Summary: A novel method for measuring termination rate coefficients, kt, in free‐radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator‐derived radicals. During subsequent polymerization, radical concentration is monitored by time‐resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain‐length dependence of kt to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example.
Two straight lines provide a very satisfactory representation of the chain‐length dependence of kt over the entire chain‐length region (cR = radical concentration). 相似文献
The effects of non‐ideal initiator decomposition, i.e., decomposition into two primary radicals of different reactivity toward the monomer, and of primary radical termination, on the kinetics of steady‐state free‐radical polymerization are considered. Analytical expressions for the exponent n in the power‐law dependence of polymerization rate on initiation rate are derived for these two situations. Theory predicts that n should be below the classical value of 1/2. In the case of non‐ideal initiator decomposition, n decreases with the size of the dimensionless parameter α ≡ (ktz /kdz) √rinkt, where ktz is the termination rate coefficient for the reaction of a non‐propagating primary radical with a macroradical, kdz is the first‐order decomposition rate coefficient of non‐propagating (passive) radicals, rin is initiation rate, and kt is the termination rate coefficient of two active radicals. In the case of primary radical termination, n decreases with the size of the dimensionless parameter β ≡ kt,srin1/2/kp,sMrt,l1/2, where kt,s is the termination rate coefficients for the reaction of a primary (“short”) radical with a macroradical, kt,l is the termination rate coefficients of two large radicals, kp,s is the propagation rate coefficient of primary radicals and M is monomer concentration. As kt is deduced from coupled parameters such as kt /kp, the dependence of kp on chain length is also briefly discussed. This dependence is particularly pronounced at small chain lengths. Moreover, effects of chain transfer to monomer on n are discussed. 相似文献
Aspects of applying n‐pulse periodic initiation in pulsed laser polymerization/size‐exclusion chromatography (PLP/SEC) experiments are studied via simulation of molecular weight distributions (MWDs). In n‐pulse periodic PLP/SEC, sequences of n laser pulses at successive time intervals Δt1 up to Δtn are periodically applied. With the dark time intervals being suitably chosen, n‐modal MWDs with n well separated peaks occur. The n‐pulse periodic PLP/SEC method has the potential for providing accurate propagation rate coefficients, kp. Among several measures for kp, the differences in molecular weights at the MWD peak positions yield the best estimate of kp under conditions of medium and high pulse laser‐induced free‐radical concentration. Deducing kp from n dark time intervals (corresponding to n regions of free‐radical chain length) within one experiment at otherwise identical PLP/SEC conditions allows addressing in more detail a potential chain‐length dependence of kp. Simulations are compared with experimental data for 2‐pulse periodic polymerization of methyl methacrylate.
Measured MWD (solid line) and associated first derivative curve (dotted line) for a 2‐pulse periodic bulk polymerization experiment of MMA at 20 °C. 相似文献
1,3‐Dipolar cycloaddition of methyl diazoacetate to methyl acrylate was investigated by kinetic 1Н NMR spectroscopy. It was established that the mechanism of the process includes parallel formation of trans‐ and cis‐dimethyl‐4,5‐dihydro‐3H‐pyrazol‐3,5‐dicarboxylates as a result of [3 + 2]‐cycloaddition of methyl diazoacetate to methyl acrylate; the corresponding rate constants were denoted k1t and k1c. The reaction rate of the isomerization of 3Н‐pyrazolines to 4,5‐dihydro‐1H‐pyrazol‐3,5‐dicarboxylate (3Н → 1Н‐pyrazoline rearrangement) was found to be sensitive to both the methyl acrylate (k2t, k2c) and 1Н‐pyrazoline concentrations (k3t, k3c). Kinetic analysis showed that the proposed scheme is valid for various reagent concentrations. The numerical solution of the system of differential equations corresponded to the reaction scheme and was used to determine the complete set of reaction rate constants (k (× 105 M–1·s–1), 298 K; solvent, benzene‐d6): k1t = 2.3 ± 0.3, k1c = 1.6 ± 0.2, k2t = 1.1 ± 0.3, k2c = 1.8 ± 0.5, k3t = 1.2 ± 0.4, k3c = 2.2 ± 0.7. 相似文献
For the first time, a 1000 Hz pulse laser has been applied to determine detailed kinetic rate coefficients from pulsed laser polymerization–size exclusion chromatography experiments. For the monomer tert‐butyl acrylate, apparent propagation rate coefficients kpapp have been determined in the temperature range of 0–80 °C. kpapp in the range of few hundreds to close to 50 000 L·mol–1·s–1 are determined for low and high pulse frequencies, respectively. The apparent propagation coefficients show a distinct pulse‐frequency dependency, which follows an S‐shape curve. From these curves, rate coefficients for secondary radial propagation (kpSPR), backbiting (kbb), midchain radical propagation (kptert), and the (residual) effective propagation rate (kpeff) can be deduced via a herein proposed simple Predici fitting procedure. For kpSPR, the activation energy is determined to be (17.9 ± 0.6) kJ·mol–1 in excellent agreement with literature data. For kbb, an activation energy of (25.9 ± 2.2) kJ·mol–1 is deduced.
Chain length distributions have been calculated for polymers prepared by pulsed laser polymerization (PLP) under the condition that not only chain termination but also chain propagation is subject to chain length dependence. The interplay between these two features is analyzed with the chain length dependence of the rate coefficient of termination kt introduced in the form of a power law and that of propagation kp modeled by a Langmuir‐type decrease from an initial value for zero chain length to a constant value for infinite chain lengths. The rather complex situation is governed by two important factors: the first is the extent of the decay of radical concentration [R] during one period under pseudostationary conditions, while the second is that termination events are governed by [R]2 while the propagation goes directly with [R]. As a consequence there is no general recommendation possible as to which experimental value of kp is best taken as a substitute for the correct average of kp characterizing a specific experiment. The second point, however, is apparently responsible for the pleasant effect that the methods used so far for the determination of kt and its chain length dependence (i.e., plotting some average of kt versus the mean chain‐length of terminating radicals on a double‐logarithmic scale) are only subtly wrong with regard to a realistic chain length dependence. This is especially so for the quantity kt* (the average rate coefficient of termination derived from the rate of polymerization in a PLP system) and its chain length dependence. 相似文献
Chain‐length‐dependent termination rate coefficients of the bulk free‐radical polymerization of styrene at 80 °C are determined by combining online polymerization rate measurements (DSC) with living RAFT polymerizations. Full kt versus chain‐length plots were obtained indicating a high kt value for short chains (2 × 109 L · mol−1 · s−1) and a weak chain‐length dependence between 10 and 100 monomer units, quantified by an exponent of −0.14 in the corresponding power law 〈kti,i〉 = kt0 · P−b.
Double logarithmic plots of 〈kti,i〉 versus P, evaluated from experimental time‐resolved Rp data according to the procedure described in the text, for different CPDA and AIBN concentrations. The best linear fit for (10 < P < 100) is indicated as full line. 相似文献