Polyacetal cyclic

The divinylacetal of 2-furaldehyde was polymerized in vacuo with free-radical initiators at 80 °C to give low yields of the cyclic polyacetal 31132, commonly known as poly(vinylfurfural). 

Among the more common thermoplastics from ring opening polymerization of interest in composite processing are polylactams, polyethers, polyacetals, and polycycloolefins. It has also been shown that polycarbonates can be produced from cyclic carbonates [22], Anionic ring opening polymerization of caprolactam to nylon 6 is uniquely suited to form a thermoplastic matrix for fiber-reinforced composites, specifically by the reaction injection pultrusion process [23-25]. The fast reaction kinetics with no by-products and the crystalline... 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

Of all the selective, deprotection procedures that are available to carbohydrate chemists, the partial hydrolysis of polyacetals is probably the most familiar. Articles by de Beider4,5 and Brady6 contained examples of this type of reaction for aldose and ketose derivatives, respectively, and an article by Barker and Bourne7 gave useful information from the early literature on the graded, acid hydrolysis of acetal derivatives of polyols. A discussion of the stereochemistry of cyclic acetals of carbohydrates was included in an article by Mills 70 and in one by Ferrier and Overend,76 and a survey of the formation and migration of carbohydrate cyclic acetals was made by Clode.7c... 

Ethylene glycol in the presence of an acid catalyst readily reacts with aldehydes and ketones to form cyclic acetals and ketals (60). 1,3-Dioxolane [646-06-0] is the product of condensing formaldehyde and ethylene glycol. Applications for 1,3-dioxolane are as a solvent replacement for methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane, and methyl ethyl ketone as a solvent for polymers as an inhibitor in 1,1,1-trichloroethane as a polymer or matrix interaction product for metal working and electroplating in lithium batteries and in the electronics industry (61). 1,3-Dioxolane can also be used in the formation of polyacetals, both for homopolymerization and as a comonomer with formaldehyde. Cyclic acetals and ketals are used as protecting groups for reaction-sensitive aldehydes and ketones in natural product synthesis and pharmaceuticals (62). 

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) ... 

Another monomer that belongs to this group is cyclic trimer of formaldehyde, 1,3,5-trioxane. Cationic polymerization of 1,3,5-trioxane leading to polyoxymethylene (polyformaldehyde, polyacetal) is one of the few industrially important processes in cationic ring-opening polymerization. 

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... 

Kern and Jaacks and independently Enikolopyan ) determined the ratio kt/kp for the polymerization of 1,3,5-trioxane as being dose to 1.0. This high value is attributed to the known fact that the linear polyacetals are more basic than the cyclic ones. In these systems, the interaction of the growing spedes with the polymer segments is certainly a reversible process thus, these systems cannot be quantitatively described by the kinetic Scheme (138). Nevertheless, if reversibility is tentatively ne ected, then for [Mlo 1.0 mole 1 and [IJo = 10 4mole 1 the complete conversion of the originally formed active species would take place only after 1.4% of conversion of monomer into polymer. 

In the polymerization of several cyclic acetals and ethers autoacceleration was observed. There might be a number of reasons for this behaviour, and some of these have already been discussed, namely the preinitiation equilibria and the inequality ki enhanced reactivity of the hydroxy end group in polyacetals toward active species (in comparison with acetal groups) is another, not yet considered, reason for induction periods. When the pol5mierization d ee increases with conversion the proportion of the active tertiary oxonium ions also increases, at the cost of the less reactive secondary oxonium ions (cf. Ref. 164), because the b k-biting or intermolec-ular transfer become more important than the end-biting. Thus, there b no need to make speculative assumptions about the nature of the active species and to propose the two stage polymerization of acetals in order to explain the induction... 

