Polyacetals preparation

Heller et al.201,202) showed that the polyacetals prepared by the condensation of a divinyl ether and a diol are only marginally useful for the release of norethindrone because hydrolysis of the acetal linkage at pH 7.4 is too slow. On the other hand the... 

We shall start, however, this review of the progress in the polymerization of cyclic acetals from a brief description of the new polyacetals prepared, and from summarizing of the new data on the thermodynamics of polymerization of substituted 1,3-dioxolans. 

Polyacetals prepared from formaldehyde are engineering thermoplastics, which have found use in traditional metal applications. Some trade names of these polymers are Delrin acetal homopolymer (DuPont) Celcon acetal copolymer (Celanese/Hoechst) Duracon acetal copolymer (Celanese and Diacel—joint venture) Tenac acetal homopolymer by Asahi Chemical in Japan and Ultraform acetal copolymer jointly by BASF and Degussa, in Germany. The polymers have the basic structure shown below ... 

Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

Similar polyacetals were prepared by BASF scientists from CO-aldehydic aUphatic carboxyUc acids (189,190) and by the addition of poly(hydroxycarboxyhc acid)s such as tartaric acid to divinyl ethers (191) as biodegradable detergent polymers. 

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 oxygens of methoxynaphthazarin 247 (Scheme 65) (white arrows) can be selectively ortho acetylated/methylated. Starting from the acetates or polyacetates of Scheme 65, it is possible to prepare a variey of polycyclic hydroxyquinone... 

A number of very useful indirect methods are available for the preparation of /3-D-glycosides. Polyacetate esters of 2-amino-2-deoxy-D-glucosyl bromide can be used,146 but, for general use, the more stable chlorides are to be preferred, since there is then little tendency for acyl migration147 and they can yield a wider range of glycosides.148,149 Under certain circum-... 

The objectives of this study were to extend these synthetic methods to the preparation of low molecular weight, hydroxy-terminated polymers containing acetylenic bonds and evaluating these prepolymers in castable solid propellant formulations. Since 2-butyne-l,4-diol was commercially available, the formation of polyacetals from this glycol seemed to be an attractive route to the desired polymers. It was believed that the rigid triple bond would inhibit the cyclic acetal formation observed by the earlier workers with the lower members of the saturated glycol series. Thus, in an acetal interchange reaction a linear polymer (I) should be favored over the cyclic acetal (II) ... 

Polyacetal polymers containing polyethylene glycol have been prepared which are stable at physiological pH but which readily degrade at lower pH s. Bolton Hunter reagent conjugates prepared from these materials showed a favorable biodistribution profile of the product. 

Bleach resistant polyacetals consisting of 98% trioxane and 2% dioxolane was prepared by Notorgiacomo [1] and used in molding compositions. 

Polyacetal resin compositions having excellent wear resistance were prepared by Kim [4], These resins consisted of polyoxymethylene polymer, ethylene vinylacetate, melamine, triethyleneglycol-bis-3-(3-t-butyl-4-hydroxy-5-methylphenyl)-propionate, and hydroxyl pentaerythritol fatty acid ester. 

Cationic polymerizations are not only important commercial processes, but, in some cases, are attractive laboratory techniques for preparing well-defined polymers and copolymers. Polyacetal, poly(tetramethyl-ene glycol), poly(e-caprolactam), polyaziridine, polysiloxanes, as well as butyl rubber, poly(N-vinyl carbazol), polyindenes, and poly(vinyl ether)s are synthesized commercially by cationic polymerizations. Some of these important polymers can only be prepared cationically. Living cationic polymerizations recently have been developed in which polymers with controlled molecular weights and narrow polydispersity can be prepared. 

If R = R (bifunctional polymers), reaction (119) does not affect the functionality but leads to the broadening of the molecular weight distribution, which is occurring anyway, due to the reversibility of propagation. Thus, several bifunctional polymers of 1,3-dioxolane were prepared and used, for example, to form the networks containing degradable and hydrolyzable polyacetal blocks (cf., Section IV.B). Reaction (119), however, may effectively prohibit the preparation of monofunctional polymers, e.g., macromonomers. Indeed, two recent attempts to prepare macromonomers by cationic polymerization of cyclic acetals led to nearly statistical... 

Polyacetals form a different subclass of compounds with oxygen in the backbone chain. In this group are included polymers that contain the group -0-C(R2)-0- and can be formed from the polymerization of aldehydes or ketones. A typical example of a polymer from this class is paraformaldehyde or polyformaldehyde or polyoxymethylene (CH20)n. Polyoxymethylene can be prepared by anionic catalysis from formaldehyde in an inert solvent. Acetylation of the -OH end groups of the polymeric chain is common since it improves the thermal stability of the polymer. Some results reported in literature regarding thermal decomposition of these polymers are indicated in Table 9.2.1 [1]. 

Acetals are one of a few groups of monomers for which the best general polymerization conditions can be given. Thus, if the aim is the preparation of high-molecular-weight polyacetals we would advise to use <0.1 mol % trifluoromethanesulfonic anhydride well below 0 °C in bulk. If the polymerization proceeds violently under these conditions and the time for mixing is insufficient to obtain uniform solution, both temperature and initiator concentration should be decreased. 

Thermally stable polyacetals are prepared by cationic TXN copolymerization with a small amount of comonomer (DXL, EO). Thus, the mechanism of TXN copolymerization was more frequently studied than its homopolymerization. 

One may prepare new products differing in properties from the basic polyacetal by functionalization of polyTXN. For example, the following approach may be used ... 

Preliminary results indicate the possibility of preparing ABA block copolymers containing polyacetal (as a middle block) and polyamine blocks 121). When N-t-butylaziridine or 2-phenyl-2-oxazoline are added to a solution of living polyDXP, further polymerization ensues and the products have considerably higher molecular weights than the original polyDXP. NMR analysis confirmed the block character of the product. 

As described in the previous section, under suitable reaction conditions one can prepare living polyacetals (poly-l,3-dioxolane and poly-1,3-dioxepane) in almost quantitative yields having well defined molecular weights (DPn = ([M]0 — [M]e)/[I]0), and a low content of cyclic fraction (a few percent). 

Though useful polymers can be made by these reactions, their low ceiling temperatures (see p. 599) and consequent tendency to undergo facile depolymerization by an unzipping mechanism pose serious limitations. To overcome this problem the technique of end-capping or end-blocking may be used. Thus poly-oxymethylene (polyacetal), an engineering plastic, prepared from the cyclic acetal... 

An acetal (e.g., 3 Figure 13.1) can be prepared by the equilibrium reaction of two equivalents of an alcohol and one equivalent of a compound possessing an aldehyde (or ketone to generate a ketal). The hemiacetal intennediate 2 is hydrolytically labile to both base and acid, but the resulting acetal product 3 is only labile to hydrolysis at acidic pH values. When polyacetals are prepared by acid catalysis, it is important to remove or neutrahze any residual add to ensure the polymer is stable enough to isolate and for storage. 

Polyacetals that have been examined for biomedical applications are often prepared by step or condensation polymerizations. Utilizing a diol monomer and an aldehyde to prepare a polymer requires removal of 1 equivalent of water per acetal (Figure 13.1). Acetal exchange reactions can be used where the small molecule is an alcohol with a lower boiling point than water... 

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