Subject polymerization isomerism

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

Reacting gases may be in excess if they react with solids and do not condense in liquid phases, but supercritical media are clearly not the subject of solvent-free chemistry and deserve their own treatment. For practical reasons, this book does not deal with homogeneous or contact-catalyzed gas-phase reactions. Furthermore, very common polymerizations (except for solid-state polymerizations), protonations, solvations, complexations, racemizations, and other stereo-isomerizations are not covered, to concentrate on more complex chemical con-... 

Lewis acids readily isomerize both 1,3-dioxolanes and 1,3-oxathiolanes in ether solution. The reaction proceeds by coordination with the oxygen atom in the latter case since 1,3-dithiolanes do not isomerize under the same conditions. With trityl carbonium ion, an oxidative cleavage reaction takes place as shown in Scheme 6. Hydride extraction from the 4-position of 2,2-disubstituted 1,3-dioxolanes leads to an a-ketol in a preparatively useful reaction. 1,3-Oxathiolanes are reported to undergo similar cleavage but no mention of products other than regeneration of the ketone has been made (71CC861). Cationic polymerization of 1,3-dioxolane has been initiated by a wide variety of proton acids, Lewis acids and complex catalytic systems. The exact mechanism of the polymerization is still the subject of controversy, as is the structure of the polymer itself. It is unclear if polymerization... 

The kinetics of olefin polymerization are the subject of several studles>104,153-156,162,182,221,226,240,241,246,252,255,266,28 12 and of an excellent book by Keii.17 The most relevant studies will be discussed below. However, we first note that the precise description of the kinetics of catalytic olefin polymerization under industrially relevant polymerization conditions has proved to be very difficult. For a given catalytic system, one has to identify all possible insertion, chain-release, and chain-isomerization reactions, and their dependence on the polymerization parameters (most importantly, temperature and monomer concentration). Once the kinetic laws for each elementary step have been determined, they have to be combined in one model in order to be able to predict the catalyst performance. This has been attempted for both ethylene and propylene polymerizations. The case of propylene polymerization with a chiral, isospecific zirconocene is shown in Figure 14.162... 

The carbanionic character of 2 is shown from UV and HNMR spectra. The method is not of synthetic significance, since cisjtrans mixtures of 4 arise trans strongly predominate), and 4 is usually accompanied by dimers, polymers, vinylacetic acids (from uncyclized 2) and other byproducts. Stereochemical information in 1 is lost because allylmagnesium halides 2 are subject to fast EjZ isomerization. Heating of 2, neat or in solution, does not lead to 3, and gives polymeric products. ... 

A solution of bicyclo[3.2.0]hepta-2,6-diene (4 g, 43 mmol) in benzene (24 g, 308 mmol) was irradiated in a quartz tube with 253.7 nm radiation from three modules of a Rayonet Type RS RPR-208 photoreactor. After 40 h, the solution was distilled and the distillate reirradiated. From eight such successive irradiations, the residues were collected. Any unreacted benzene and bicycloheptadiene were removed by distillation at atmospheric pressure and the residue was subjected to bulb-to-bulb distillation at ca. 1 Torr to remove polymeric material. The distillate ( 4g), which was light yellow, was then separated by preparative GC (UCON-550X column, 9 f), at 155 °C) to give, in addition to 1 g (15 %) of isomeric pentacyclodecadienes, the title compound yield 2.2 g (30%). 

The hydrosilylation of alkenes and alkynes catalyzed by transition metal complexes is often accompanied by side reactions such as isomerization, polymerization, and hydrogenation of unsaturated compounds and/or redistribution and dehydrogenation of silicon hydrides as well as reactions in which both substrates take part, such as dehydrogenative silylation (4,13). The latter reaction, which in certain conditions permits direct production of unsaturated silyl compound, has been a subject of intense study over the last two decades. 

Diacetylenes in phospholipid bilayers have been the subject of extensive studies in our laboratory, not only because of the highly conjugated polymers they form, but also because of their ability to transform bilayers into interesting microstructures. Consequent to our synthesis and characterization of several isomeric diacetylenic phospholipids, we have found that the polymerization in diacetylenic bilayers is not complete. In order to achieve participation of all diacetylenic lipid monomer in the polymerization process, diacetylenic phospholipid was mixed with a spacer lipid, which contained similar number of methylenes as were between the ester linkage and the diacetylene of the polymerizable lipid. Depending upon the composition of the mixtures different morphologies, ranging from tubules to liposomes, have been observed. Polymerization efficiency has been found to be dependent on the composition of the two lipids and in all cases the polymerization was more rapid and efficient than the pure diacetylenic system. We present the results on the polymerization properties of the diacetylenic phosphatidylcholines in the presence of a spacer lipid which is an acetylene-terminated phosphatidylcholine. 

Included among the many types of vinyl monomers that have been subjected to photoinitiated cationic polymerization are styrene," substituted styrenes, a-methylstyrenes, N-vinylcarbazole, alkyl vinyl ethers, prop-l-en-l-yl ethers, ketene acetals, and alkoxyallenes. Most useful in the crosslinking photopolymerizations employed for UV curing applications are multifunctional vinyl ethers and multifunctional prop-l-en-l-yl ethers. A number of multifunctional vinyl ether monomers are available from commercial sources, while multifunctional prop-l-en-l-yl ethers can be readily prepared by catalytic isomerization from their corresponding allyl ether precursors. The photoinitiated cationic... 

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