Methods of structuring thermoreversible urethane block copolymers on the basis 3,3-bis-(azidomethyl)oxetane (BAMO) and 3-(azidomethyl)-3-methyloxetane (AMMO) were developed for the first time. Type (AB)n copolymers of BAMO and AMMO have an amorphous crystalline structure but differ from the type B(AB)n copolymers in that they contain less crystal phase and have a higher glass transition temperature and better physicomechanical properties. 相似文献
Statistical polymers and copolymers were firstly synthesized by polymerization of 3,3-bis(azidomethyl) oxetane (BAMO) and 3-azidomethyl-3-methyloxetane (AMMO) in a triisobutylaluminum–water catalytic system, and their physicochemical, physicomechanical, and themochemical properties were studied. It was found that with increasing fraction of poly-AMMO in the BAMO/AMMO copolymers the strength properties of the copolymers enhance and the degree of crystallinity decreases. 相似文献
Chain transfer constants were obtained for styrene, methyl methacrylate, methyl acrylate and vinyl acetate, polymerized in methyl oleate and methyl stearate at 60°C. Transfer constants increased in the order: methyl methacrylate < styrene < methyl acrylate ? vinyl acetate in both solvents. Average values of the transfer parameters were: for methyl oleate, Qtr = 2.04 × 10?4, etr = 1.08; for methyl stearate, Qtr = 0.373 × 10?4, etr = 1.01. Indication that polar species predominate in the transition state is supported by the observed order of reactivity. The usual rate dependence appeared to be followed by all of the monomers except vinyl acetate, which was retarded, severely in methyl oleate. Transfer in methyl oleate was about 5.8 times greater than that found in methyl stearate for these four monomers. The internal allylic double bond of methyl oleate had about the same reactivity in transfer as had the terminal unsaturation in N-allylstearamide at 90°C. Rough estimates were obtained of the monomer transfer constants for the long side-chain homologs of these four monomers from the respective monomer transfer constants and the experimental transfer constants, corrected for transfer to the labile groups of the solvent. It was concluded that the rate of polymerization would determine in large measure the degree of polymerization for the reactive 18-carbon homologs but that the molecular weight of poly(vinyl stearate) and (oleate) will be regulated primarily by transfer to monomer. 相似文献
Two crystal modifications have been found for poly(3,3-bisethoxymethyl oxetane) [poly- (BEMO)] by wide-angle x-ray powder diffraction and differential scanning calorimetry, while only one modification has been found for poly(3,3-bisazidomethyl oxetane) [poly(BAMO)]. Melting temperatures for the two polymers were nearly the same, varying from about 70 to about 90°C depending on the thermal treatment; higher crystallization temperatures resulted in higher melting temperatures. The equilibrium melting temperature T was found to be 125 and 128°C for poly(BEMO) and poly(BAMO), respectively, by using the Hoffman-Weeks extrapolation procedure. Measurement of the melting-point depression of Poly(BEMO) and poly(BAMO) in dibutyl phthalate yielded enthalpy of fusion values of 2.25 and 12.8 kcal/mol, respectively. The percent crystallinity for poly(BEMO) and poly(BAMO), respectively, was calculated to be 55-60 and 13-30% based on DSC and x-ray analysis. 相似文献
The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction: For reaction (1) the detailed rate constants [k(ν′ = 1) = 0.30;k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <?′ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH3Cl to CH3Br to CH3I, viz <?′ν> = 0.50 (1a), <?′ν> = 0.58 (1b), <?′ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <?′ν> = 0.50 (1a), <?′ν> = 0.23 (2a), <?′ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <?′ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH2Cl, CHCl2, CCl3, due to increase in the number of secondary encounters between HF and the departing radical. 相似文献
