Virtually cross-linked

When reacting either ethylene carbonate or propylene carbonate with an aliphatic diamine, a polyurethane can be produced (Figure 2.14). Polyethylene ether carbonate) diols (Harris et al., 1990), when fabricated into polyurethanes using MDI and BDO, produce elastomers that have polyester polyol features. This was shown using 13C NMR. The structure gives rise to potential for a very high virtual cross-linking density. These carbonate-derived polyesters have superior hydrolysis resistance compared to the traditional materials. 

Schollenberger (Goodrich)—Virtually cross-linked polyurethane elastomers. 

In 1958 Schollenberger, Scott, and Moore (37) of Goodrich reported on a polyurethane elastomer, Estane, which was a linear polyurethane described as being "virtually cross-linked" by secondary rather than by primary valence forces. 

Density has been found to decrease in the order of brominated polyurethanes (1.26 g cm ) > chlorinated polyurethanes (1.15 g cm ) > alkoxyl-ated polyurethanes (1.11 g cm ) > polyurethanes (1.09 g cm ). The value increases with an increase of the NCO/OH ratio which may be due to an increase in rigidity in the structure and to an increase in virtual cross-linking by H-bonding and other molecular interactions.However, the viscosity is dependent on the nature of the polyurethanes which is related to the physical and chemical structures, molecular weight and distribution of the polymers. For example, resinous polyurethanes prepared for the surface... 

Muller E, Petersen S and Bayer O (1944) German patent 76 584, to IG Farben. Petersen S, Muller E, and Bayer O (12 Apr 1944) Ger Pat 77 229, to IG Farben. Bayer O, Muller E, Petersen S, Piepenbrink H F and Windemuth E (1950) Polyurethanes. VI. New highly elastic synthesis. Vulcollans , Angew Chem 62 57-66. Schollenberger C S, Scott H and Moore G R (1958) Polyurethan VC, a virtually cross-linked elastomer. Rubber World 137 549-55. 

Cast urethane elastomers can be made by a variety of routes similar to those used for foams (Table 17.5). A popular schane with polyester diols is to make a prepolymer with excess diisocyanate and then to chain-extend or cross-link the prepolymer with a small molecule such as a diamine or a polyol. Two such reactants are shown in Table 17.5, IV. Thermoplastic urethane elastomers have been called virtually cross-linked polymers because they appear at room tanperature to be cross-linked and thus resist creep and have rather low hysteresis. However, the cross-links are microphase-separated domains of the urethane polar segments of the poly(urethane) and disappear as the temperature is raised, so that the polymer can be molded or extruded like an ordinary thermoplastic resin (see Section 17.10). The cross-links may be hydrogen bonds between urethane groups as well as allo-phanate bonds. They re-form on cooling. A typical elastomer is made by combining a polyester diol (adipic acid with 1,4-butanediol) with methylene-bis-diisocyanate (MDI). The segmented nature of the polymer, with polyester sections separating... 

Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers which go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins, for which typical structures are given by structures [IV] and [V], respectively ... 

Polymer films that are sensitive to light, x-rays, or electrons— known as photoresists—are nsed extensively to transfer the pattern of an electronic circuit onto a semiconductor surface. Such films must adhere to the semiconductor surface, cross-link or decompose on exposure to radiation, and nndergo development in a solvent to achieve pattern definition. Virtually all aspects of photoresist processing involve surface and interfacial phenomena, and there are many outstanding problems where these phenomena mnst be controlled. For example, the fabrication of multilayer circuits requires that photoresist films of about 1-pm thickness be laid down over a semiconductor surface that has already been patterned in preceding steps. 

Numerous resin supports are commercially available for solid-phase synthesis and some allow the acquisition of quite reasonable quality spectra of compounds bonded to them - and some don t. The resins to avoid (if you intend trying to monitor your reactions by MAS-NMR) are any that are based purely on cross-linked polystyrene. These are too rigid and afford little or no mobility to any bound compound. These resins are relatively cheap and have high specific loadings but will give very poor spectra even in a MAS probe. We see little point in running spectra of compounds on these resins as the quality of the spectra make them virtually useless - and perhaps worse - potentially misleading. 

Investigations into the molecular weight distribution showed that C—C bond break down randomly [126]. Thus, in the PS oxidized at 473 K, the ratio Mw/A/n initially increased from 1.06 to 1.50 (in this case, Mn decreased from 400,000 to 90,000) and remained virtually unchanged during further oxidation. The situation was different with PE residual oxygen at concentrations as low as 0.3% promoted the destruction of this polymer [127]. Moreover, anaerobic conditions at 588 K induced cross-linking processes in PE, while in the presence of 0.34% 02 the destructive processes became predominant. The decrease in Mn at 628 K under anaerobic conditions was not accompanied by changes in the ratio Mw/Mn 3. Conversely, this ratio increased in the course of the oxidative destruction of PE. 

---