solution of benzene‐d₆ using benzoyl peroxide (BPO) as the initiator at 60°C. Kinetic studies of this copolymerization reaction were investigated by on‐line ¹H‐NMR spectroscopy. Individual monomer conversions vs. reaction time, which was followed by this technique, were used to calculate the overall monomer conversion, as well as the monomer mixture and the copolymer compositions as a function of time. Monomer reactivity ratios were calculated by various linear and nonlinear terminal models and also by simplified penultimate model with r _2(VA)=0 at low and medium/high conversions. Overall rate coefficient of copolymerization was calculated from the overall monomer conversion vs. time data and k _p  . k _t ^?0.5 was then estimated. It was observed that k _p  . k _t ^?0.5 increases with increasing the mole fraction of MA in the initial feed, indicating the increase in the polymerization rate with increasing MA concentration in the initial monomer mixture. The effect of mole fraction of MA in the initial monomer mixture on the drifts in the monomer mixture and copolymer compositions with reaction progress was also evaluated experimentally and theoretically.     

Polymers from renewable resources. II. A study in the radio chemical grafting of poly(styrene‐alt‐maleic anhydride) onto cellulose extracted from pine needles

A binary mixture of styrene and maleic anhydride has been graft copolymerized onto cellulose extracted from Pinus roxburghii needles. The reaction was initiated with gamma rays in air by the simultaneous irradiation method. Graft copolymerization was studied under optimum conditions of total dose of radiation, amount of water, and molar concentration previously worked out for grafting styrene onto cellulose. Percentage of total conversion (Pg), grafting efficiency (%), percentage of grafting (Pg), and rates of polymerization (R_p), grafting (R_g), and homopolymerization (R_h) have been determined as a function of maleic anhydride concentration. The high degree of kinetic regularity and the linear dependence of the rate of polymerization on maleic anhydride concentration, along with the low and nearly constant rate of homopolymerization suggest that the monomers first form a complexomer which then polymerizes to form grafted chains with an alternating sequence. Grafting parameters and reaction rates achieve maximum values when the molar ratio of styrene to maleic anhydride is 1 : 1. Further evidence for the alternating monomer sequence is obtained from quantitatively evaluating the composition of the grafted chains from the FT‐IR spectra, in which the ratio of anhydride absorbance to aromatic (CC) absorbance for the stretching bands assigned to the grafted monomers remained constant and independent of the feed ratio of maleic anhydride to styrene. Thermal behaviour of the graft copolymers revealed that all graft copolymers exhibit single stage decomposition with characteristic transitions at 161–165°C and 290–300°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1763–1769, 1999     

Organometallic polymers. XVI. Copolymerization of styrenetricarbonylchromium and the reaction of polystyrene with chromium hexacarbonyl

Styrenetricarbonylchromium (IV) has been synthesized. Monomer IV did not homopolymerize with free-radical initiation but copolymerized with styrene, methyl acrylate, and vinylcymantrene. The copolymerizations were carried out in benzene solutions at 70°C with azobisisobutyronitrile as the initiator. The relative reactivity ratios were determined for the styrene and methyl acrylate copolymerizations. They were (defining M₁ as monomer IV) r₁ ? 0, r₂ ? 1.39 for styrene copolymerizations and r₁ ? 0, r₂ ? 0.75 for methyl acrylate copolymerization. Polystyrene reacted with chromium-hexacarbonyl in refluxing DME to produce a polymer in which about 32% of the benzene rings were complexed with ? Cr(CO)₃ units. The use of a polystyrene of narrow molecular weight distribution in this reaction demonstrated that no decomposition of the polystyrene chains occurred.     

