Oxymethylene

The good mechanical properties of this homopolymer result from the ability of the oxymethylene chains to pack together into a highly ordered crystalline configuration as the polymers change from the molten to the solid state. 

Pentaerythritol, 2,2p [methylenebis(oxymethylene)]bis(2-hydr oxymethyl-l,3-propanediol) [6228-26-8]... 

Polyoxyethylene. Synthetic polymers with a variety of compositionaHy similar chemical stmctures are as follows. Based on polarity, poly(oxymethylene) (1) would be expected to be water soluble. It is a highly crystalline polymer used in engineering plastics, but it is not water-soluble (see... 

Polyacetaldehyde, a mbbery polymer with an acetal stmcture, was first discovered in 1936 (49,50). More recentiy, it has been shown that a white, nontacky, and highly elastic polymer can be formed by cationic polymerization using BF in Hquid ethylene (51). At temperatures below —75° C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline, isotactic polymer is obtained (52). This polymer also has an acetal [poly(oxymethylene)] stmcture. Molecular weights in the range of 800,000—3,000,000 have been reported. Polyacetaldehyde is unstable and depolymerizes in a few days to acetaldehyde. The methods used for stabilizing polyformaldehyde have not been successful with poly acetaldehyde and the polymer has no practical significance (see Acetalresins). 

The term "acetal resins" commonly denotes the family of homopolymers and copolymers whose main chains are completely or essentially composed of repeating oxymethylene units (—CH2—O—). The polymers are derived chiefly from formaldehyde or methanal [50-00-00] either directly or through its cychc trimer, trioxane or 1,3,5-trioxacyclohexane [110-88-3]. 

Formaldehyde homopolymer is composed exclusively of repeating oxymethylene units and is described by the term poly oxymethylene (POM) [9002-81-7]. Commercially significant copolymers, for example [95327-43-8] have a minor fraction (typically less than 5 mol %) of alkyUdene or other units, derived from cycHc ethers or cycHc formals, distributed along the polymer chain. The occasional break in the oxymethylene sequences has significant ramifications for polymer stabilization. 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Chemical Structure and Properties. Homopolymer consists exclusively of repeating oxymethylene units. The copolymer contains alkyhdene units (eg, ethyUdene —CH2—CH2—) randomly distributed along the chain. A variety of end groups may be present in the polymers. Both homopolymer and copolymer may have alkoxy, especially methoxy (CH3 O—), or formate (HCOO—) end groups. Copolymer made with ethylene oxide has 2-hydroxyethoxy end groups. Homopolymer generally has acetate end groups. 

The main intermediates in the pentaerythritol production reaction have been identified and synthesized (50,51) and the intermediate reaction mechanisms deduced. Without adequate reaction control, by-product formation can easily occur (52,53). Generally mild reaction conditions are favored for optimum results (1,54). However, formation of by-products caimot be entirely eliminated, particularly dipentaerytbritol and the linear formal of pentaerythritol, 2,2 -[meth5lenebis(oxymethylene)]bis(2-hydroxymethyl-1,3-propanediol) [6228-26-8] ... 

Formaldehyde is produced and sold as water solutions containing variable amounts of methanol. These solutions are complex equiUbrium mixtures of methylene glycol, CH2(OH)2, poly(oxymethylene glycols), and hemiformals of these glycols. Ultraviolet spectroscopic studies (13—15) iadicate that even ia highly concentrated solutions the content of unhydrated HCHO is <0.04 wt%. 

Formaldehyde gas (Oxymethylene) HCHO 430 7.0-73.0 1.1 -21 Colourless Water soluble gas producing formalin solutions Suffocating odour Polymerizes readily Highly toxic Respiratory sensitizer... 

With some important exceptions, drugs in this class are conceptually related to the phenyl ethanol amines by the interposition of an oxymethylene group between the aromatic ring and the benzyl alcohol. 

Prenalterol (73) interestingly exhibits adrenergic agonist activity in spite of an interposed oxymethylene... 

It has by now been reasonably well established that an ethanolamine function appropriately linked to an aromatic ring is a prerequisite for beta adrenergic activity and/or antagonism. Examples have been met above where those two moieties are attached directly as well as examples where the funetions are separated by an oxymethylene fragment. It has reeently been found that beta blocking activity is retained even when a earbonyl is inserted between the extra oxymethylene... 

It has bsen prepared artificially by Semmler and McKenzie by oxidation of oxymethylene-menthone, CjjHjgO, a diketone, CjgHjgOg, resulting, which is inverted by means of alkalis to diosphenol. 

The same compound is obtained when proceeding according to the following alternative procedure. A mixture of 39.4 parts by weight of 2,4-dioxo-3,3-diethyl-5-oxymethylene-piperidine and 27 parts by weight of dibutylamine are heated to 150°C in a closed vessel. The 2,4-dioxo-3,3-diethyl-5-dibutylamino-methylene-piperidine formed melts at 77°C after having been recrystallized in petroleum ether. 

Random copolymers are similar to PEO but when the regular helical structure of the chains is demolished, the crystallinity is also destroyed. One of the simplest and most successful amorphous host polymers is an oxyethylene- oxymethylene structure in which medium length but statistically variable EO units are interspersed with methylene oxide groups. First described in 1990 [37], aPEO has the general structure... 

Nakase, Kurijama, I. and Odajima, A. Analysis of the Fine Structure of Poly(Oxymethylene) Prepared by Radiation-Induced Polymerization in the Solid State. Vol. 65, pp. 79-134. 

A number of thermoplastics undergo depolymerisation on heating. These include poly(styrene), poly(methyl methacrylate), and poly(oxymethylene). Such depolymerisation will occur regardless of the prevailing oxygen concentration and under well aerated conditions will provide a ready source of fuel for sustained combustion. 

Polyphosphazenes bearing crown ethers (12-crown-4,15-crown-5 and 18-crown-6) as single or as mixed substituents with trifluoroethoxy or methoxy-ethoxyethoxy groups were synthesized by Cowie [601,602] and Allcock [484] and their conductivity studied because it was shown that the incorporation of crown ether molecules into a polymer electrolyte could increase their ionic conductivity. In these macromolecules, the crown ether units were linked to the backbone through oxymethylene spacer groups. 

A monocarbonyl complex of zirconocene has been proposed by Floriani and co-workers as a transient intermediate in the carbonylation of the bridging oxymethylene complex (Cp2ZrCl)2(Ai-CH20) (57) (104,105). The assignment of 58 as the proposed intermediate to the final carbonylation product, [Cp2Zr(/i-CHO=CHO)]2 (59), was based on the appearance of a metal carbonyl band at 1970 cm-1 together with the subsequent CO-induced loss of Cp2ZrCl2. 

The [nri] of microgel solutions decreases with increasing degree of neutralization of the carboxyl acid groups of the EUP (Fig. 19) because the emulsifier concentration increases and, accordingly, the micelles or microemulsion droplets become smaller. In this case an external emulsifier poly(oxymethylene) octylphenyl ether was added to insure complete solubilization over the whole range of neutralization. 

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