Polymerization of vinyl chloride in the presence of precipitants. II.

The kinetics of the radiation-induced polymerization of vinyl chloride in the presence of precipitants has been successfully described by a one-parameter equation as follows, where ?₀ is initial monomer volume fraction, X is conversion, t is time, and k is reaction constant. The equation was confirmed for extensive conditions of temperatures and monomer concentrations in the case of polymerization in methanol. The degree of polymerization was related with the reaction constant k, initial monomer volume fraction ?₀, monomer chain transfer constant C_m, conversion X, and the initiation rate I as follows, The factors which determine the value of the reaction constant k were elucidated through measurements of the reaction constant k and the degree of polymerization DP _n.     

Crystallization of Poly(ε‐caprolactone)/Poly(vinyl chloride) Miscible Blends Under Strain: The Role of Molecular Weight

Summary: The effect of poly(ε‐caprolactone) (PCL) molecular weight on the orientation of crystalline PCL in miscible poly(ε‐caprolactone)/poly(vinyl chloride) (PCL/PVC) blends, melt crystallized under strain, has been studied by a combination of wide angle X‐ray diffraction (WAXD) and small angle X‐ray scattering (SAXS) studies. An unusual crystal orientation with the b‐axis parallel to the stretching direction was observed in miscible PCL/PVC blends with PCL of high molecular weight (>21 000). SAXS showed the presence of nanosize confined PCL in the PCL/PVC blends, which could be preserved at temperatures higher than the T_m of PCL but lower than the T_g of PVC. A mechanism based on the confinement of PCL crystal growth was proposed, which can explain the formation of b‐axis orientation in PCL/PVC blends crystallized under strain.

SAXS pattern of stretched PCL/PVC blend after annealing at 90 °C for 5 min.     

Kinetics of some reactions of vinyl and isopropyl radicals in the gas phase

Vinyl and isopropyl radicals were generated by the pyrolysis of azoisopropane in the presence of acrolein at 473–563 K. Reaction products were analyzed by gas chromatography. Rate constant ratios k₂/k₁ = 0.02 ± 0.01 and k₄/k₃ = 0.01 ± 0.005 are suggested for the following reactions: The rate constant ratio of reactions (7) and (c) obeys the Arrhenius equation The Arrhenius equation was derived for (k₈ + k₉).     

The kinetics of the gas phase decomposition reactions of the hexafluoro-t-butoxy radical

Hexafluoro-t-butoxy radicals have been generated by reacting fluorine with hexafluoro-2-methyl isopropanol: Over the temperature range of 406–600 K the hexafluoro-t-butoxy radical decomposes exclusively by loss of a CF₃ radical [reaction (-2)] rather than by loss of a CH₃ radical [reaction (-1)]: (1) The limits of detectability of the product CF₃COCF₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_-1 of ～80. The implications of this finding in relation to the reverse radical addition reactions to the carbonyl group are briefly discussed. A thermochemical kinetic calculation reveals a discrepancy in the kinetics and thermodynamics of the decomposition and formation reactions of the related t-butoxy radical:      

Laser Single Pulse Initiated RAFT Polymerization for Assessing Chain‐Length Dependent Radical Termination Kinetics

Summary: A novel method combining RAFT polymerization with pulsed‐laser initiation for determining chain‐length dependent termination rate coefficients, k_t, is presented. Degenerative chain‐transfer in RAFT enables single‐pulse pulsed‐laser polymerization (SP‐PLP) traces to be measured on systems with a narrow radical distribution that remains essentially unchanged during the experiment. SP‐PLP‐RAFT experiments at different polymerization times allow for determining k_t as a function of chain length via classical kinetics assuming chain‐length independent k_t.

Single‐pulse pulsed‐laser polymerization trace for BMPT‐mediated RAFT polymerization of butyl acrylate.     

Extended Chemical CrossLinking of a Thermoplastic Polyimide: Macroscopic and Microscopic Property Development

Polyimides are well established as gas separation membranes due to their intrinsically low free volume and correspondingly high H₂ selectivity relative to other gases such as CO₂. Prior studies have established that H₂/CO₂ selectivity can be improved by crosslinking polyimides with diamines differing in spacer length. In this work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long crosslinking times (t_x) with 1,3‐diaminopropane. According to spectroscopic analysis, the crosslinking reaction saturates after ≈24 h, whereas tensile, nanoindentation and stress relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases with increasing t_x. Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing t_x, permeation studies indicate that the permeabilities of H₂ and CO₂ decrease, while the H₂/CO₂ selectivity increases markedly, with increasing t_x. At long t_x, the polyimide becomes impermeable to CO₂, suggesting that it could be used as a barrier material.

