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1.
The free‐radical copolymerization of styrene and butyl acrylate has been carried out in benzene at 50 °C. The lumped k p/k parameter (where k p and k t are the average copolymerization propagation and termination rate constants, respectively) has been determined. Applying the implicit penultimate unit model for the overall copolymerization propagation rate coefficient and the terminal unit effect for the overall copolymerization termination rate coefficient and using the homopolymerization kinetic coefficients, we have found good qualitative agreement between the experimental and theoretical k p/k values. The variation of the copolymerization rate in solution with respect to the values previously found in bulk has been ascribed to a chain length effect on the copolymerization termination rate coefficient. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 130–136, 2004  相似文献   

2.
A study was carried out in aqueous solutions using luminescence technique to investigate the effects of pH, salt concentration, and temperature on the polyacrylic acid/uranyl ion (PAA/UO) complex formation as well as competitive phenomena of enhancement and quenching effects on photoexcited state of uranyl ions. It was found that excess of H+ and OH? is not favorable for complexation between uranyl ions and polymer. Added nitrate salts of Na+ and K+ had significant enhancement effect on emission spectra of PAA/UO complex. These results indicated that the metal ion/polymer chain complex collapsed by addition of salts and then complex became more compact with consequent phase separation. No significant effect of temperature on the PAA/UO complex stability has been observed between 25–50 °C. The quenching rate constants obtained from Stern–Volmer plots were found to be in the order of kq(H+) >> kq(K+) > kq(Na+). © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2737–2744, 2005  相似文献   

3.
Unusual difficulties are faced in the determination of propagation rate coefficients (kp) of alkyl acrylates by pulsed‐laser polymerization (PLP). When the backbiting is the predominant chain transfer event, the apparent kp of acrylates determined in PLP experiments for different frequencies should range between kp (propagation rate coefficient of the secondary radicals) at high frequency and k at low frequency. The k value could be expressed from kinetic parameters: , where kfp is the backbiting rate coefficient, kp2 is the propagation rate coefficient of mid‐chain radicals, and [M] is the monomer concentration.

Apparent propagation rate coefficients determined for different frequencies by simulating the PLP of n‐butyl acrylate at 20 °C. Horizontal full lines show the values of kp and k.  相似文献   


4.
The aqueous solution of a thermoresponsive polymer, poly[2‐(2‐ethoxy) ethoxyethyl vinyl ether] poly(EOEOVE), contains a tiny amount of large polymer aggregates at low polymer concentrations far below the lower critical solution temperature (~40 °C). The molar mass Mw,slow, radius of gyration 〈S2〉, and hydrodynamic radius RH,slow of the aggregating component of poly(EOEOVE) were obtained by simultaneous static and dynamic light scattering as functions of the polymer concentration and temperature, while the weight fraction wslow of the component was estimated by size‐exclusion chromatography. The Mw,slow dependencies of 〈S2〉 and RH,slow, as well as the ratio 〈S2〉/RH,slow, indicated that the poly(EOEOVE) aggregate takes a sparsely branched polymer‐like conformation. We have analyzed the structure of the aggregate, using the branched polymer model of random type. The Mw,slow dependence of 〈S2〉 obtained was favorably compared with this model with reasonable structural parameters. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1179–1187, 2006  相似文献   

5.
The unperturbed chain dimensions (〈R2o/M) of cis/trans‐1,4‐polyisoprene, a near‐atactic poly(methyl methacrylate), and atactic polyolefins were measured as a function of temperature in the melt state via small‐angle neutron scattering (SANS). The polyolefinic materials were derived from polydienes or polystyrene via hydrogenation or deuteration and represent structures not encountered commercially. The parent polymers were prepared via lithium‐based anionic polymerizations in cyclohexane with, in some cases, a polymer microstructure modifier present. The polyolefins retained the near‐monodisperse molecular weight distributions exhibited by the precursor materials. The melt SANS‐based chain dimension data allowed the evaluation of the temperature coefficients [dln 〈R2o/dT(κ)] for these polymers. The evaluated polymers obeyed the packing length (p)‐based expressions of the plateau modulus, G = kT/np3 (MPa), and the entanglement molecular weight, Me = ρNanp3 (g mol?1), where nt denotes the number (~21) of entanglement strands in a cube with the dimensions of the reptation tube diameter (dt) and ρ is the chain density. The product np3 is the displaced volume (Ve) of an entanglement that is also expressible as pd or kT/G. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1768–1776, 2002  相似文献   

