共查询到20条相似文献,搜索用时 15 毫秒
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van As BA van Buijtenen J Mes T Palmans AR Meijer EW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(29):8325-8332
The well-known dynamic kinetic resolution of secondary alcohols and esters was extended to secondary diols and diesters to afford chiral polyesters. This process is an example of iterative tandem catalysis (ITC), a polymerization method where the concurrent action of two fundamentally different catalysts is required to achieve chain growth. In order to procure chiral polyesters of high enantiomeric excess value (ee) and good molecular weight, the catalysts employed need to be complementary and compatible during the polymerization reaction. We here show that Shvo's catalyst and Novozym 435 fulfil these requirements. The optimal polymerization conditions of 1,1'-(1,3-phenylene) diethanol (1,3-diol) and diisopropyl adipate required 2 mol% Shvo's catalyst and 12 mg Novozym 435 per mmol alcohol group in the presence of 0.5 M 2,4-dimethyl-3-pentanol as the hydrogen donor. With these conditions, chiral polyesters were obtained with peak molecular weights up to 15 kDa, an ee value up to 99% and with 1-3 % ketone end groups. Also with the structural isomer, 1,4-diol, a chiral polyester was obtained, albeit with lower molecular weight (8.3 kDa) and slightly lower ee (94%). Aliphatic secondary diols also resulted in enantio-enriched polymers but at most an ee of 46 % was obtained with molecular weights in the range of 3.3-3.7 kDa. This low ee originates from the intrinsic low enantioselectivity of Novozym 435 for this type of secondary aliphatic diols. The results presented here show that ITC can be applied to procure chiral polyesters with good molecular weight and high ee from optically inactive AA-BB type monomers. 相似文献
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Akihiro Ando Kunio Kimura Yuko Onda Yuhiko Yamashita 《Macromolecular rapid communications》2005,26(2):98-102
Summary: The stereoselective polymerization of partial rac‐lactide was attempted by using crystallization during polymerization. Polymerizations were carried out with trifluoromethanesulfonic acid in a mixed solvent of toluene and octane. The molar content of L ‐lactate in the feed was 95%. Polymerization proceeded with the elimination of D ‐lactate by means of crystallization. Poly(L ‐lactic acid) (PLLA) crystals were first precipitated and then plate‐like poly(lactic acid) (PLA) crystals were finally obtained, of which the composition of L ‐lactate was 97.6% and the was 1.68 × 104.
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Deon DE Wet‐Roos Aletta DU Toit Dawie J. Joubert 《Journal of polymer science. Part A, Polymer chemistry》2006,44(23):6847-6856
Homogeneous tandem catalysis of the bis(diphenylphoshino)amine‐chromium oligomerization catalyst with the metallocenes Ph2C(Cp)(9‐Flu)ZrCl2 and rac‐EtIn2ZrCl2, is discussed. GC, CRYSTAF, and 13C NMR analysis of the products obtained from reactions at constant temperatures show that during tandem catalysis, α‐olefins, mainly 1‐hexene and 1‐octene, are produced from ethylene by the oligomerization catalyst and subsequently built into the polyethylene chain. At 40 °C the Cr/PNP catalyst acts as a tetramerization catalyst while the polymerization catalyst activity is low. Copolymerization of ethylene and the in situ produced α‐olefins have also been carried out by increasing the temperature from 40 °C, where primarily oligomerization takes place, to above 100 °C, where polymerization becomes dominant. The melting temperature of the polymer is dependent on the catalyst and cocatalyst ratios as well as on the temperature gradient followed during the reaction, while the presence of the oligomerization catalyst reduces the activity of the polymerization catalyst. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6847–6856, 2006 相似文献
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Tsuji H 《Macromolecular bioscience》2005,5(7):569-597