     

Kinetic Simulations of Atom Transfer Radical Polymerization (ATRP) in Light of Chain Length Dependent Termination

Kinetic simulations using the composite k_t model allows a better understanding of the effects of the persistent radical affecting ATRP or for that matter any activation–deactivation system. It also provides a better fit to experimental data in either bulk or solution conditions for ATRP polymerizations carried out at 110 °C. The results suggest that the composite model has broad utility over a wide range of experimental conditions and temperatures. The advantage of incorporating an accurate k_t model is that one can then use simulations as predictive tool to obtain polymers with higher chain‐end fidelity or polymers with low PDI values. This becomes important when attempting to use the chain‐ends for further functionalization to make complex polymer architectures. This model can also be used in simulations of miniemulsion or seeded emulsions to determine the effect of compartmentalization with particle size.

     

Rheology of poly(vinyl chloride) melts. II. Shear rate-dependent properties

Four samples containing 40, 60, 80, and 97 wt-% of poly(vinyl chloride), the rest being plasticizer and stabilizer, were tested by using the Weissenberg Rheogoniometer in the steady-shearing mode at temperatures between 155 and 235°C and rates of shear \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma = 0.01 - 400 $\end{document} sec^?1. The viscosity η versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} follows Graessley's theoretical dependence for infinitely entangled system. The primary normal-stress difference coefficient ψ versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} is well described by the same theoretical function, used with the square of its argument. The temperature dependence of η₀ and ψ₀ shows discontinuities at T = T_b. The numerical values of T_b can be calculated from the theory of the melting point depression due to diluent. The activation energy of viscous flow E_η below T_b is 5–9 times as large as above this temperature. The activation energy of normal stress is found to be Eψ ≈ 5E_η. The characteristic relaxation times τ_o, ψ_p, calculated from superposition of η versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} and ψ versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} data, respectively, onto Graessley's master curves, and τ_N, computed from zero shear parameters η₀ and ψ₀, differ in their sensitivity to the melting of microcrystalline regions. It is postulated that in the systems investigated, aggregates with long lifetimes are being formed, increasing the effective molecular weight and introducing changes in the effective polydispersity.     

Radiation-induced graft polymerization of styrene to poly(vinyl chloride)

The radiation-induced graft polymerization of styrene to poly(vinyl chloride) (PVC) was investigated. Relations between the rate of grafting and the dose rate when the polymer is irradiated in liquid monomer or in monomer vapor, and between the rate of grafting and monomer concentration absorbed in the polymer have been investigated. The rate of grafting in monomer vapor was found to be far larger than that in liquid monomer. A high rate of grafting in monomer vapor was thought to result from a lower concentration of monomer in PVC during irradiation. An experiment carried out on PVC containing the monomer at various concentrations showed that the rate is largest at a monomer concentration of about 3.5 mole/l. and is smaller for higher and lower concentrations. On the assumption that the theory of homogeneous homopolymerization can be applied to this grafting reaction, the value of k_p²/k_t has been obtained, where k_p and k_t are propagation constant and termination constant, respectively. The value of k_t greatly increases when the monomer concentration exceeds 3.5 mole/l. This increase of k_t can be accounted for if it is assumed that the monomer absorbed in the polymer works as a plasticizer and increases the molecular motion of the polymer. A measurement of the elastic modulus of PVC containing the monomer at various concentrations showed that this is, in fact, the case.     