6.
Triphenylbismuthonium 1,2,3,4‐tetraphenylcyclopentadienylide in 1,4‐dioxan initiated radical polymerization of methyl acrylate to ~30% conversion without gelation because of autoacceleration. The polymer had a viscosity‐average molecular weight of 200,000. The kinetic expression was Rpα[I]0.3[M]1.16, that is, the system followed nonideal kinetics because of primary radical termination and degradative chain‐transfer reactions. The values of kkt and the energy of activation were computed as 3.12 × 10?5 Lmol?1s?1 and 28 kJ/mol, respectively. The ylide dissociated to form a phenyl radical, which brought about polymerization of methyl acrylate. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2060–2065, 2004  相似文献   

7.
In carbocationic polymerization and copolymerization, a recent publication concluded that the substituent effect on carbocation reactivity is much larger than its effect on monomer reactivity, and this by a factor 106 in the case of the rate constant k12capp for p‐methylstyrene addition (monomer M2) on, respectively, poly(p‐methoxystyrene)± or poly(p‐methylstyrene)± (M). This conclusion is disputed, as well as the assumption that the rate constants of capping (k12capp) obtained in deactivation reactions of poly(p‐methoxystyrene)± are identical with cross propagation rate constants in copolymerization (k12copol). It is shown that the large calculated k12capp are based on propagation constant values for p‐methylstyrene (k ≈ 109) obtained by the diffusion‐clock method. They are 104 times smaller as found for all styrenes, that is, between 104 and 105 when they are based on the ionic species concentrations. In such a case, the available data are still in agreement with an approximate compensation between the reactivities of a monomer and of the corresponding carbocation. It is also shown that copolymerization data for styrenes are not compatible with k values near to diffusion control, and that variations of log k12capp and log k12copol with the nucleophilicity parameter N of the monomers indicate a much lower selectivity of the monomers in the case of copolymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2666–2680, 2010  相似文献   

8.
Soluble microgels with several pendant vinyl groups were synthesised by radical copolymerization of methyl methacrylate (MMA) with p-divinyl benzene (p-DVB). The competitive reactions of intermolecular and intramolecular crosslinkings of these microgels were carried out at 40°C in the presence of 1-buten-3-ol as a degradative chain transfer agent. The rate constant of intermolecular crosslinking (k) was estimated by GPC (gel permeation chromatography) analysis on the polymer produced from intermolecular propagation between bimolecules. The k depended strongly on the internal structure of microgels. Network formation was discussd inclusive of informations for the rate constant of intramolecular crosslinking (k).  相似文献   

9.
Following earlier suggestions the values for the rate coefficient of chain termination kt in the bulk polymerization of styrene at 25°C were formally calculated (a) from the second moment of the chainlength distribution (CLD) and (b) from the rate equation for laser-initiated pseudostationary polymerization (both expressions originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation kp. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of kt vs. the number-average degree of polymerization of the chains at the moment of their termination yielded exponents b of 0.16–0.18 in the power-law kt = A · Pn −b, A ranging from 2.3 × 108 to 2.7 × 108 L · mol−1 · s−1. These data are only slightly affected if termination is not assumed to occur by recombination only and a small contribution of disproportionation is allowed for.  相似文献   

10.
The values for the rate coefficient of chain termination kt in the bulk polymerization of methyl methacrylate at 25°C were formally calculated (i) from the second moment of the chain-length distribution and (ii) from the rate equation for laser-initiated pseudostationary polymerization (both expressions were originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation kp. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of kt vs. the average degree of polymerization of the chains at the moment of their termination v′ yielded exponents b of 0.16–0.17 in the power-law k t = A · v−b, A ranging from 1.1 × 108 to 1.3 × 108 (L · mol−1 · s−1). A 70% contribution of disproportionation to overall termination has been assumed in the calculations.  相似文献   

11.
12.
For polymerization initiated by an arbitrary sequence of laser pulses a numerical technique for calculating molecular weight distributions (MWDs) is developed, which takes into consideration the chain length dependence of the termination rate constant kt. The MWDs for methyl methacrylate and styrene are calculated by use of α and k0 values (for the law k = k0(i)−α of termination of radicals with chain length i) and averages $ \overline {(i,{\rm }j)} $ (for rate constants k = k0$ \overline {(i,{\rm }j)} $ of termination of radicals with different degrees of polymerization) taken from the literature. The dependences of the overall termination constant 〈kt〉 on initiation parameters (pulse repetition rate (v) and pulse intensity for initiation by periodic laser pulses) are presented. Two methods are proposed for α and k0 determination: (a) by experiments on polymerization with periodic laser pulses where monomer-to-polymer conversions per pulse are determined for different v; (b) by experiments on polymerization with packets of pulses where the constant kp (the rate constant of propagation), α and k0 can be determined simultaneously from MWD. For both methods simple analytical equations are derived for evaluation of the constants. The limits of application of the methods are determined by use of the numerical technique for MWD calculation.  相似文献   