Poly(lactide)s [i.e. poly(lactic acid) (PLA)] and lactide copolymers are biodegradable, compostable, producible from renewable resources, and nontoxic to the human body and the environment. They have been used as biomedical materials for tissue regeneration, matrices for drug delivery systems, and alternatives for commercial polymeric materials to reduce the impact on the environment. Since stereocomplexation or stereocomplex formation between enantiomeric PLA, poly(L-lactide) [i.e. poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e. poly(D-lactic acid) (PDLA)] was reported in 1987, numerous studies have been carried out with respect to the formation, structure, properties, degradation, and applications of the PLA stereocomplexes. Stereocomplexation enhances the mechanical properties, the thermal-resistance, and the hydrolysis-resistance of PLA-based materials. These improvements arise from a peculiarly strong interaction between L-lactyl unit sequences and D-lactyl unit sequences, and stereocomplexation opens a new way for the preparation of biomaterials such as hydrogels and particles for drug delivery systems. It was revealed that the crucial parameters affecting stereocomplexation are the mixing ratio and the molecular weight of L-lactyl and D-lactyl unit sequences. On the other hand, PDLA was found to form a stereocomplex with L-configured polypeptides in 2001. This kind of stereocomplexation is called "hetero-stereocomplexation" and differentiated from "homo-stereocomplexation" between L-lactyl and D-lactyl unit sequences. This paper reviews the methods for tracing PLA stereocomplexation, the methods for inducing PLA stereocompelxation, the parameters affecting PLA stereocomplexation, and the structure, properties, degradation, and applications of a variety of stereocomplexed PLA materials. 相似文献
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Fanny Bonnet Marc Visseaux Angela Pereira Frdric Bouyer Denise Barbier‐Baudry 《Macromolecular rapid communications》2004,25(8):873-877
Summary: The neodymium trisborohydride Nd(BH4)3(THF)3 (THF = tetrahydrofuran) has been used as a catalyst precursor for isoprene polymerization for the first time. Associated to an excess of Al(Et)3, the resulting catalyst is moderately active, giving a mixture of cis‐ and trans‐ polymer. Addition of a stoichiometric amount of MgBu2 to Nd(BH4)3(THF)3 affords a stereospecific catalyst providing trans‐1,4‐polyisoprene, more than 96% regular. That dual component Nd/Mg system also shows a better efficiency and good control of the molecular weights. A molecular structure is tentatively attributed to a bimetallic active species, based on 1H NMR experiments.
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Kittichote Musikabhumma Thomas P. Spaniol Jun Okuda 《Journal of polymer science. Part A, Polymer chemistry》2003,41(4):528-544
The synthesis of branched polyethylenes by ethylene polymerization with new tandem catalyst systems consisting of methylaluminoxane‐preactivated linked cyclopentadienyl‐amido titanium catalysts [Ti(η5:η1‐C5Me4SiMe2NR)Cl2 (R = Me or tBu)] supported on pyridylethylsilane‐modified silica (PySTiNMe and PySTiNtBu) and homogeneous dibromo nickel catalyst having a pyridyl‐2,6‐diisopropylphenylimine ligand (PyminNiBr2) in the presence of modified methylaluminoxane was investigated. Ethylene polymerization with only PyminNiBr2 yielded a mixture of 1‐ and 2‐olefin oligomers with methyl branches [weight‐average molecular weight (Mw) ~ 460)] with a ratio of about 1:7. By the combination of this nickel catalyst with PySTiNtBu, polyethylenes with long‐chain branches (Mw = 15,000–50,000) were produced. No incorporation of 2‐olefin oligomers was observed in the 13C NMR spectra. Unexpectedly, the combination of the nickel catalyst with PySTiNMe produced lower molecular weight polyethylenes with only methyl branches. The molecular weight distributions of branched polyethylenes obtained with both PySTiNMe and PySTiNtBu combined with the nickel catalyst were broad (weight‐average molecular weight/number‐average molecular weight < 9). Bimodal gel permeation chromatography (GPC) curves were clearly observed in the PySTiNMe system, whereas GPC curves with small shoulders in low molecular weight areas were observed for PySTiNtBu. The synthesis of branched polyethylenes with tandem catalyst systems of corresponding homogeneous titanium catalysts and the nickel catalyst was also investigated for comparison. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 528–544, 2003 相似文献
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Philippe Zinck Denise Barbier‐Baudry Andr Loupy 《Macromolecular rapid communications》2005,26(1):46-51
Summary: Isoprene polymerisation performed under microwave irradiation with [Nd(BH4)3(THF)3]/Mg(Bu)2 and [Nd(BH4)3(THF)3]/Al(Et)3 showed an enhancement in reactivity, with selectivity being only slightly modified. An explanation of the observed effect is proposed based on our current knowledge of the catalytic mechanism and by considering the alkylated complex as an ion pair. An analogy is proposed with the pseudoanionic polymerisation of oxygenated monomers. Finally, depolymerisation is observed under microwave irradiation at high temperature.
