Polyacetals structure

FTIR, 1H NMR and 13C NMR analysis support the polyacetal structure containing intact cyclopropyl rings cited by previous workers (5,6)... 

Amorphous polyacetaldehyde was obtained for the first time in 1936 by Travers (9) and Letort (10) independently by cooling acetaldehyde vapor to liquid air temperature in high vacuum. The polyacetal structure of this polymer was suggested by Staudinger (//). This poly-... 

Crotonaldehyde, like acrolein, can be expected to yield polymers with structures derived from 1,2 3,4 or 1,4 additions. Anionic catalysts, however, yield predominantly polyacetal structures. ... 

Similar polyacetal structures were prepared by BASF scientists from general dialdehydes and aliphatic hydroxycarboxylic acids derived from sugars [156, 157], shown in Scheme 15. Alternatively, carboxypolyacetals are available by the addition of polyhydroxy carboxylic acids, tartaric acid, for example, to divinyl ethers [158]. 

Triacetic acid lactone is the simplest of all polyacetate structures. Its formation incorporates an essential feature of the synthesis of acetate-derived phenols, since it is formed by condensation from acetyl-CoA and malonyl-CoA via an intermediate acetoacetyl-enzyme (Nixon et al., 1968 Yalpani et al., 1969) or acetoacetyl-ACP stage (Brock and Bloch, 1966), at which point... 

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

Furaldehyde is a classical example of such thermodynamically unfavoured monomers . Its strong conjugation with the ring is well represented by a carbonyl frequency at about 1670 cm-1 and the best indication of its reluctance to polymerize is simply the fact that, despite many attempts and some claims of success, no one has in reality been able to prepare a polyacetal with the structure given below ... 

The intriguing structural complexity and often potent biological activity exhibited by many polyacetate-derived natural products have generated a continuing interest in the development of efficient, stereoselective approaches to the assembly of repeating l -diol subunits. One such method is the alkylation and reduc-... 

Poly(4-phenoxybenzoyl-1,4-phenylene) (PPBP), sulfonated, 23 718 Polyacetal, antioxidant applications, 3 121 Polyacetaldehyde, 1 103 Polyacetal fiber, 13 392 Polyacetylene, 7 514-515 26 953 conduction in, 7 527 22 208 molecular structure of, 22 211 optical band gap, 7 529t Peierls distortion in, 22 203, 208 room temperature conductivity, 7 532 synthesis of, 22 213... 

Commercial polymers of formaldehyde are also produced using cationic polymerization. The polymer is produced by ring opening of trioxane. Since the polyacetal, POM, is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (structure 5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a... 

Another stable polyacetal (POM Celcon) is produced by the cationic copolymerization of a mixture of trioxane and dioxolane (structure 5.23). 

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. a-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary structure. 

In the stable forms of the polyacetates of the aldohexopyranosyl and aldohexofuranosyl halides, the halogen atom at Cl is trans to the group at the ring junction when the structural formulas are written according to the Haworth convention, (XIV) to (XVII). 

Inasmuch as the pentosan structure represents a polyacetal, the acid hydrolysis of pentosan corresponds to the hydrolysis of acetals [3]. 

Figure 5 Structure of the dimeric polyacetate alkaloid psylloborine from the ladybird beetle Psyllobora vigintiduopunctata (12).46...

While biosynthetic studies have yet to report on the epothilones, their structure is presumably elaborated from a polyacetate pathway with extra pendant methyl groups added from L-methionine the structure also contains an interesting gem dimethyl grouping at C-4. The unusual thiazole starter unit can be predicted to arise either from cysteine directly with post-PKS addition of a C2 unit (probably acetate) and cyclisation, or from an N-acetylcysteine, or from a... 

Ladybird beetles produce a wide variety of polyacetate or fatty acid-derived alkaloids, many of which closely resemble the ant venom alkaloids described in the previous section. The original discovery of coccinelline (75) and precoccinelline (100) (Figure 32) in blood of the European ladybird beetle, C. 1-punctata had been motivated in part by these beetles aposematic coloration and their tendency to reflex-bleed when molested. Many ladybird beetle species share these characteristics and correspondingly are chemically protected by alkaloids as well. The carbon skeletons of almost all of these alkaloids are based on simple unbranched chains. The structures, biology, and laboratory syntheses of ladybird beetle alkaloids have been reviewed previously by Daloze et al and by Glisan King and Meinwald. ... 

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