Symmetrical polyesters

Crystalline polyesters are highly important as adhesive raw materials. They are normally crystalline waxes and are highly symmetrical in nature, which can aid the crystallization process [26]. Poly(hexamethylene adipate) and poly(caprolactone), shown in Table 2, are only two of the many crystallizable backbones. Poly(ethylene adipate) and poly(letramethylene adipate) are also commonly used in urethane adhesives. The crystalline polyesters are used in curing hot melts, waterborne polyurethanes, thermoplastic polyurethanes, and solvent-borne urethane adhesives. The adipates are available mostly as diols. The poly(caprolactones) are available as diols and triols. 

J. Low molecular weight symmetrical polyester polyols may be liquids as a result of many chain ends that disrupt crystalline packing entirely. 

The thermoplastic polyurethane (TPU) adhesives must, of necessity, contain low gel content because they must be processable in an extruder. Most adhesives are relatively linear, with a functionality of 2.0, although small amounts of branching may be introduced, usually at the expense of a lower melt flow. Good physical properties of TPU s are obtained when the thermoplastic urethanes have molecular weights of 100,000 or higher (see p. 56 in [63]). Most TPU adhesives are based on symmetrical polyesters with a fast crystallizing backbone or a backbone slightly modified to increase the open time. 

Enzymatic enantioselective oligomerization of a symmetrical hydroxy diester, dimethyl /Lhydroxyglutarate, produced a chiral oligomer (dimer or trimer) with 30-37% ee [24]. PPL catalyzed the enantioselective polymerization of e-substituted-e-hydroxy esters to produce optically active oligomers (DP < 6) [25]. The enantioselectivity increased with increasing bulkiness of the monomer substituent. Optically active polyesters with molecular weight of more than 1000 were obtained by the copolymerization of the racemic oxyacid esters with methyl 6-hydroxyhexanoate. 

A more general strategy to approach symmetrical telechelic polyesters consists of the control of both initiation and termination steps. Indeed, combination of a functional initiator for the (di)lactones ROP with an effective coupling... 

As in the case of polyphenylenes, the Tg s of symmetrical polyester dendrimers up to 46 rings demonstrated a steady increase with increasing molecular weight, and there was a large difference in the range of Tg s... 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

Wholly para substituted polyesters are normally infusible, unless disruptive groups are substituted onto the aromatic rings, e.g. methyl or chlorine. Commercially, it has proved more attractive to use symmetrical monomers which are able to disrupt crystallinity because they are significantly different in size. Sequences therefore match at infrequent intervals along the chain. Xydar is an example of a commercial LCP using this principle. 

Linear unsaturated and epoxidized polyesters via enzymatic polymerization were reported as well [58]. For this long-chain symmetrically unsaturated a,co-dicarboxylic acid dimethyl esters (C18, C20, C26) were synthesized using metathesis techniques from 9-decanoic, 10-undecanoic, and 13-tetradecanoic acid methyl esters, respectively. The dicarboxylic acid dimethyl esters were epoxidized via chemoenzymatic oxidation with hydrogen peroxide/methyl acetate and Novozym... 

Polyesters. The first synthetic fiber forming polymer produced by Carothers and coworkers was an aliphatic polyester made from trimethylene glycol and a hexadecamethylene dicarboxylic acid. This polymer was low-melting (70°C) and hydrolytically unstable. In 1941, Whinfield and Dickson synthesized the first high-melting symmetrical linear aromatic polyester, polyethylene terephthalate, and found it to be a very useful product. ... 

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