Cationic graft copolymerization of polystyrene onto poly-2,6-dimethoxystyrene by the termination reaction

Poly-2,6-dimethoxystyrene (P26) was used as the polymeric backbone and was obtained by a new synthetic route involving the Wittig reaction. The grafting reaction was studied by polymerizing styrene with stannic chloride as catalyst in the presence of various concentrations of preformed P26 in solution with carbon tetrachloride and nitrobenzene at 60, 45, 30, and 0°C. The product was fractionated with cyclohexane and the fractions were analyzed by ultraviolet absorption spectroscopy and density-gradient ultracentrifugation. The molecular termination constant of P26 at 30°C was 0.106 much larger than the value of 0.016 which had been obtained for poly-p-methoxystyrene at 0°C. The amount of grafting and grafting efficiency generally increased with increasing temperature, contrary to results using the cationic initiation reaction. Anomalous results were obtained at 0°C as the product was no longer completely soluble. The ratio of grafted to ungrafted polystyrene chains was proportional to the concentration of P26 and generally increased with temperature. The grafting reaction was competitive with the normal termination reaction, and the energy of activation of the grafting reaction was estimated to be larger by ca. 7 kcal/mole.     

Grafting of styrene onto low molecular weight polymers and copolymers of butadiene

The grafting of styrene onto low molecular weight polybutadienes and butadiene–styrene co-polymers was studied. A mathematical method was used for the design of experiments and for the determination of the optimum grafting conditions with respect to the conversion of styrene and the efficiency of grafting. The reaction parameters were temperature (65–105°C), time (2–10 hr), concentration of the initiator, polymer to monomer ratio (10/90–90/10) and dilution by solvent (toluene). The optimum grafting conditions were chosen under which 50–60 wt-% of styrene was grafted onto backbone polymer at a high conversion of the monomer. It was found that the reactions producing graft copolymer prevailed over the styrene homopolymerization when the temperatures employed were lower (65–85°C), and the reaction time (8–10 hr), backbone polymer/monomer ratio, and the dilution by solvent were higher. The efficiency, density, and degree of grafting were found to increase with the increase in the molecular weight of the backbone polymer. The efficiencies and densities of grafting onto low molecular weight polybutedienes were higher than those of grafting onto low molecular weight butadiene–styrene copolymers. Grafting efficiencies and grafting densities were in the ranges 37.8–61.6 wt % and 0.06–0.26, respectively, in the studied range of number-average molecular weights (M?_n = 2400–6000).     

Radiation-induced grafting of styrene vapor to polyethylene

The radiation-induced grafting of styrene vapor to low-density polyethylene film of 0.063 mm thickness was studied at 23°C at a dose rate of 1.98 × 10⁴ rad/hr. The concentration C of monomer in the film was measured as a function of pre-irradiation exposure time to monomer vapor. The concentration-dependent diffusion coefficient of styrene in polyethylene was calculated to be 4.9 × 10^?9 exp {2.0C/C₀} cm²/sec, where C₀ is the saturation concentration of styrene in the film, and a linear boundary diffusion coefficient for styrene vapor into polyethylene film was found to be 2.0 × 10^?7 cm/sec. The rate of grafting was determined as a function of the concentration of styrene absorbed in the film. The maximum graft yield was obtained with an initial styrene concentration in the film of 4 wt-%. Under conditions of low initial monomer concentration, the grafting rate increases with irradiation time. The results are compared with previously published data on grafting of polyethylene from methanol–styrene solutions. They are explained in terms of the viscosity of the amorphous region as a function of styrene content and the resistance to the diffusion of monomer at the film–vapor interface.     

Introduction of 2-methoxycarbonylallyl end group by copolymerization of methyl α-(phenoxymethyl)acrylate accompanying with addition-fragmentation reaction

Copolymerizations of methyl α-(phenoxymethyl)acrylate (MPMA) with methyl acrylate, methyl methacrylate, styrene, and methyl α-ethylacrylate were carried out. Addition of a polymer radical to MPMA followed by the subsequent fragmentation of poly(MPMA) radical resulted in the 2-methoxycarbonylallyl end group and phenoxy radical in the course of the copolymerization. The extent of the fragmentation determined by ¹H-NMR spectroscopy depends on reactivity of the MPMA radical toward the reference monomers. An increase in the addition rate of the MPMA radical to the reference monomer brought about suppression of the fragmentation. The addition of the MPMA radical to styrene seems to be sufficiently fast to prevent the fragmentation. Since the rate of the fragmentation relative to the propagation was considerably accelerated by raising the temperature to 110°C, MPMA can be used as a novel chain transfer agent to control molecular weight and end group at a temperature above 100°C. © 1993 John Wiley & Sons, Inc.     

