Chain transfer to macromolecules with chain scission is the most often observed “side” reaction in the polymerization of heterocyclics. In our previous works we analysed quantitatively the intramolecular chain transfer to the own macromolecule (back-biting). This paper gives a general treatment of the kinetics of polymerization with propagation and intermolecular chain transfer to macromolecules, accompanied with chain scission. The numerical solution developed allows determining the kp/ktr ratio from the dependence of m̄w/m̄n on monomer conversion. This treatment was applied to the polymerization of L,L-lactide and kp/ktr ratios were measured for covalent alcoholate active species bearing Al, Fe, Ti, Sm, and La. In this way selectivities of active species (expressed with kp/ktr) were for the first time measured and finally correlated with the atomic number of the corresponding metal atoms, related to the strength of the bond involved in the monomer addition. 相似文献
Vinyl acetate was polymerized in bulk and in benzene at 50°C using a wide range of concentrations of azobisisobutyronitrile. Values of fk (the efficiency of initiator) and kprt/kikp (the characteristic constant of primary radical termination) were found to be 0.53 and 2.00 × 104 respectively from data for bulk polymerization. In solution polymerization, the initiator exponent is a function of initiator concentration ranging from 0.35 at high concentration to- about 0.65 at low concentration. This result has been explained on the basis of degradative chain transfer to solvent and primary radical termination. The results have been treated according to mathematical formulations already developed; the characteristic constant of degradative chain transfer and the transfer constant of the solvent have been determined. The results have been compared with literature values and discrepancies explained. 相似文献
A general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation (kp) to the rate constant of the bimolecular transfer (k(2)tr) from the dependence of the MWD on monomer conversion. Numerical values of kp/k(2)tr equal to ≈ 102 and 25 were thus determined for the polymerization of L , L -lactide (L , L -dilactide) initiated with aluminium tris(isopropoxide) trimer ({Al(OiPr)3}3) and tributyltin ethoxide (nBu3SnOEt), respectively. 相似文献
The effects of triphenyl phosphite (TPP) on the radical polymerization of styrene (St) and methyl methacrylate (MMA) initiated with α,α,-azobisisobutyronitrile (AIBN) was investigated at 50°C. The rate of polymerization of St and MMA at a constant concentration of TPP was found to be proportional to the monomer concentration and the square root of the initiator concentration. The rate of polymerization and the degree of polymerization of both St and MMA increased with increasing TPP concentration. The accelerating effect was shown to be due to the decrease of the termination rate constant kt with an increase in the viscosity of the polymerization systems. The chain transfer constant Ctr of TPP in St and MMA systems was determined from the degree of polymerization system. The Ctr of TPP was almost zero in the St system and 6.5 × 10?5 in the MMA system. 相似文献
The ethylene polymerization by Cp2ZrCl2/MAO (Cp = η5: cyclopentadienyl; MAO = methyl aluminoxane) and CpZrCl3/MAO have been studied. The MW and PD (= Mw/Mw) of polymers obtained after 2.5-60 min are the same, which indicate short chain lifetime. The values of rate constants for Cp2ZrCl2 at 70°C are: kp = 168?1670 (M s)?1 and ktrA1 = 0.012-0.81 s?1 depending upon [Zr] and [MAO,] ktrβ = 0.28 s?1, and ktrH = 0.2 M?1 torr?1/2 s?1. These chain transfer rate constant values are two to three orders of magnitude greater than the corresponding values found for MgCl2 supported titanium catalysts. One significant difference between the heterogeneous and homogeneous catalysts is that the former decays according to an apparent second order kinetics, whereas the latter decay is simple first order at 0°C and biphasic first order at higher temperatures. The productivity of the catalysts depends weekly on temperature while the MW decreases strongly with increase of temperature above 30°C. All the active species were formed upon mixing Cp2ZrCl2 with MAO while it took up to 20 min for the CpZnCl3/MAO system. The productivity of the former increase more strongly with the decrease of [Zr] than the latter. Otherwise, the two catalyst systems have all their kinetic parameters differing less than a factor of two. 相似文献
Pulsed‐laser induced polymerization is modeled via an approach presented in a previous paper.[1] An equation for the time dependence of free‐radical concentration is derived. It is shown that the termination rate coefficient may vary significantly as a function of time after applying the laser pulse despite of the fact that the change in monomer concentration during one experiment is negligible. For the limiting case of t ≫ c–1 (kpM)–1, where c is a dimensionless chain‐transfer constant, kp the propagation rate coefficient and M the monomer concentration, an analytical expression for kt is derived. It is also shown that time‐resolved single pulse‐laser polymerization (SP–PLP) experiments can yield the parameters that allow the modeling of kt in quasi‐stationary polymerization. The influence of inhibitors is also considered. The conditions are analyzed under which M (t) curves recorded at different extents of laser‐induced photo‐initiator decomposition intersect. It is shown that such type of behavior is associated with a chain‐length dependence of kt. 相似文献
