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1.
Summary: The influence of the Laplace pressure in polymer emulsion particles during aqueous heterogeneous free radical polymerization on the polymerization kinetics has been investigated. Calculations were carried out based on experimentally reported pressure dependences of propagation and termination rate coefficients. The results suggest that in most cases the effects are not likely to be significant, although under conditions of very small particles (diameter <20 nm) and high interfacial tensions effects of the order of a few percent on propagation (increase in rate) and termination (decrease in rate) were predicted.

(kp/kt0.5)/(kp/kt0.5)0 as a function of particle radius as a result of the Laplace pressure.  相似文献   


2.
Two different equations are presented describing the frequency distribution (fl) of linear chains in step‐growth (SG) polymerizations. Two related equations for the weight distribution (wl) are formulated as well. All equations contain either an exponent α that represents the reaction rates (or probabilities) of chain growth versus cyclization reactions or they contain a cyclization factor β′. Plots of fl and wl versus conversion or degree of polymerization are presented and discussed. Distribution curves of individual oligomers versus conversion and versus β′ are described. Furthermore, the consequences for hyperbranched polymers resulting from polycondensations of abn monomers are discussed. Analogous to Flory's theory all equations concern kinetically controlled SG polymerizations.

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3.
In acrylate polymerizations both SPRs and tertiary MCRs occur. Via pulsed laser polymerization, using a wide range of LPRRs, in conjunction with aqueous‐phase size‐exclusion chromatography, the polymerization of 1.35 mol · L−1 acrylic acid in aqueous solution has been investigated at 6 °C. The sigmoidal decrease in the apparent propagation rate coefficient, k, towards lower LPRRs is in line with recent predictions. At the highest LPRRs, k approaches the rate coefficient of SPR propagation, k, whereas the limiting value of k at low LPRRs approaches the effective propagation rate coefficient, k, which allows for an estimate of the fraction of MCRs under polymerization conditions, xMCR.

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4.
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   


5.
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   


6.
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   


7.
Summary: Spontaneous ring‐opening polymerization of macrocyclic aromatic thioether ketones [ 1,4‐SC6H4COC6H4 ]n (n = 3 and 4), in which the thioether linkages are para to the ketone, occurs during rapid, transient heating to 480 °C, to afford a soluble, semi‐crystalline poly(thioether ketone) of high molar mass (ηinh > 1.0 dL · g−1). Corresponding macrocyclic ether ketones, and a macrocyclic thioether ether ketone in which the thioether linkage is para to the ether rather than to the ketone, show no evidence of polymerization under analogous conditions.

The uncatalysed ring‐opening polymerization of macrocycle 1 , within the pores of an alumina microfiltration membrane, leads to formation of polymer 3 with the microstructure shown in the above scanning electron micrograph.  相似文献   


8.
Termination kinetics of methyl methacrylate (MMA) bulk polymerization has been studied via the single pulsed laser polymerization–electron paramagnetic resonance method. MMA‐d8 has been investigated to enhance the signal‐to‐noise quality of microsecond time‐resolved measurement of radical concentration. Chain‐length‐dependent termination rate coefficients of radicals of identical size, k, are reported for 5–70 °C and up to i = 100. k decreases according to the power‐law expression . At 5 °C, kt for two MMA radicals of chain‐length unity is k = (5.8 ± 1.3) · 108 L · mol−1 · s−1. The associated activation energy and power‐law exponent are: EA(k) ≈ 9 ± 2 kJ · mol−1 and α ≈ 0.63 ± 0.15, respectively.

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9.
Calculations of polymerization kinetics and molecular weight development in the dithiolactone‐mediated polymerization of styrene at 60 °C, using 2,2′‐azobisisobutyronitrile (AIBN) as initiator and γ‐phenyl‐γ‐butirodithiolactone (DTL1) as controller, are presented. The calculations were based on a polymerization mechanism based on the persistent radical effect, considering reverse addition only, implemented in the PREDICI® commercial software. Kinetic rate constants for the reverse addition step were estimated. The equilibrium constant (K = kadd/k‐add) fell into the range of 105–106 L · mol?1. Fairly good agreement between model calculations and experimental data was obtained.

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10.
Summary: N‐methacryloyl‐1‐aminopropane ( 1 ), N‐methacryloyl‐1‐aminobutane ( 2 ), N‐methacryloyl‐1‐aminopentane ( 3 ), and N‐methacryloyl‐1‐aminohexane ( 4 ) are synthesized and treated with an aqueous solution of randomly methylated β‐cyclodextrin (Me‐β‐CD) to form the water‐soluble host‐guest complexes 1a – 4a . In case of the aqueous polymerization of the free monomers 1 – 4 the initial polymerization rate increases with increasing water solubility. The opposite effect is observed in the case of the polymerizations of the Me‐β‐CD‐complexed methacrylamide monomers 1a – 4a . The polymerization rates are increased with increasing alkyl chain length of the complexed monomers 1a – 4a and decreasing water solubility of the free monomers 1 – 4 .

Initial polymerization rate v0 of CD‐complexed monomers 1a – 4a (○) vs. water solubilities of monomers 1 – 4 (▪).  相似文献   


11.
Summary: The nitroxide‐mediated controlled/living free radical copolymerization of styrene and divinylbenzene using a polystyrene‐TEMPO macroinitiator in aqueous miniemulsion and in bulk have been investigated. The crosslink densities were estimated based on the content of pendant vinyl groups as determined by 1H NMR. Considerably lower crosslink densities were revealed in the miniemulsion than in the corresponding bulk system. The rate of polymerization in the miniemulsion increased with decreasing particle size, and was significantly higher than in bulk.