Emulsion copolymerization of styrene and methyl methacrylate

Chain transfer constants to monomer have been measured by an emulsion copolymerization technique at 44°C. The monomer transfer constant (ratio of transfer to propagation rate constants) is 1.9 × 10^?5 for styrene polymerization and 0.4 × 10^?5 for the methyl methacrylate reaction. Cross-transfer reactions are important in this system; the sum of the cross-transfer constants is 5.8 × 10^?5. Reactivity ratios measured in emulsion were r₁ (styrene) = 0.44, r₂ = 0.46. Those in bulk polymerizations were r₁ = 0.45, r₂ = 0.48. These sets of values are not significantly different. Monomer feed compcsition in the polymerizing particles is the same as in the monomer droplets in emulsion copolymerization, despite the higher water solubility of methyl methacrylate. The equilibrium monomer concentration in the particles in interval-2 emulsion polymerization was constant and independent of monomer feed composition for feeds containing 0.25–1.0 mole fraction styrene. Radical concentration is estimated to go through a minimum with increasing methyl methacrylate content in the feed. Rates of copolymerization can be calculated a priori when the concentrations of monomers in the polymer particles are known.     

Characterization and Evaluation of Thermal,Morphological, and Physicochemical Properties of Chemically Modified Lignocellulosic Biomass

A novel redox system, ascorbic acid-hydrogen peroxide, was employed to initiate graft copolymerization of ethyl acrylate and methyl methacrylate binary monomer mixtures onto Abelmoschus esculentus fibers at a temperature of 45°C for 90 min in an aqueous medium. Factors affecting grafting such as feed molarity and comonomer composition were investigated. Contrary to the lower affinity of methyl methacrylate for grafting on Abelmoschus fibers, a synergistic effect of ethyl acrylate on methyl methacrylate was observed when graft copolymers were prepared using different feed compositions (f_MMA). The percentage of grafting increased from 40.2% to 89.74% at 0.4 mole fraction of f_MMA. The graft copolymers were characterized by FT-IR, TGA, and SEM techniques.     

GRAFT COPOLYMERIZATION OF METHYL ACRYLATE ONTO CHITOSAN INITIATED BY POTASSIUM DIPERIODATONICKELATE (IV)

ABSTRACT

A novel redox system, potassium diperiodatonickelate [Ni (IV)]‐chitosan, was employed to initiate the graft copolymerization of methyl acrylate (MA) onto chitosan in alkali aqueous solution. The effects of reaction variables such as monomer concentration, initiator concentration, reaction time, pH and temperature were determined. By means of a series of copolymerization, the grafting conditions were optimized. The maximum grafting percentage obtained was 404.1% when 0.3 g chitosan was copolymerized with 1.8 mL monomer at 35°C for 5 hours with [Ni (IV)]=9.4×10^?4 M and the total volume was 20 mL. Ni (IV)-chitosan system is found to be an efficient redox initiator for this graft copolymerization. A single electron transfer mechanism is proposed to explain the formation of radicals and the initiation. The grafted copolymers were characterized by IR and X-ray diffraction diagrams. The thermal stability of chitosan and chitosan-g-PMA was studied by thermogravimetric analysis (TGA).     

Amphiphilic polymer gel electrolytes. I. preparation of gels based on poly(ethylene oxide) graft copolymers containing different ionophobic groups

Amphiphilic graft copolymers were prepared via the radical copolymerization of poly(ethylene oxide) (PEO) macromonomers with fluorocarbon or hydrocarbon acrylates in toluene with 2,2′‐azobisisobutyronitrile (AIBN) as an initiator. ¹H NMR spectroscopy confirmed that the composition of the graft copolymers corresponded well to the monomer feed. For gel electrolytes prepared from the amphiphilic copolymers, the nature of the ionophobic parts of the amphiphilic graft copolymers had a great influence on the ion conductivity. Gel electrolytes based on graft copolymers containing fluorocarbon side chains showed significantly higher ion conductivity than electrolytes based on graft copolymers containing hydrocarbon groups. The ambient‐temperature ion conductivity was about 2.6 mS/cm at 20 °C for a gel electrolyte based on an amphiphilic graft copolymer consisting of an acrylate backbone carrying PEO and fluorocarbon side chains. Corresponding gels based on graft copolymers with PEO side chains and hydrocarbon groups showed an ambient‐temperature ion conductivity of about 1.2 mS/cm. The gel electrolytes contained 30 wt % copolymer and 70 wt % 1 M LiPF₆ in an ethylene carbonate/γ‐butyrolactone (2/1 w/w) mixture. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2223–2232, 2001     