There is no settled convention for the representation of polyacetals and other complex cyclic derivatives of carbohydrates. To facilitate the comparison of polycyclic derivatives of carbohydrates with alicyclic compounds, the carbohydrate derivatives will be drawn in perspective, with single rings and fused rings placed in the plane of the paper, and with darkened or broken lines used to show the orientation of substituents and bridges above or below the plane of the rings, respectively, as is customary for terpenes and steroids. 

Chain growth polymerizations very often contain a double bond however, cyclic ethers will polymerize in this manner [5], POM (polyoxymethylene) made by the Celanese method shown in Figure 3.6 is an example of a cyclic ether with this method. The Celanese route for the production of polyacetal yields a more stable copolymer product via the reaction of trioxane, a cyclic trimer of formaldehyde, and a cyclic ether (e.g., ethylene oxide or 1,3 dioxalane). 

Several papers57"59 were devoted to investigating a complex process such as the cationic copolymerization of monomeric formaldehyde with dioxolane in the gas, liquid, and gas-liquid phases. It is known that polyacetal resins are industrially produced by copolymerizing cyclic acetals (trioxane, 1,3,5,7-tetraoxane), or by anionic homopolymerization of monomeric formaldehyde with subsequent modification of end groups. 

Polymerization of 1,3,5-trioxane (TXN) gives linear polyoxymethylene (POM), a homopolymer of formaldehyde 39). This is the only polyacetal made on the technical scale. Two methods are used for the industrial production of stable, high-molecular-weight POMs. This is either the anionic polymerization of formaldehyde or the cationic copolymerization of the cyclic trimer of formaldehyde TXN with ethylene oxide or 1,3-dioxolane (DXL) ... 

Thermal stability of polyacetal is achieved by incorporating into the backbone —CH2—CH2— units, i.e., disrupting the sequence of —CH2—O— units susceptible to unzipping. This is done, by copolymerization of TXN with other cyclic ethers or acetals, preferably with 1,3-dioxolane or ethylene oxide (<5%mol)42,53). 

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... 

Polyoxymethylene polymers, POM, commonly known as polyacetals or Acetal resins are linear thermoplastic polymers containing predominantly the -CH -O- repeat unit in their backbone. There are two types of acetal resins available commercially (1) homopolymers made by the polymerization of formaldehyde, followed by endcapping, (2) copolymers derived from the ring opening polymerization of trioxane (a cyclic trimer of formaldehyde), and a small amount of a comonomer such as ethylene oxide. Acetal resins are... 

Copolymer Structure and Sequence Distribution. The free radical polymerization of cyclic ketene acetals had two possibilities forming two different structures. One was the ring opening producing a polyester, and the other was ring retention producing polyacetals. The free radicd polymerizations of monomer MDO... 

Polyoxymethylenes have a marked tendency to undergo thermal depolymerization with loss of formaldehyde. To prevent thermal depolymerization, polyoxymethylenes are structurally modified, the two possibilities being acetylation to block the reactivity of the end groups of co-polymerization with cyclic ethers, e.g., ethylene oxide. Polyacetals are also sensitive towards autoxidation, which invariably leads to depolymerization as a result of chain scission. The formaldehyde released by depolymerization is very likely to be oxidized to formic acid, which can catalyze further depolymerization. 

Polyethers are obtained from three different classes of monomers, namely, carbonyl compounds, cyclic ethers, and phenols. They are manufactured by a variety of polymerization processes, such as polymerization (polyacetal), ring-opening polymerization (polyethylene oxide, polyprophylene oxide, and epoxy resins), oxidative coupling (Polyphenylene oxide), and polycondensation (polysulfone). 

Polyoxymethylene (polyacetal) is the polymer of formaldehyde and is obtained by polymerization of aqueous formaldehyde or ring-opening polymerization of trioxane (cyclic trimer of formaldehyde, melting point 60-60°C), the latter being the preferred method [52]. This polymerization of trioxane is conducted in bulk with cationic initiators. In contrast, highly purified formaldehyde is polymerized in solution using using either cationic or anionic initiators. 

