Polyethers chain extension

This process is based on the very high reactivity of the isocyanate group toward hydrogen present ia hydroxyl groups, amines, water, etc, so that the chain extension reaction can proceed to 90% yield or better. Thus when a linear polymer is formed by chain extension of a polyester or polyether of molecular weight 1000—3000, the final polyurethane may have a molecular weight of 100,000 or higher (see Urethane polymers). 

In recent years there has been some substitution of TDI by MDI derivatives. One-shot polyether processes became feasible with the advent of sufficiently powerful catalysts. For many years tertiary amines had been used with both polyesters and the newer polyethers. Examples included alkyl morpholines and triethylamine. Catalysts such as triethylenediamine ( Dabco ) and 4-dimethyla-minopyridine were rather more powerful but not satisfactory on their own. In the late 1950s organo-tin catalysts such as dibutyl tin dilaurate and stannous octoate were found to be powerful catalysts for the chain extension reactions. It was found that by use of varying combinations of a tin catayst with a tertiary amine... 

Polyether-based thermoplastic copolyesters show a tendency toward oxidative degradation and hydrolysis at elevated temperature, which makes the use of stabilizer necessary. The problem could be overcome by incorporation of polyolehnic soft segments in PBT-based copolyesters [31,32]. Schmalz et al. [33] have proposed recently a more useful technique to incorporate nonpolar segments in PBT-based copolyesters. This involves a conventional two-step melt polycondensation of hydroxyl-terminated PEO-PEB-PEO (synthesized by chain extension of hydroxyl-terminated hydrogenated polybutadienes with ethylene oxide) and PBT-based copolyesters. 

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

Linear polyols of various molecular weights can be used in the chain extension of prepolymers. Their main use is to adjust the hardness of the final compound. They act as a reactive plasticizer, and their functionality and molecular weight must be taken into account in the curative calculations. Either polyether or polyester polyols can be used. The most important point is that they must be dry. 

Following the early developments using NDI, it was found that by using TDI instead, a far more stable prepolymer could be made. Stable prepolymers are normally made using either polyesters or polyethers that have been reacted with a slight excess of a diisocyanate such as toluene diisocyanate (TDI) or methylene diisocyanate (MDI). Provided the storage is moisture free, the stable prepolymer may be kept for months before use. The polyurethane is prepared by chain extension with diols or diamines. 

Segmented polyurethanes were synthesized from the a, to diol polyethers listed in Table I and the diisocyanates IV, V, and VI by the two-step process shown in Figure 1 or the three-step process in Figure 2. In all cases, chain extension of isocyanate-terminated prepolymers was accomplished with ethylenediamine. The synthesis took place in a 2 1 (v/v) mixture of dimethyl sulfoxide and 4-methyl-2-pentanone at 60°C. 

Novel polyether-urethane ureas—chain extension with tertiary alcohols [58]. 

The first commercial thermoplastic elastomers (TPE) were the thermoplastic urethanes (TPU). Their general structure is A-B-A-B, where A represents a hard crystalline block derived by chain extension of a diisocyanate with a glycol. The soft block is represented by B and can be derived from either a polyester or a polyether. Figure 5.1 shows typical TPU structures, both polyester and polyether types. 

The production of liquid short-chain difunctional polymers by anionic polymerization is of considerable technological interest and importance, and has attracted much attention in recent years, since it offers an analogous technology to that of the polyethers and polyesters used in urethane polymers. Such liquid telechelic polydienes could thus lead, by means of chain extension and crosslinking reactions, directly to castable polydiene networks [161,162]. 

The use of catalysts in the manufacture of urethane elastomers is to be treated with caution as the hydrolytic stability of the final product is usually adversely affected, especially when it is polyester-based. Trace amounts of catalysts are usually present in raw-material manufactured polyesters and polyethers and these must be considered by either their removal or neutralization, otherwise the balance between chain extension, crosslinking and hydrolytic stability will be disturbed. 

The quadrupole hydrogen-bonded unit can be employed in the chain extension of telechelic ohgomers such as polysiloxanes, polyethers, polyesters, and polyacrbonates (Fohner et al., 2000). Thus the electrophilic isocyanate group (-NCO) of synthon (3 in Fig. 1.5) can be reacted with common nucleophilic end groups (-OH or -NH2) of telechelic ohgomers, resulting in supramolecular polymers by chain extension (Fig. 1.6). Thus the material properties of telechelic polymers have been shown to improve dramatically upon functionahzation with synthon, and materials have been obtained that combine many of the mechanical properties of conventional macromolecules... 

Polyurethane fibres of a kind different to those described above have become important within the last decade these are elastomeric fibres, which are commonly called spandex fibres. These products are made either by solution spinning or by reaction spinning. In the first process, a hydroxy-terminated polyester (e.g., an adipate) or polyether (e.g., poly(oxytetramethylene) glycol) is treated with an excess of diisocyanate (e.g., tolylene diisocyanate) to give an isocyanate-terminated pre-polymer similar to those used for cast elastomers (Section 14.6.1). The pre-polymer is dissolved in a strongly polar solvent (e.g., dimethylformamide) and treated with an aliphatic diamine or hydrazine to effect chain extension with hydrazine the following reaction occurs ... 

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