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Mark R. Ten Breteler Zhiyuan Zhong Pieter J. Dijkstra Anja R. A. Palmans Joris Peeters Jan Feijen 《Journal of polymer science. Part A, Polymer chemistry》2007,45(3):429-436
The ring‐opening polymerization (ROP) of ε‐caprolactone (ε‐CL), 4‐methyl‐ε‐caprolactone (4‐MeCL), and 6‐methyl‐ε‐caprolactone (6‐MeCL) with a single‐site chiral initiator, R,R′‐(salen) aluminum isopropoxide (R,R′‐[1]), was investigated. The kinetic data for the ROP of the three monomers at 90° in toluene corresponded to first‐order reactions in the monomer and propagation rate constants of kε‐CL > k4‐MeCL ? k6‐MeCL. A notable stereoselectivity with a preference for the R‐enantiomer was observed in the ROP of 6‐MeCL with R,R′‐[1], whereas for 4‐MeCL, no stereoselectivity was found. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 429–436, 2007. 相似文献
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Design of supports containing a hyperbranched backbone and active transition metal centers at the periphery is described. Such nanoarchitectures can be easily assembled from 3,5-dihydroxybenzyl alcohol and dimethylsilylamine using a divergent synthetic methodology. Following a controlled reaction pathway, construction of dendrimers of up to generation 5 is achieved, while a simple mix of reagents in one-pot or with sequential additions yields analogous hyperbranched polymers. Subsequent functionalization at the periphery with phosphine followed by Rh(I) centers yields the desired metallodendritic materials. The efficiency of the latter in catalytic hydrogenation of decene under varied conditions is surveyed. 相似文献
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Mariusz Majchrzak Monika Ludwiczak Malgorzata Bayda Bronislaw Marciniak Bogdan Marciniec 《Journal of polymer science. Part A, Polymer chemistry》2008,46(1):127-137
The novel trans‐stereo‐regular silylene–thiophene derivatives ( 4 , 5 ) with perfect consecutive silylene–arylene–silylene–vinylene linkage were synthesized via silylative coupling polycondensation of 2,5‐bis(vinyldimethylsilyl)thiophene ( 2 ) or 5,5′‐bis(vinyldimethylsilyl)‐2,2′‐bithiophene ( 3 ) catalyzed by ruthenium‐hydride complex [RuHCl(CO)(PCy3)2] ( 1 ). Their spectroscopic, absorption, and luminescence properties were characterized and compared with those of model compounds containing thiophene or bithiophene chromophores. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 127–137, 2008 相似文献
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Kouki Matsubara Chihiro Terata Hiromichi Sekine Kenji Yamatani Tatsuo Harada Kiyoka Eda Misato Dan Yuji Koga Munehisa Yasuniwa 《Journal of polymer science. Part A, Polymer chemistry》2012,50(5):957-966
Ring‐opening polymerization of D,L ‐lactide was stereoselectively achieved using newly designed aluminum alkoxide complexes as initiators. These half‐SALEN aluminum complexes bearing tridentate nonchiral Schiff‐base ligands are racemates, which provide chirality in the aluminum centers, efficiently afforded a stereoblock copolymer of D,L ‐LA. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
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Sriram Venkataramani Tharanikkarasu Kannan Palash Jyoti Das Ganga Radhakrishnan 《先进技术聚合物》2009,20(12):892-898
Novel AB crosslinked polymer (ABCP) networks were synthesized from telechelic 4‐vinylbenzyl carbamate terminated polyurethanes and monomers such as styrene, 4‐vinylpyridine, methyl methacrylate and butyl acrylate. Telechelic 4‐vinylbenzyl carbamate terminated polyurethanes were synthesized from polypropylene glycol‐based NCO‐terminated polyurethane and vinylbenzyl alcohol. Effect of changing the molecular weight of polypropylene glycol on the static and dynamic mechanical properties of ABCP networks from polyurethane‐polymethyl methacrylate was studied in detail. Dynamic mechanical thermal analysis results show that polymethyl methacrylate and polystyrene‐based ABCPs have good damping over a broad temperature range. ABCP networks prepared from 4‐vinylbenzyl carbamate terminated polyurethane and different monomers such as methyl methacrylate, butyl acrylate and styrene exhibit single tan δmax value which implies excellent interlocking between the two polymers present in the ABCP networks. Static mechanical studies showed that methyl methacrylate and styrene‐based ABCP networks exhibit better tensile properties compared to other ABCP networks from butyl acrylate and 4‐vinyl pyridine monomers. Thermogravimetric analysis results revealed that the ABCP networks showed an improved thermal stability. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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The development of efficient methods to access complex molecules with multistereogenic centers has been a substantial challenge in both academic research and industrial applications. One approach to this challenge is catalytic asymmetric tandem transformations, which allow a rapid increase in molecular complexity from readily available starting materials to produce enantiopure compounds. In recent years, considerable efforts have been directed towards the development of asymmetric tandem transformations. This Minireview highlights recent developments and the applications of metal-catalyzed and organocatalytic asymmetric tandem transformations triggered by conjugate additions. 相似文献
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A Cu(Ⅰ)-catalyzed one-pot tandem reaction of 2-bromobenzyl bromides with 1,3-dicarbonyl compounds leading to 4H-chromene derivatives has been developed.This new approach toward 4H-chromenes combines several reactions in one pot and builds molecular complexity from readily available starting materials. 相似文献