We showed you glucose as on p. 137 an example of a stable, cyclic hemiacetal. matic hydrolysis of starch or cellulose, which are themselves polyacetals made... 

Cross-linked cyclic polyacetals have been examined in recent years for use in regenerative medicine [125] in an effort to minimize the inflammatory response often caused by degradation side products (e.g., acids from polyesters). One example is the copolymer hydrogel poly[poly(ethylene glycol)-ct -cyclic-acetal] 42 (Figure 13.8). The diol 39 is prepared from trimethanol... 

FIGURE 13.8 Monomer 39 is used to prepare cyclic polyacetal hydrogels such as 42 for potential use in tissue regeneration applications [125,132]. 

These reactions often lead to cyclic acetals that interfere with the formation of high molecular weight products. Useful polyacetals, however, can be formed fix)m pentaeiythritol and acetals of dialdehydes ... 

Acetal, (Polyacetal) Poly-oxymethylene (POM) Acetal is a polymer obtained through an addition reaction of formaldehyde — (CH2—0) . It excels in mechanical performance and is regarded as a prominent engineering polymer. It appeared in 1959 with the commercial name Delrin . A short time later a useful copolymer was also developed with a cyclic ether like ethylene oxide. The monomer formaldehyde is a gas produced mostly by oxidizing methanol, and it is very useful in thermoset polymers like phenol, urea and melamine-formaldehydes. For high purity it is initially converted to trioxane or paraformaldehyde. The polymerization is carried out by ionic mechanism, wherein the monomer is dispersed in an inert liquid (heptane). The molecular weights reach 20,000 to 110,000. 

The main chains of polyacetals consist of strictly alternating carbon-oxygen bonds of the type —CHR—O—. They are produced by polymerization of aldehydes or their cyclic trimers. The cyclic trimers and tetramers of formaldehyde are, respectively, known as trioxane and tetroxane. The formaldehyde oligomer with 6-10 monomeric units produced spontaneously in aqueous formaldehyde solutions is known as paraformaldehyde. The cyclic trimer of acetaldehyde is called paraldehyde and the cyclic tetramer is called metaldehyde. 

Poly(oxymethylene), with the monomeric unit -(rCUiO, can be produced from formaldehyde, HCHO, or its cyclic trimer, trioxane (1,3,5-trioxacyclohexane). The commerical polymer obtained from formaldehyde is also known as polyacetal homopolymer, and that from trioxane is known as polyacetal copolymer. 

Polyacetals contain (—CHR—O—) groups in the chain. Like their low-molecular-weight counterparts, they are resistant to alkalis but not to acids. Macromolecular polyacetals have been known for some time under various trivial names as modifications of the corresponding monomers, but their macromolecular nature was first recognized by H. Staudinger. Paraformaldehyde, for example, is a low-molecular-weight poly(oxy-methylene), -f-O—with n 6-100. Metaldehyde is an acetaldehyde oligomer, -(-O—CH(CH3)-)-4.6. Paraldehyde is the cyclic trimer of acetaldehyde and trioxane is the cyclic trimer of formaldehyde. 

Polyacetal can be divided into two basic types, acetal homoploymer and acetal copolymer. Both homopolymer and copolymer are available in a range of molecular weights (M = 20 000-100 000). The homopolymer is a polymer of formaldehyde with a molecular structure of repeated oxymethylene units (Staudinger, 1932). Large-scale production of polyformaldehyde, i.e. polyacetal, commenced in 1958 in the USA (US Patent 2 768 994,1956) (British patent 770 717,1957). Delrin (1959) was the first trade mark for this polymer by Du Pont Company. The copolymers were introduced by the Celanese Corporation of America, and the first commercial product named Celcon (1960). One of the major advantages of copolymerization is to stabilize polyacetal because the homopolymer tends to depolymerize and eliminate formaldehyde. The most important stabilization method is structural modification of the polymer by, for example, copolymerization with cyclic ether. 

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