In this paper, the basic principle and a Monte Carlo method are described for numerically simulating the chain-length distribution in radical polymerization with transfer reaction to monomer. The agreement between the simulated and analytical results shows that our algorithm is suitable for systems with transfer reaction. With the simulation algorithm, we confirm that transfer reaction has a similar effect as disproportionation on the molecular weight distribution in radical polymerization with continuous initiation. In the pulsed laser (PL) initiated radical polymerization with transfer reaction, the ‘waves’ on the chain-length distribution profile become weaker as the ratio of transfer reaction rate constant, ktr, to the propagation rate constant, kp, is increased in the case with either combination-type or disproportionation-type termination. Moreover, it seems that the combination termination has a broadening effect on the waves. Therefore, kp can also be determined by precisely locating the inflection point Lo on the chain-length distribution profile for radical polymerization with transfer reaction, unless ktr is large enough to smear out the waves on the chain-length distribution. 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
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 solubilities of N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS) or N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropanesulfonic
acid (TAPSO) in water and in aqueous solutions of CH3COOK (KAc), KBr, KCl, or NaCl were determined from density measurements at 298.15 K. The solubilities of TAPS in aqueous solution
decrease with increasing concentration of the salts (salting-out effect), whereas those of TAPSO increase with increasing
concentration of the salts (salting-in effect). The solubility and density data were further used to calculate the apparent
transfer Gibbs energies, ΔtrG, and transfer molar volumes, DtrVfo\Delta_{\mathrm{tr}}V_{\phi}^{\mathrm{o}}, of these buffers from water to aqueous electrolyte solutions at 298.15 K. The contributions of various functional groups
of TAPS, TAPSO, and the related buffers (tris(hydroxymethyl)aminomethane, TRIS, and N-tris[hydroxymethyl]-4-amino-butanesulfonic
acid, TABS) to the transfer properties were systematically estimated from the calculated ΔtrG and DtrVfo\Delta_{\mathrm{tr}}V_{\phi}^{\mathrm{o}}. 相似文献
Polymerisation of methyl methacrylate in the presence of two catalytic chain transfer agents has been investigated. Several factors were examined to see how they would affect the chain transfer process. Oxygen and initiator impurities added to the system were found to be detrimental. The transfer process was highly intolerant of initiator impurities, but was able to endure a large excess of oxygen. The temperature dependencies of the chain transfer process were determined for cobaloxime boron fluoride (COBF) and cobalt(meso‐Ph4‐porphyrin) (CoP). The results showed that a study of MW, or ktr would be a better gauge of the catalyst activity. Activation energies were determined at two concentrations of initiator and it was discovered that the transfer process has a dependence on the initiator concentration. A mechanism for this dependency is proposed that involves a reduction in the active catalyst concentration. As a result, the measured values for Ctr, and ktr, must be apparent values. An additional consequence of the initiator dependence is that the MW of the polymer products will be directly dependent on the concentration of the initiator. 相似文献
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
Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC)
and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser
flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature,
radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide
type radicals ∼CH2–NO•–CH2∼ and/or ∼CH2–NO•–CH3∼ at the first and methyl •CH3 and formyl •CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination
rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g.,
Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol
and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained
by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO. 相似文献
The macroradical decay in poly(methyl methacrylate) samples with different thermal histories was investigated in the temperature interval 20–100 °C using ESR spectroscopy and the second order kinetic model. The rate constants exhibit two different regimes with the transitions atTtr=68±1°C which are independent of thermal treatment. ForT<Ttr andT>Ttr the rate constants as well as the corresponding activation parameters are sensitive to history because of different physical microstructures. The compensation law, i.e., the linear relation between lnko, eff andEeff, was analyzed in terms of the so-called compensation quantitieskc andTc and a proximity betweenTc=Ttr andTo=53±3 °C — Vogel temperature for -segmental dynamics was found. A comparison of kinetic and dynamic data suggests that the decay of terminal macroradicals in the low-temperature region is controlled by secondary relaxations and that the -mobility contributes to a more rapid decay at higher temperatures belowTg. 相似文献