Thermal stability and high glass transition temperature of 4-chloromethyl styrene polymers bearing carbazolyl moieties

A new styrene derivative monomer, 4-(N-carbazolyl)methyl styrene (CzMS), was synthesized by reacting 4-chloromethyl styrene with carbazole in the presence of sodium hydride. Then, CzMS was homopolymerized and copolymerized with different monomers such as methyl methacrylate (MMA), ethyl methacrylate (EMA), methyl acrylate (MA), ethyl acrylate (EA) and n-butyl acrylate (BA) by free radical polymerization method in N,N-di-methylformamide (DMF) solution at 70 ± 1 °C using azobisisobutyronitrile initiator to give the copolymers I-V in good yields. The structure of all the resulted polymers was characterized and confirmed by FT-IR, ¹H NMR and ¹³C NMR spectroscopic techniques. The average molecular weight and glass transition temperature of polymers were determined using gel permeation chromatograph (GPC) and differential scanning calorimeter (DSC) instruments, respectively. It was found that these polymers with carbazole moieties have high thermal stability and the presence of bulk carbazole groups in polymer side chains leads to an increase in the rigidity and glass transition temperature of polymers.     

Synthesis of branched polystyrenes via cationic polymerisation

Branched polystyrenes have been prepared rapidly (within 15 min) and in good yields (40-99%) in dichloromethane solution at 0 °C via cationic copolymerisation of styrene (St) with divinylbenzene (DVB) using SnCl₄ as the initiator. All reaction components were deliberately used as supplied to evaluate whether such a simple approach could provide a facile synthesis of branched polymers and this has proved to be the case. The only additional experimental precaution was to avoid condensation of atmospheric moisture during the reactions. No additional chain regulating species was required to avoid crosslinking providing the St/DVB mole feed ratio was ?100/5. The intrinsic chain transfer to monomer reaction seems to be sufficient to reduce the length of the primary polymer chains and hence inhibit crosslinking and gelation in the case of the above mole feed ratios. The branching architecture of the products has been evaluated by ¹H NMR and MALS-SEC analyses.     

60Co-initiated copolymerization of styrene with vinyl acetate

The kinetic behavior of the ⁶⁰Co-initiated copolymerization at 25°C of styrene with vinyl acetate at 1100 and 2000 rad/hr was studied. As in the case of thermal and photochemical copolymerizations of these monomers, the growing chains are particularly rich in styrene units, and the overall rate is affected by a diluent effect due to the vinyl acetate monomer. However, in the case of the radiation copolymerization, this effect is partially counterbalanced by an increase of the initiation rate with the vinyl acetate concentration; the polymerization rate curve shows a maximum at a vinyl acetate molar fraction of 0.25. This effect is due to the very different free radical yields of these two monomers. The experimental results may be understood on the basis of a kinetic scheme which involves an energy transfer process from the excited vinyl acetate molecules to the styrene monomer and a termination reaction of the growing chains by very short styrene radicals when the mixture is rich in vinyl acetate.     

以双硫酯为链转移剂的活性自由基聚合

合成并研究了两种双硫酯链转移剂的纯化方法 ,进行了多种单体以双硫酯为链转移剂的活性自由基聚合及嵌段共聚 .发现以PhC(S)SC(CH3) 2 Ph为链转移剂的效果比PhC(S)SCH(CH3)Ph好 ,聚合产物的多分散性系数较小 .引发剂与链转移剂的摩尔数比为 1∶3 5～ 1∶4 2时 ,得到多分散性系数小 ,实测分子量与理论分子量相近的聚合产物 .聚合物的分子量随时间和转化率的增加而增加 ,加入第二单体形成嵌段共聚物 ,具有活性聚合特征 .聚甲基丙烯酸酯大分子引发剂引发丙烯酸酯单体聚合时 ,聚合速度最快 .