Polymer modifications epoxidation

Recently, a series of polymer-anchored tungsten carbonyl catalysts based on modified polystyrenes was prepared [86]. Polymer modification was carried out by reaction of chloromethylated polystyrene (2% cross-linked with DVB) with diphosphines, di- and triamines, pyrazine, 4,4 -bipyridine, and imidazole. The polymers were treated with W(CO)6(TEIF) and their catalytic performance was evaluated in the epoxidation of cyclooctene. Different solvents and oxidants were tested and epoxide yields up to 98% were obtained using the system CH3CN/ H2O2. A detailed catalyst recycling study was carried out and the catalyst containing 4,4 -bipyridine units kept constant activity over 10 reactions whereas other catalysts revealed deactivation. 

Influence of the double bond polymer nature in the reaction direction and the polymer modification degree is vividly revealed in the epoxidation reaction of the syndiotactic 1,2-PB, which is carried out under peracids (performic, peracetic, meta-chloroperbenzoic, trifluoroperacetic ones) [21-27], tret-huty hydroperoxide [21-23] and other reagents [28] (Table 3.1). 

Terminal modification Epoxide on PPE, phenoxy, PMMA Anhydride on PS, PP, PSF, PAES Carboxylic acid on PS, NBR ester on PPS Hydroxyl on NBR, PS, PSF, PA6 Amine on NBR, PS, PIP, SAN Isocyanate on polyamides, carbodiintide on PPE For both addition and condensation polymers... 

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

The third and newest modified natural mbber available is epoxidized natural mbber (ENR). This modification was actually discovered as early as 1922 (50), although the elimination of ring opening and side reactions to give a purely epoxidized material took another 50 years or so to achieve (51). The resulting polymer is a new material, with properties totally different from natural mbber, as iadicated ia Table 5. 

The most important inference is that Chemisorption is a direct response to carboxyl group concentration indicated by the XPS photopeak component at 288.7 eV. It seems likely that weak add functionality is of minor import to applications for surface treatments, while interfacial phenomena such as practical adhesion may be sensitive to small concentrations of very high site energies. Interphase modification in epoxy resins, for example, can occur by direct reaction of epoxide groups with surface carboxyls (17), or by accelerated cure chemistry near the surface (39). Carboxyl groups on carbon surfaces may interact with basic moieties in polymers such as polycarbonate or poly(ethylene)oxide (40=42), or promote interfacial crystallinity that improves impact strength and other aspects of composite performance (43, M)-... 

It has recently been demonstrated that this reaction could be used for the epoxidation of a broader class of enones and excellent enantioselectivities were obtained provided the enone was substituted at the 3-position [48,49,50]. However, essentially no asymmetric induction was observed with cyclic enones [51]. Further practical improvements in this reaction have recently been made. Roberts found that the use of anhydrous urea-hydrogen peroxide with DBU in THF as a two-phase system (polymer and organic phase) resulted in rapid epoxidation with high levels of asymmetric induction [52]. This modification solves many of the problems associated with the original procedure (oxidant decomposition, long reaction times, work up) and provides a very practical oxidation system (Scheme 17). 

The physical and mechanical properties of polyanhydrides can be altered by modification of the polymer structure with a minor change in the polymer composition. Several such modifications include the formation of polymer blends, branched and crosslinked polymers, partial hydrogenation and reaction with epoxides. 

Solution reactions of polydienes illustrate the diversity of chemical modifications on polymers (Scheme 11). Such polymers can be isomerized, cyclized, hydrogenat, epoxidized, and reacted with carbenes or enophiles. Most of these transformations increase the Tg s of the polymers with a consequent change in the physical/mechanical properties of the materials. 

New interesting applications have been in the epoxidation of difficult olefin compounds, side-chain chlorination of substituted toluenes, diazotization, polymer manufacturing and modification, and in organometallic and anal5Tical chemistry. 

Although DGEBA resins provide the backbone of most epoxy formulations, they may be blended with other types to achieve modifications. Epoxy novolacs, having higher functionality, increase the cross-linking density, which improves heat resistance but decreases impact resistance. Incorporation of epoxidized oils increases flexibility at the expense of heat and chemical resistance. Low-viscosity polyfunctional epoxies based on polyols or polyhydric phenols reduce viscosity and can increase functionality without impairing cured properties. Monofunctional reactive diluents will also decrease viscosity and form part of the polymer backbone, to impart a measure of flexibility without the possibility of migration. Properties of commercially available epoxy resins and diluents from various suppliers are listed in Table 1. 

---