Solvolytic Stability

Polyurethanes, particularly those based on polyesters, are well known for their oil and grease resistance however, they are susceptible to attack by moisture. The structure of the backbone has a profound influence on the hydrolytic stability. The more hydrophobic the backbone, the greater the resistance of the polyurethane towards hydrolysis. Thus, polyether polyurethanes are inherently more stable to hydrolysis than polyester-based materials, and HTPBD-based polyurethanes are even more stable than the polyether materials. Polyurethanes prepared from polycarbonate diols have also been reported to display very good hydrolytic stability.  

From the above discussion, it can be seen that there are several factors which can affect the stability of a polyurethane. Unfortunately, a given structural factor is not able to provide maximum stability against all mechanisms of degradation. For example, polyether polyurethanes are more resistant than polyester polyurethanes to hydrolysis, but the opposite is true regarding oxidative stability. 

The performance of a polyurethane can often be enhanced by a modification of the polymer backbone or via the use of additives. Their thermal resistance can be improved by the incorporation of linkages more 

Thermooxidative degradation, which is more of a problem with polyether polyurethanes, can be inhibited by the addition of antioxidants. Mathur et al. have studied the effectiveness of several commercial antioxidants for stabilizing polyether polyurethanes against UV and thermally induced oxidation. Stabilizers against thermooxidation include hindered phenols, aromatic amines, and phosphites. Commercially available antioxidants have recently been reviewed by Allbee. Replacement of some, or all, of the ether linkages in polyether polyurethanes by silicone has led to a marked improvement in the thermooxidative stability of the resulting polymer.  

As already mentioned, the hydrolytic stability of polyurethanes is influenced by the nature of the macroglycol used in their preparation. Polyesters prepared from sterically hindered glycols, such as neopentyl glycol (18), and a long-chain diacid, or aromatic diacid, for example, terphthalic acid (19), will maximize moisture stability. Polyester polyols have been developed which are claimed to be similar in hydrolytic stability to PTMO polyurethanes. Increasing the crosslink density in polyester polyurethanes has been shown to improve hydrolytic stability as well however, this may not always be a viable solution, since the crosslink density can have a profound influence on other properties of the polymers as already described. 

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Substitution of the cage with the methyl groups increases the solvolytic stability of the silatranes13,58,298. The symmetric isomer 40 of l-phenyl-3,7,10-trimethylsilatrane shows little reaction in acetic acid containing 5% water after 90 h, whereas the half-life of the asymmetric isomer 40a is about 2-3 h. The half-live of 1-phenyl-3,3,7,7,10,10-hexamethylsilatrane 58 bearing the sterically hindered oxygen atoms is at least one year58 ... 

The hydrolytic and solvolytic stability increases with the number of Me substituents. The penta- and hexamethyl derivatives can be treated in water. 

Bis(arene) complexes of ruthenium (99), first prepared in relatively low yield by the Fischer-Hafiier synthesis (see Fischer-Hafner Synthesis) (MCb/AlCb/arene) are now readily available through solvento cations [(arene)RuS] + (S = acetone, nitromethane), which in turn are made from the dimeric half-sandwich halides [(arene)RuCl2]2 (Section 6.3) with Ag+ with the appropriate arene as solvent (Scheme 23). This method allows a whole range of functionalities to be incorporated to at least one of the arenes. The dications (99) + are of relatively high solvolytic stability and are decomposed by DMSO only to form [(arene)Ru(DMSO)3] +. 

We have investigated the solvolytic stability and reactivity of polymer-bound borohydrides and have evaluated these materials in several applications such as solvent purification, arsine generation, and metal reduction. These polymer-bound borohydrides offer several advantages over sodium or tetraethylammonium borohydride. The primary advantages are the convenience of use and the minimal introduction of ionic species or organic by-products into the treated bulk media. With the polymer-bound borohydrides, the cation is bonded covalently to the insoluble resin while the borohydride anion or its oxidation product (borate) is retained by ionic bonding. Typically, boron at levels of less than 5 ppm is the only impurity introduced into the treated medium. 

Solvolytic Stability. Solvolytic stability of all four polymer-bound borohydrides was investigated in 100% ethanol as a function of temperature and in water as a function of pH. Comparative studies for sodium borohydride and tetraethylammonium borohydride were conducted. Solvolytic decomposition of the borohydride group results in the generation of hydrogen gas. Stability measurements were obtained by observing volume of hydrogen gas evolved as a function of time. Percent loss of hydride as a function of time was calculated from resin weight and initial hydride content. 

Thus, both of the above considerations would be expected to sharply retard subsequent attack by any other nucleophile. The well-known enhanced solvolytic stability of the alkoxy- and halo-silatranes9 strongly attests to the validity of this assertion . 

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