Alkoxide chain

However, in most cases, propagation takes place through carboxylate chain-ends, although coexistence with alkoxide chain ends in the beginning of ROP... 

The coordination polymerisation of lactones with a six- and seven-membered ring (5- and e-lactones respectively) occurs via ring opening at the C(0)-0 linkage to generate metal alkoxide chain terminals, following a reaction analogous to that presented by scheme (9). 

Unlike anionic initiators or anionically growing alkoxide chains which can only grow (or terminate), cationic initiators (Lewis, Bronsted acids or preformed initiators) or the cationically growing chain may cause acetal-interchange reactions. These reactions are also called transacetalization and cause rearrangement in the molecular weight distribution in homopolymers. The rates of transacetalization are relatively slow compared to that of polymerization except at high temperatures. In the presence of cyclic ethers or cyclic formals, for example, dioxolane, polyformaldehyde can incorporate randomly the co-monomer polyoxyethylene units into the polymer under transacetalization conditions. 

The polymeric structure of [Sn(ONeoPent)2]oo is shown in Figure 2.12.10. Within linear oligometallic alkoxide chains, each tin center is coordinated by four ONeoPent ligands through /u-2-0, forming a chain. 

Because the monomer was not a vinyl compound and the active chain end was an alkoxide, this reaction was not considered an important case of anionic polymerization. Ironically, this reaction actually is a very good example of the anionic mechanism and can be satisfactorily studied because it is a homogeneous reaction. In fact, it was Flory (6) who first pointed out the unique consequences that arise from such a polymerization in which presumably the alkoxide chain end does not undergo any "side reactions," that is, termination. Flory remarked that in such a situation in which all the growing chains have equal access to the monomer, the chains will tend to reach similar lengths, that is, the molecular weight of the polymer will have the very narrow Poisson distribution ... 

On the other hand, in the purely anionic polymerization of five- or higher-membered cyclic esters, the carbonyl carbon of the monomer is attacked with subsequent acyl-oxygen bond scission and reformation of the alkoxide anion. In the coordination polymerization, this is also the carbonyl carbon that is now first coordinated with alkoxide species and then the acyl-oxygen bond is broken with reforming of the covalent alkoxide chain end. In the already formed macromo-lecular chains, the same ester bonds are present as those being the site of the nucleophilic attack in the monomer molecules. These processes are illustrated in Scheme 12, where the active centers are shown as. ..-OMt, for both anionic and covalent centers. 

With ethylene oxide, the active alkoxide chain end can abstract a hydrogen from the chain, leading to terminal unsaturation and a new chain-initiation site. 

This reaction is important in the manufacture of long-chain alcohols by means of hydrolysis of the aluminum alkoxide. Examples of oxidation of metal alkoxides (40,42) include ... 

PZN-PT, and YBa2Cug02 g. For the preparation of PZT thin films, the most frequently used precursors have been lead acetate and 2irconium and titanium alkoxides, especially the propoxides. Short-chain alcohols, such as methanol and propanol, have been used most often as solvents, although there have been several successful investigations of the preparation of PZT films from the methoxyethanol solvent system. The use of acetic acid as a solvent and chemical modifier has also been reported. Whereas PZT thin films with exceUent ferroelectric properties have been prepared by sol-gel deposition, there has been relatively Httle effort directed toward understanding solution chemistry effects on thin-film properties. 

Reactions of the Side Chain. Benzyl chloride is hydrolyzed slowly by boiling water and more rapidly at elevated temperature and pressure in the presence of alkaHes (11). Reaction with aqueous sodium cyanide, preferably in the presence of a quaternary ammonium chloride, produces phenylacetonitrile [140-29-4] in high yield (12). The presence of a lower molecular-weight alcohol gives faster rates and higher yields. In the presence of suitable catalysts benzyl chloride reacts with carbon monoxide to produce phenylacetic acid [103-82-2] (13—15). With different catalyst systems in the presence of calcium hydroxide, double carbonylation to phenylpymvic acid [156-06-9] occurs (16). Benzyl esters are formed by heating benzyl chloride with the sodium salts of acids benzyl ethers by reaction with sodium alkoxides. The ease of ether formation is improved by the use of phase-transfer catalysts (17) (see Catalysis, phase-thansfer). 

These alkoxides are liquids or sublimable solids and, unless the steric effects of the alkyl chain prevent it, apparently attain octahedral coordination of the titanium by polymerization (Fig. 21.6). The lower alkoxides are especially sensitive to moisture, hydrolysing to the dioxide. Application of these organic titanates (as they are frequently described) can therefore give a... 

Substitution of an additional nitrogen atom onto the three-carbon side chain also serves to suppress tranquilizing activity at the expense of antispasmodic activity. Reaction of phenothia zine with epichlorohydrin by means of sodium hydride gives the epoxide 121. It should be noted that, even if initial attack in this reaction is on the epoxide, the alkoxide ion that would result from this nucleophilic addition can readily displace the adjacent chlorine to give the observed product. Opening of the oxirane with dimethylamine proceeds at the terminal position to afford the amino alcohol, 122. The amino alcohol is then converted to the halide (123). A displacement reaction with dimethylamine gives aminopromazine (124). ... 

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

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