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991.
Benoît Barrire Michel Glotin Ludwik Leibler 《Journal of Polymer Science.Polymer Physics》2000,38(24):3201-3209
Chlorinated poly(vinyl chloride) (CPVC) obtained through a dry chlorination of PVC films or grains can show a heterogeneous repartition of its chlorine atoms because the reaction process is limited by the mass transfer of the chlorine gas in the material. In order to describe the evolution of such a system, a set of coupled equations is derived where only two dimensionless constants have to be determined: KS, which depends on the solubility of the chlorine gas in the PVC, and KT, the ratio between the diffusion and the reaction characteristic times. The kinetics of chlorination obtained for the different regimes matches the available experimental data, and the corresponding concentration profiles for the chlorinated PVC chains are displayed to demonstrate how a heterogeneous chlorination can arise from this dual process. In particular, a sharp interface appears in the diffusion‐limited regime that separates the chlorinated region from an unchlorinated core and is shown to progress deeper into the film with the square root of time. To a larger extent, this analysis shows how heterogeneity of reaction and nonlinear effects can arise from a coupling between a diffusive phenomenon and a reactive phenomenon. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3201–3209, 2000 相似文献
992.
Patrick Cancouët Elodie Daudet Grard Hlary Michel Moreau Georges Sauvet 《Journal of polymer science. Part A, Polymer chemistry》2000,38(5):826-836
Poly(hydrogenmethylsiloxane‐co‐dimethylsiloxane)s of various compositions have been prepared by cationic ring‐opening polymerization of octamethylcyclotetrasiloxane (D4) and 1,3,5,7‐tetramethylcyclotetrasiloxane (D) in the presence of hexamethyldisiloxane as end‐blocker or by rearrangement of poly(hydrogenmethylsiloxane) in the presence of D4. These copolymers were examined by high resolution 1H NMR (500.13 MHz) and 29Si NMR (99.37 MHz) spectroscopies. Triad effects were observed by 1H and up to heptad effects by 29Si NMR. The chemical shifts were assigned for these stereosequences. The intensities of the triad signals were used to calculate the quantitative parameters describing the microstructure of the copolymer chains: number‐average block length (L̄) and persistence ratio (η). The values of these parameters for copolymers prepared in various experimental conditions show that the time necessary for redistribution reactions (backbiting) is much larger than the time required to establish the equilibrium between linear polymer and cyclic oligomers. However, redistribution is fast enough to prevent the formation of block copolymers even in the case of the rearrangement of poly(hydrogenmethylsiloxane) in the presence of D4. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 826–836, 2000 相似文献
993.
The grafting of polystyryl lithium onto poly(chloroethyl vinyl ether) chains has been investigated. The reaction proceeds cleanly and quantitatively thus allowing the synthesis of comblike polymers. Since the dimensions of the polystyrene branches and of the poly(chloroethyl vinyl ether) backbone can be controlled by living polymerizations, both the length and the number of branches of the graft copolymers can be tuned. The latter behave as star polymers. The possibility to initiate a new cationic polymerization of chloroethyl vinyl ether from polystyrene branches bearing acetal termini in order to prepare the corresponding stars with poly(chloroethyl vinyl ether-b- styrene) branches is also examined. Finally access to hyperbranched polymers of controlled architecture and dimensions by deactivation of a second amount of polystyryl lithium onto the last blocks of poly(chloroethyl vinyl ether) is also reported. 相似文献
994.
Bernadette Charleux Michel Moreau Jean-Pierre Vairon Savvas Hadjikyriacou Rudolf Faust 《Macromolecular Symposia》1998,132(1):25-35
The use of non-homopolymerizable monomers such as 1,1-diphenylethylene (DPE) in the synthesis of functional polymers and block copolymers by cationic polymerization has been recently reported. The most important parameters in this process, the kinetics and extent of ionization and capping as well as the stability of the cation, were investigated by studying the reaction with TiCl4 of 1-methoxy-1,1-diphenyl-3,3,5,5-tetramethylhexane, a model for DPE-capped polyisobutene chain-end. This study was performed using 1H NMR and the high-purity stopped-flow device coupled with UV-visible spectroscopy. 相似文献
995.
Aihemaiti Kayishaer Sami Fadlallah Louis M. M. Mouterde Aurlien A. M. Peru Yasmine Werghi Fanny Brunois Quentin Carbou Michel Lopez Florent Allais 《Molecules (Basel, Switzerland)》2021,26(24)
Levoglucosenone (LGO) is a cellulose-derived molecule that is present commercially on a multi-ton/year scale. Taking advantage of the α,β-conjugated ketone of LGO, a new citronellol-containing 5-membered lactone (HBO-citro) was synthesized through a one-pot two-step pathway involving oxa-Michael addition and Baeyer-Villiger oxidation. The solvent-free treatment of HBO-citro with NaBH4 at room temperature led to the full reduction of the lactone moiety which gave a novel fully renewable triol monomer having a citronellol side chain (Triol-citro). Noticeably, by simply changing the reducing agent, temperature and reaction duration, the partial reduction of HBO-citro can be achieved to yield a mixture of 5- and 6-membered Lactol-citro molecules. Triol-citro was chosen to prepare functional renewable polyesters having citronellol pendant chains via polycondensation reactions with diacyl chlorides having different chain lengths. Good thermal stability (Td5% up to 170 °C) and low glass transition temperatures (as low as −42 °C) were registered for the polyesters obtained. The polymers were then hydrolyzed using a commercial lipase from Thermomyces lanuginosus (Lipopan® 50 BG) to assess their biodegradability. A higher degradation profile was found for the polyesters prepared using co-monomers (acyl chlorides) having longer chain lengths. This is likely due to the decreased steric hindrance around the ester bonds which allowed enhanced accessibility of the enzyme. 相似文献
996.
997.
Catherine Pale-Grosdemange Corinna Feil Michel Rohmer Karl Poralla 《Angewandte Chemie (International ed. in English)》1998,37(16):2237-2240
Tetracyclic triterpenes (for example, 3 ) are formed as minor products of the cyclization of squalene ( 1 ) by squalene/hopene cyclase; they have also been found in vivo. The identification of these side products offers new insight into the complex biosynthesis of the hopane skeleton, and proves that the enzyme has no absolute control of the conformation of the acyclic substrate squalene and that tetra- and pentacyclic carbocationic intermediates are involved (for example, 2 ) in the cyclization process. 相似文献
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