13.
We investigate relaxation dynamics in a series of six‐arm star/linear 1,4‐polybutadiene blends with mechanical rheometry measurements. Blend systems are formulated to systematically probe constraint release and arm relaxation dynamics. Zero shear viscosity and terminal relaxation times of star/linear polymer blends with fixed star arm molecular weights (Ma) and compositions (?S) are found to follow nonmonotonic dependencies on the linear polymer molecular weight (ML). At low values of ?S, at least two scaling regimes are apparent from the data (ξ0M and ξ0M), where ξ0 refers to the zero shear viscosity or terminal relaxation time of the blend. The two regimes are separated by a critical linear polymer molecular weight M* that is more than 20 times larger than the critical molecular weight for entanglements. When the linear polymer contribution to blend properties is removed, a clear transition from dilution dynamics, ξ0M, to Rouse‐like constraint‐release dynamics, ξ0M, is apparent at low values of ?S. At higher ?S values, a new activated constraint‐release dynamic regime is evident in which ξ0M and ξ0 ~ ?, where α changes continuously from approximately 2 to 0.5 as ?S increases and β varies from 2.0 to 1.0 as ML increases. The experimental results are compared with theoretical predictions based on a drag coupling model for entangled polymer liquids. All features observed experimentally are captured by this model, including the value of M* for the transition from dilution to Rouse constraint‐release dynamics. Predictions of the drag coupling model are also compared with published data for the zero shear viscosity and terminal relaxation time in bidisperse linear polymer blends and pure entangled starlike molecules. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2501–2518, 2001  相似文献   

14.
The phenomenon of chain entanglement in undiluted linear amorphous polymers is treated by calculating the probability of forming closed intramolecular loops. Adoption of the rotational isomeric state model of polymer chains permits an appropriate accounting of the detailed molecular structure to be made through the configurational characteristics of the polymer. The second (〈r0) and fourth (〈r0) moments of the vector rhk connecting groups h and k in the isolated polymer chain and averaged over all chain conformations are calculated and used to evaluate the probability Wx(0) that rhk is 0, or that an intramolecular loop of x = k ? h bonds is formed. Several linear polymers with widely differing molecular structures are treated. An attempt is made to correlate the degree of chain entanglement they manifest in the bulk with their ability to fold back upon themselves to form closed intramolecular loops.  相似文献   

15.
Acrylamide was polymerized in acetonitrile at 82 °C with a perfluorinated azo‐derivative initiator. The polymerization proceeded heterogeneously. Varying amounts of initiator and monomer were used. The activation energy was deduced from three experiments carried out at 59, 71, and 82 °C. The following kinetic law, deviating a great deal from the classical law, was obtained: R ∼ [I2][M](0.05% < [I2]o/[M]o < 1.00%) and R ∼ [I2][M](1% < [I2]o/[M]o < 7%). These results can be interpreted in light of the contribution of primary radical termination and the emergence of occlusion. The development of a new kinetic relationship allowed us to confirm the existence of both of these termination reactions. The calculation of the kprt /ki · kp ratio was also achieved. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1834–1843, 2000  相似文献   

16.
Polymerization of tetrahydrofuran (THF) in CH3NO2 solvent was initiated with 1,3-dioxolan-2-ylium cations with AsF and SbF anions, as well as with esters of FSO3H and CF3SO3H acids. Polymerization shows in this solvent a living feature: values of kp (determined directly from the semilogarithmic kinetic plots) were the same for all of the listed above initiators; thus kp is the same for AsF, CF3SO, FSO, and SbF anions. The identity of the kp values for complex and noncomplex (ester-forming) anions comes from the fact that in CH3NO2 solvent equilibrium between macroesters and macroion pairs is shifted almost completely (Ke = 33.0 at 25°C and |THF|0 = 7.0M) to the macroions side. Dissociation constants of the polytetrahydrofuranium ion pairs (CF3SO and SbF anions) were measured (e.g., KD = 2 × 10?3 M at 25°C and |THF|0 = 7.0M; i.e., at D = 22.8, ΔHD = ?3.8 ± 6 kcal mole; ΔSD = ?25 ± 2 eu). On the basis of the known values of KD, and therefore dissociation degrees α, rate constants of propagation on the free and paired THF cations (k and k) were determined for a large range of degrees of dissociation (α from 0.15 to 0.52). The rate constants k and k were found to be the same within an experimental error of measurements (± 15% of the value of kp). Apparently, the polytetrahydrofuranium cations are highly solvated or even separated from their anions by molecules of THF itself. At these conditions the reactivities of the solvated “free” and solvated (or separated) paired cations became undistinguishable.  相似文献   

