Poly dehydrogenated

About 35% of total U.S. LPG consumption is as chemical feedstock for petrochemicals and polymer iatermediates. The manufacture of polyethylene, polypropylene, and poly(vinyl chloride) requires huge volumes of ethylene (qv) and propylene which, ia the United States, are produced by thermal cracking/dehydrogenation of propane, butane, and ethane (see Olefin polymers Vinyl polymers). 

The acetone supply is strongly influenced by the production of phenol, and so the small difference between total demand and the acetone suppHed by the cumene oxidation process is made up from other sources. The largest use for acetone is in solvents although increasing amounts ate used to make bisphenol A [80-05-7] and methyl methacrylate [80-62-6]. a-Methylstyrene [98-83-9] is produced in controlled quantities from the cleavage of cumene hydroperoxide, or it can be made directly by the dehydrogenation of cumene. About 2% of the cumene produced in 1987 went to a-methylstyrene manufacture for use in poly (a-methylstyrene) and as an ingredient that imparts heat-resistant quaUties to polystyrene plastics. 

PCSs obtained by dehydrochlorination of poly(2-dilorovinyl methyl ketones) catalyze the processes of oxidation and dehydrogenation of alcohols, and the toluene oxidation207. The products of the thermal transformation of PAN are also catalysts for the decomposition of nitrous oxide, for the dehydrogenation of alcohols and cyclohexene274, and for the cis-tnms isomerization of olefins275. Catalytic activity in the decomposition reactions of hydrazine, formic acid, and hydrogen peroxide is also manifested by the products of FVC dehydrochlorination... 

Reactions with other nucleophiles follow a similar mechanism. For the reaction of Cl with poly(3-methylthiophene) in acetonitrile, the reaction stops at structure 5 (Scheme 2).128 A fully conjugated, Cl-substi-tuted product 6 can subsequently be obtained by electrochemical or chemical dehydrogenation.128 With Br and alcohols, the overoxidation... 

In the course of studying the reactions of Si-H compounds with dialkyltitanocenes, with a view to the synthesis of new hydridosilyltitanocene complexes, we adventitiously discovered that phenylsilane undergoes facile, quantitative dehydrogenative coupling to a linear poly(phenylsilylene) under the catalytic influence of dimethyltitanocene. The ease with which this reaction proceeds initially induced us to underestimate the significance of the observation. 

In an enantiomer-differentiating oxidation at a poly-(L-valine)-coated Pb02 anode, rac-2,2-dimethyl-l -phenyl-1 -propanol was partially oxidized leaving 43% optically pure (5)-alcohol [371]. At a TEMPO-modified graphite felt anode rac-1-phenyl-ethanol has been enantioselectively oxidized in the presence of (-[-sparteine leaving 46% of the (/ [-alcohol with 99.6% ee [372]. However, under the same conditions, an exclusive dehydrogenation of (-[-sparteine to the iminium salt without oxidation of the alcohol was found [373]. 

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

Alkane dehydrogenation has been demonstrated as a suitable method for the functionalization of polyolefins such as atactic poly(l-hexene) under homogeneous conditions (Equation 12.6) [23]. 

Flatanaka T, Kawahara T, Asahi N, Tsuji M (1995) Effects of the structure of poly(vinyl alcohol) on the dehydrogenation reaction by poly(vinyl alcohol) dehydrogenase from Pseudomonas sp. 113P3 T. Biosci Biotechnol Biochem 59 1229-1231... 

These polymers do not contain a functional group within the polymer chain but are classified as condensation polymers, since water is split out during the polymerization process. Another example is poly(p-xylene). which is produced by the oxidative coupling (dehydrogenation) of p-xylene ... 

Poly(2,6-dimethylphenylene ether) can be prepared by dehydrogenation of 2,6-dimethylphenol with oxygen in the presence of copper(l) chloride/pyridine as catalyst at room temperature. It is known that the mechanism involves a stepwise reaction, probably proceeding via a copper phenolate complex that is then dehydrogenated. 

Ebel s method is an adaptation of the Stoermer synthesis of benzo-[h]furans and involves the 0-alkylation of a phenolate anion (229, Scheme 58) with a 2-halocyclohexanone (230). The resultant 2-phenoxycyclo-hexanone 231 is then cyclized by poly phosphoric acid, usually at 100°C, or sometimes by concentrated sulfuric acid, to afford a 1,2,3,4-tetrahydrodi-benzofuran (232). Dehydrogenation to the dibenzofuran is often effected with palladized charcoal, but 2,3-dichloro-5,6-dicyano-l,4-benzoquinone ... 

A significant improvement was achieved when Shvo s Ru-catalyst 2 (Fig. 12b) was employed in combination with the addition of DMP to suppress dehydrogenation reactions [106]. Poly-(R)-6-MeCL, with a promising ee of 86% and Mp of 8.2 kDa, was obtained after workup starting from optically pure (5 )-6-MeCL. The low rate of reaction compared to DKR (typically complete after 48 h with the Shvo catalyst) is attributed to the low concentration of the terminal alcohol as well as to the iterative nature of the system. Racemic 6-MeCL showed comparable rates of reaction for both enantiomers, which polymerized within 220 h with complete conversion of both enantiomers, yielding polymers of high ee (92%) and Mp (9.4 kDa). Successful polymerizations with more than 100 consecutive and iterative enzymatic additions and Ru-catalyzed racemizations on one polymer chain were realized. 

One of the common chemical methods for determining carbonyl compounds consists of converting them into hydrazones [7], This has been used for (1) oxycellulose [28], (2) nylon-6 and nylon-6,6 [29], (3) dehydrogenated poly (vinyl chloride) [30], (4) in irradiated polyethylene films [31], and (5) grafted polyethylene glycol [32],... 

From this example it is clear that the selectivity for (a) dehydrogenation, (b) isomerization, and (c) cracking is likely to be related to the relative concentrations of mono-, di-, and tri-adsorbed complexes, etc. More generally, the selectivity of a catalytic reaction will depend on the relative chance for a molecule adsorbed on -surface atoms either to desorb or become adsorbed on (n + 1) surface atoms. This idea easily permits us to understand that dilution of an element A, capable of forming chemisorption bonds with a given molecule, with an inert element B will lower the ratio of poly- to monoadsorbed molecules and have an effect on catalytic selectivity. We will call this concept the primary ensemble effect. 

Organic compounds having labile hydrogen atoms, such as phenols, anilines, and acetylenes, are also oxidatively polymerized by metal-complex catalysts (Eqs. 1-3). The oxidative coupling is a dehydrogenation reaction the polymer chain produced contains the dehydrogenated monomer structure as a repeating unit. As a remarkable example, poly(phenylene ether), one of the... 

Note that each of these simple elementary reactions is reversible, and so the entire catalytic cycle is also reversible. This is known as the principle of microscopic reversibility. Consequently, if platinum is a good hydrogenation catalyst, then it must also be a good dehydrogenation catalyst. In fact, as we will see later, catalysts change only the reaction rate, not the equilibrium. Every catalyst catalyzes both the forward and the reverse reactions in the same proportions. In the above example, the reverse reaction is actually more interesting for industry, because propene is a valuable monomer for making poly(propylene) and other polymers. 

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