The addition of methyl radicals to hexafluoroacetone

The reaction of methyl radicals (Me) with hexafluoroacetone (HFA), generated from ditertiary butyl peroxide (dtBP), was studied over the temperature range of 402–433 K and the pressure range of 38–111 torr. The reaction resulted in the following displacement process taking place: where TFA refers to trifluoroacetone. The trifluoromethyl radicals that were generated abstract a hydrogen atom from the peroxide: such that k_6a is given by: where θ = 2.303RT kcal/mol. The interaction of methyl and trifluoromethyl radicals results in the following steps: Product analysis shows that k₁₇/kk = 2.0 ± 0.2 such that k₁₇ = 10^10.4±0.5M^?1 · s^?1. The rate constant k₅ is given by: It is concluded that the preexponential factor for the addition of methyl radicals to ketones is lower than that for the addition of methyl radicals to olefins.     

Kinetics of recombination,dismutation, and disproportionation reactions involving neutral ketyl radicals and radical anions

The kinetics of fast elementary recombination of neutral ketyl radicals of benzophenone and its four derivatives (BPH?), the dismutation of benzophenone radical anions, the disproportionation between BPH? and stable nitroxyl radicals, ( ), and the electron transfer have been investigated in both individual solvents and binary mixtures of different viscosities. Reaction (1) for unsubstituted BPH in water, water glycerol, and n-hexane is controlled by diffusion with 2k₁ ? k_diff. In aliphatic alcohols and toluene, which form solvation complexes with BPH?, reaction (1) is diffusion-enhanced and activation-controlled, respectively, with 2k₁ < k_diff. In a viscous solvent such as 1-propanol–glycerol mixture (100 ? η ? 450 cP) reaction (1) is diffusion-controlled. Reaction (2) in alkaline 1-propanol and alkaline 1-propanol–glycerol mixture is activation controlled. The rates of reactions (3) and (4) for benzophenone radicals and nitroxyl radicals of the imidazoline series decrease as the viscosity of the water–glycerol and 1-propanol–glycerol mixtures is increased. The reactions are molecular mobility limited; nevertheless, the numerical values of k₃ (k₄) are 2–6 times as small as the corresponding k_diff values due to the low steric factor of the reactions (therefore called pseudodiffusion-controlled reactions). The theoretical estimates of k₃ (k₄) are in good agreement with the experimental results. The elimination of spin forbiddance in the process of radical recombination in viscous solvents is discussed.     

Direct metalation of poly(2,6-dimethyl-1,4-phenylene ether)

Poly(2,6-dimethyl-1,4-phenylene ether) (I) was metalated with butyllithium in tetrahydrofuran and with the N,N,N′,N′-tetramethylethylenediamine complex of butyllithium in a variety of solvents. In these cases, metalation occurred at both the ring and side chain positions, the former being preferred initially. Subsequently, there was an isomerization in favor of the side chain. At 25°C, there is no significant amount of polymer scission or crosslinking during metalation, but some crosslinking occurs on derivatizing with dimethyl sulfate and trimethylchlorosilane for high extents of ring metalation. With sodium and potassium alkyls, only side-chain metalation was observed. The metalated polymer reacts as a typical organometallic, allowing polymer modification by a wide variety of reactions.     

Reaction Rates of t-Butyl and Pivaloyl Radicals in Solution

The rate constants of the unimolecular decomposition of the pivaloyl radical (k_D) and of the bimolecular self terminations of pivaloyl (k₁) and t-butyl radicals (k₂) in liquid methylcyclopentane are determined by ESR.-spectroscopy: The viscosity dependence of (k₂) is analysed with respect to diffusion control of the reaction. Comparison of (k_D) values of different acyl radicals reveals a strong dependence of the activation energies on radical structure.     

A Very Useful Redox Initiator for Aqueous RAFT Polymerization of N‐Isopropylacrylamide and Acrylamide at Room Temperature

Reversible addition fragmentation chain transfer (RAFT) polymerizations of N‐isopropylacrylamide (NIPAM), and acrylamide (AM) were successfully performed in aqueous media at ambient temperature using a redox initiator, potassium persulfate (K₂S₂O₈), and sodium thiosulfate (Na₂S₂O₃). The molecular weight of the polymers obtained increases linearly with monomer conversion, molecular weight distribution is narrow even at high conversion (>90%), and a linear relationship exists between ln([M]₀/[M]) and polymerization time. Moreover, self‐chain extension reaction also proceeded successfully. All the results demonstrate that K₂S₂O₈–Na₂S₂O₃ is a very efficient initiating system for the RAFT polymerizations of NIPAM and AM in aqueous media at room temperature.