17.
We report that the brittle‐ductile transition of polymers induced by temperature exhibits critical behavior. When t close to 0, the critical surface to surface interparticle distance (IDc) follows the scaling law: IDct?v, where t = 1 ? T/T (T and T are the test temperature and brittle‐ductile transition temperature of matrix polymer, respectively) and v = 2/D. It is clear that the scaling exponent v only depends on dimension (D). For 2, 3, and 4 dimension, v = 1, 2/3, and 1/2 respectively. The result indicates that the IDc follows the same scaling law as that of the correlation length (ξ), when t approach to zero. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 766–769, 2008  相似文献   

18.
Cationic polymerization of tetrahydrofuran (THF) in CH2Cl2 solvent and in mixed CH2Cl2/CH3NO2 solvent was initiated with 1,3-dioxolan-2-ylium cations with AsF and SbF anions. Dissociation constants of the polytetrahydrofuranium ion pairs into ions were measured (e.g., KD = 1.5 × 10?5M at 25°C and [THF]0 = 7.0M; CH2Cl2 solvent) and were found to be more than 100 times lower than in CH3NO2 solvent at the same [THF]0 and temperature. The rate constants k and k, measured for degrees of dissociation ranging from 0.03 to 0.35 in CH2Cl2, were the same within an experimental error of measurements (±15% of the value of kp). Dependence of k( = k = k) on the dielectric constant was a monotonous function in three different solvents, namely, CCl4, CH2Cl2, and CH3NO2, which covered a large range of dielectric constants of the medium (from D = 5 to D = 22) and degrees of dissociation of the macroion pairs, α (from 0.03 to more than 0.70). Thus a decrease in the dielectric constant increases the rate constant k in the whole range of studied polarities of the medium. This result confirms an earlier conclusion that the rate constant of propagation does not depend on the state of aggregation of ions and k = k.  相似文献   

19.
A method that utilizes reversible addition fragmentation chain transfer (RAFT) chemistry is evaluated on a theoretical basis to deduce the termination rate coefficient for disparate length radicals k in acrylate free radical polymerization, where s and l represent the arbitrary yet disparate chain lengths from either a “short” or “long” RAFT distribution. The method is based on a previously developed method for elucidation of k for the model monomer system styrene. The method was expanded to account for intramolecular chain transfer (i.e., the formation of mid-chain radicals via backbiting) and the free radical polymerization kinetic parameters of methyl acrylate. Simulations show that the method's predictive capability is sensitive to the polymerization rate's dependence on monomer concentration, i.e., the virtual monomer reaction order, which varies with the termination rate coefficient's value and chain length dependence. However, attaining the virtual monomer reaction order is a facile process and once known the method developed here that accounts for mid-chain radicals and virtual monomer reaction orders other than one seems robust enough to elucidate the chain length dependence of k for the more complex acrylate free radical polymerization.  相似文献   

20.
In the radiolysis of water vapor containing small concentrations of cyclohexane, the principal products which account for about 98% of all end products are found to be hydrogen, cyclohexene, and bicyclohexyl. Cyclohexene and bicyclohexyl yields were determined over a range of temperatures (70–200°C), total pressures (50–2400 torr), and total doses (0.15–2.0 Mrad). The disproportionation–combination ratio k/k for c-C6H11 radicals could be determined as 0.56 ± 0.01 from the ratio of cyclohexene to bicyclohexyl yield. By using c-C6D12, the ratio k/k for c-C6D11 radicals is found to be 0.38 ± 0.01. Comparison of the reactivity pattern of C6H11 and C6D11 radicals leads to (k)/(k)/(k/k) = 1.47 ± 0.02. The corresponding values for the reactions of c-C6H11 with c-C6D11 were also determined.  相似文献   

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