Crosslink density for the TEMPO‐mediated free radical copolymerization of S(1) and DVB(2) (f = 0.99, f = 0.01) at 125 °C in bulk (□) and in miniemulsions with dn = 585 nm (○) and 53.3 nm (•).  相似文献   


12.
The kinetics of ternary complex formation involving Cu(5-X-1, 10-phen) and threonine (CuAL, A=5-X-1, 10-phen; L=threonine or represented by O-N; X=NO2, Cl, H, CH3) has been studied by temperature-jump and stopped-flow methods. The formation rate constants, kf(M?1·s?1), for the complexation reaction, CuA + L CuAL, are as follows; X=NO2, 8.68×108; X = Cl, 7.13×108; X=H, 6.12×108; X=CH3, 5.42×108. The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability (logKCuACuAL) of the complexes CuAL and log kf as follows: logKCuACuAL = 0.13+0.83 logkf, r = 0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron-donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log(kRf/KOf = 0.097σ, r = 0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L.  相似文献   

13.
14.
Kinetic simulations of reversible chain transfer catalyzed polymerization (RTCP) were performed using the program package Predici. Mimicking the RTCP of styrene in bulk at 80 °C, the full molecular weight distributions, the polydispersities of resulting polymer and the time evolutions of monomer conversion and participating species were simulated. The influence of the kinetic coefficients governing the RTCP equilibrium – specifically, the rate coefficients of activation, ka, and deactivation, kda – on the controlled polymerization behavior was probed in detail by varying their respective simulation input values over five orders of magnitude. It was found that optimum results for molecular weight control are obtained for K = ka/kda in the range 1 to 10 and with ka and kda being of the order of 106 L · mol−1 · s−1 or above. The influence of degenerative chain transfer on the process was found to be significant only in poorly controlled systems, but is small in well‐controlled RTCP. Based on the finding that the catalyst is depleting during the polymerization due to cross‐termination, guidelines for obtaining high molecular weight material via repeated addition of catalyst were developed.

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15.
Summary: A novel method combining RAFT polymerization with pulsed‐laser initiation for determining chain‐length dependent termination rate coefficients, kt, 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 kt as a function of chain length via classical kinetics assuming chain‐length independent kt.

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


16.
High‐pressure atom transfer radical polymerization (ATRP) of n‐butyl acrylate (BA) is performed in acetonitrile (MeCN) with CuIBr/TPMA [TPMA: tris(2‐pyridylmethyl)‐amine] as the catalyst up to 5 kbar. Increasing either pressure or temperature significantly enhances the rate of polymerization, while retaining control over the polymerization. The polymerizations under high pressure could be efficiently performed with very low levels of Cu catalyst in the absence of any reducing agents. For example, 100 ppm Cu is sufficient to catalyze the polymerization of BA with targeted degree of polymerization (DPT) = 1000. The conversion reached 79% in 3.0 h at 80 °C providing PBA with M n = 112 000, M w/M n = 1.12. Since the initial CuI‐to‐initiator molar ratio is 0.05:1, the molar percentage of terminated chains should remain <5%. For DPT = 10 000 using only 50 ppm Cu catalyst, a polymer with molecular weight M n = 612 000 (DP = 4800) was obtained at 67% conversion.

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17.
A facile two‐step synthesis for branched poly(isoprene)s (PI) based on polyaddition of ABn‐type macromonomers is described. The synthesis of the macromonomers was achieved by anionic polymerization of isoprene and subsequent end‐capping of the polymers by addition of chlorodimethylsilane to the living carbanions. This led to PI‐based macromonomers with narrow polydispersity ( / < 1.15) and molecular weights in the range of 1 700 – 22 100 g · mol−1. Synthesis of the branched polymers was carried out by a hydrosilylation‐based polymerization of the macromonomers. Characterization via SEC, SEC‐MALLS, coupled SEC‐viscosimetry and 1H‐NMR‐spectroscopy supported the formation of branched structures. Interestingly, these branched polymers exhibited α‐values that were similar to those reported for hyperbranched polymers based on AB2‐monomers.

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18.
The decomposition of ethane sensitized by isopropyl radicals was studied in the temperature range of 496–548°K. The rate of formation of n-butane, isopentane, and 2,3-dimethylbutane were measured. The expression k1/k2½ was found to be where k1 and k2 are rate constants of The decomposition of propylene sensitized by isopropyl radicals was studied between 494 and 580°K by determination of the initial rates of formation of the main products. The ratio of k13/k21/2 was evaluated to be where k13 is the rate constant for The isomerization of the isopropyl radical was investigated by studying the decomposition of azoisopropane. The decomposition of the iso-C3H7 radical into C2H4 and CH3 was followed by measuring the rate of formation of C2H4. On the basis of the experimental data, obtained in the range of 538–666° K, k15/k2½ was found: where k15 is the rate constant of   相似文献   

19.
This article concerns the synthesis and characterization of new epoxy and aldehyde telechelic polyisobutylenes, that is, and . The synthesis of the epoxy derivative was achieved by quantitative epoxidation of α,ω-di(isobutenyl) polyisobutylene with m-chloroperoxybenzoic acid and that of the dialdehyde by quantitative isomerization of the epoxy termini with zinc bromide. Infrared (IR) and 1H-NMR analysis of these new telechelic polymers and ultraviolet (UV) analysis of the 2,4-dinitrophenylhydrazine derivative of the dialdehyde indicate quantitative conversions and yields, that is, essentially theoretical functionalization (F n = 1.95 ± 0.05).  相似文献   

20.
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 k6a 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 = 1010.4±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.  相似文献   

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