Chain transfer processes (ktr) taking place in the polymerization (anionic and pseudoanionic) of cyclic esters (lactones) are reviewed. These reactions are mostly responsible for the departure of these systems from the fully controlled (living) polymerizations. The ratios of kp/ktr have been determined for a number of initiating systems and the structures of the growing species are related to their selectivity, expressed by kp/ktr. It has been shown that the less reactive and more sterically crowded active species polymerize more selectively. 相似文献
In the cationic polymerization of 3,3-bis(chloromethyl)oxetane induced by BF3 the solvent polarity (toluene, methylene chloride, ethylene chloride, nitrobenzene, and nitromethane) does not influence the ktr/kp ratio, where ktr stands for the rate constant of chain transfer to polymer. Increase of the overall polymerization rate is due mainly to the increase of ki. The application of the steady-state conditions in which the slow formation of the active centers is compensated by the unimolecular chain transfer to polymer allowed the determination of ktr/kp ratios for several chain-transfer agents of low molecular weight. Alcohols and ethers of different basicities were used. It was established that the ktr/kp ratio is a linear function of ?pKa of the chain-transfer agents. 相似文献
Catalytic chain transfer copolymerisation of MMA and HEMA (70:30) has been carried out under semi‐continuous emulsion polymerisation conditions, using CoBF as catalyst. It has been shown that macromonomers of low molar mass can be synthesised with an apparent chain transfer constant, CSE, of ca. 1300 down to a threshold value of ca. 20 ppm of CoBF. Below this value an initial 20% shot of monomer/catalyst mixture was necessary to prevent events involved in the catalytic chain transfer process becoming diffusion controlled and to prevent the reaction to proceed under monomer starved conditions. Analysis of the Co(II) species by SQUID has been carried out. CoBF shows a value for its effective magnetic susceptibility of 1.77μB. It was found that a correction for the response of the sample container is essential for reliable data to be achieved. Diffusion ordered 2D‐NMR spectroscopy (DOSY) has been used as a method to study the catalyst diffusion dependence for the rate coefficient of chain transfer. However, the apparent values of the found diffusion coefficients are an order of magnitude above the natural limit for center of mass diffusion. 相似文献
In bulk polymerization and copolymerization of trioxane with ethylene oxide, it has been shown that p-chlorophenyldiazonium hexafluorophosphate is a superior catalyst as compared to boron trifluoride dibutyl etherate (BF3 · Bu2O). Polymers and copolymers of significantly higher molecular weight have been obtained. The higher molecular weight has been attributed primarily to less inherent chain transfer during propagation, which in turn can be attributed to the superior gegenion PF6?. The polymerization proceeds via a clear period followed by sudden solidification. Faster polymerization and higher molecular weight polymers have been observed for homopolymerization than for copolymerization. The polymer yield obtained after solidification is determined by both rate of polymerization and rate of crystallization of polymers. These rates, in turn, are dependent on the catalyst concentration. The molecular weight is determined both by polymer yield and extent of inherent chain transfer. In the range of monomer to catalyst mole ration [M]/[C] = (0.5–20) × 104 investigated, it has been found that in the higher range, the polymer yield is independent of the catalyst concentration and the extent of inherent chain transfer is inversely proportional to the half power of catalyst concentration: [M]/[C] = (0.5–8) × 104 for homopolymerization and (0.5–3) × 104 for copolymerization with 4.2 mole % ethylene oxide. In the lower range, the yield decreases with catalyst concentration and the extent of inherent chain transfer is inversely proportional to higher power of catalyst concentration. The dependence of molecular weight of polymers on catalyst concentration has been shown to be a complex one. The molecular weight goes through a maximum as the catalyst concentration is decreased. The maximum molecular weights have been obtained at [M]/[C] ≈ 8 × 104 for homopolymerization and ~3 × 104 for copolymerization with 4.2 mole % ethylene oxide. Prior to reaching maximum the molecular weight is inversely proportional to the half power of catalyst concentration indicating it is primarily controlled by inherent chain transfer. Upon further decrease of catalyst, molecular weight decreases as a result of both a decrease in polymer yield and an increase in inherent chain transfer. In copolymerization of trioxane and ethylene oxide, it has been ascertained that methylene chloride exhibits a favorable solvating effect. Although higher inherent chain transfer takes place in copolymerization than in homopolymerization, the extent of chain transfer is independent of ethylene oxide concentration. The difference in polymer yield and molecular weight a t different ethylene oxide concentrations is attributed primarily to the difference in kp/kt ratio. It also has been demonstrated that end capping of polymer chains can be accomplished by the use of a chain transfer agent—methylal. 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