     

Molecular confirmation and crystal structure of the β form of poly(ethylene oxybenzoate)

The crystal structure of the β form of poly(ethylene oxybenzoate) was analyzed by x-ray diffraction. Four nearly extended molecular chains pass through a unit cell with parameters a = 8.19 Å, b = 11.07 Å, c (fiber axis) = 19.05 Å, β = 114.8°, and the space group P2₁/n-C. The structural difference between the α and β forms is mainly due to the internal rotation angles for the virtual bond and the ? CH₂? CH₂? bond. They are essentially in trans confirmation in the β form, while the α form contains cis and gauche (? CH₂? CH₂? ) conformations.     

Rotating sector study of the gas phase photolysis of the 1,1,1-trichloro-2,2,2-trifluoroethane–cyclohexane system

The rate constant for the combination of 1,1-dichloro-2,2,2-trifluoroethyl radicals in the gas phase has been measured by applying the rotating sector technique to the 1,1,1-trichloro-2,2,2-trifluoroethane–cyclohexane photochemical system. The combination rate constant, k₅, was found to be 6.6 × 10¹² cc mole^?1 sec^?1. Arrhenius parameters for the reaction, are given by the expression log k₄ = 11.81 – (9700/2.3RT).     

Copolymerization of vinyl ketones with allylbenzene in the presence of zinc chloride

Copolymerization of 3-buten-2-one (MVK) and 3-methyl-3-buten-2-one (MIPK) with allylbenzene (AB) in the presence of zinc chloride (ZnCl₂) was studied. The apparent monomer reactivity ratio decreased with an increasing concentration of ZnCl₂ added. By assuming a terpolymerization system as free vinyl ketone (M₁), vinyl ketone complexed with ZnCl₂ (M₂), and AB(M₃), the absolute monomer reactivity ratios were determined by using the least squares method: k₁₁/k₁₂ = 0.0038, k₁₁/k₁₃ = 11.77, k₂₂/k₂₁ = 1.37, and k₂₂/k₂₃ = 3.77 for the MVK-AB system, and k₁₁/k₁₂ = 0.0070, k₁₁/k₁₃ = 55.47, k₂₂/k₂₁ = 1.84, and k₂₂/k₂₃ = 11.16 for the MIPK-AB system, respectively. It is seen from these values that the complexed vinyl ketones are more reactive to both polymer radicals which have free and complexed vinyl ketone units at its terminal position than the free vinyl ketones. Q and e values for MVK and MIPK complexed with ZnCl₂ were found to be Q = 850, ^e_(MVK)2ZnCl₂ = 2.97, Q = 2.62, respectively.     

The kinetics of the thermal bromination of CF3I. Determination of the bond dissociation energy D(CF3−I)

The kinetics of the thermal bromination reaction have been studied in the range of 173–321°C. For the step we obtain where θ=2.303RT cal/mole. From the activation energy for reaction (11), we calculate that This is compared with previously published values of D(CF₃?I). The relevance of the result to published work on k_c for a combination of CF₃ radicals is discussed.     

Modification of poly(vinyl alcohol) through reaction with phosphorus-containing reactants

Crosslinked products of the form: and have been formed from the interfacial condensation of phosphorus diacid halides with poly(vinyl alcohol). Product yield and amounts of phosphorus reactant included in the product increases as the amount of base increases. Product stability in aqueous systems decreases in the order neutral > base > acid.     

Cyclodextrins in Polymer Synthesis: Crosslinking Water‐Soluble Unsaturated Polyester Resins Using a Cyclodextrin/Styrene Complex in Aqueous Medium

A new method was developed to crosslink water‐soluble unsaturated polyester resins prepared from maleic anhydride and poly(ethylene glycol) in water as the solvent. Crosslinking was carried out with various molar ratios of the host‐guest complex consisting of styrene as the guest and methylated β‐cyclodextrin as the host. Polymerizations were performed in water with K₂S₂O₈/Na₂S₂O₅ as free radical initiator at 25 °C. Thermal properties of the networks obtained depend on the amount of styrene incorporated into the polymer.

Acceleration effect of me‐β‐CD during crosslinking of an unsaturated polyester with styrene in water: (a) monomer complexed with me‐β‐CD, and (b) with uncomplexed monomer.