Possible crosslinking reactions

Scheme 1.17. Possible crosslinking reactions of urea-formaldehyde resins.

Scheme 1.40. Various possible crosslinking reactions during grafting.

Figure 19.9 Possible crosslinking reaction between PVA and maleic add.

The different possible adducts formed between mitomycin C and DNA have been isolated by degradation of DNA after in vitro alkylation/crosslinking reactions and structurally characterized. Monoadduct 21 (Scheme 11.3), derived from alkylation at C-l only [53], and monoadducts 22 [54] and 23 [55, 56] (derived from C-10 alkylation by 16 at N-7 or N-2 of guanine, respectively) have been isolated, together with bisadducts 24 [57] and 25 [58], derived from interstrand and intrastrand crosslinks, respectively, and adduct 26 [59], formed by addition of a molecule of water to C-10 instead of the second guanine. All of these adducts have also been isolated from DNA after in vivo crosslinking [60, 61]. 

Due to the viscous and highly exothermic characteristics of the hydrogenation reactions, the reactor should have superior mixing capabilities in order to avoid possible crosslinking induced by hot spots in the reactor. 

LC oligoester diols can be crosslinked with HMMM by carrying out the crosslinking reaction at temperatures between Tm and Tj. Crosslink density is high. Certain film properties of enamels made from LC diols are far superior to those of enamels made from non-LC diols. Films are hardened without substantially increasing Tg, and they retain the elasticity associated with low Tg. The mechanism of this property enhancement is uncertain, but the effect is substantial. Inclusion of LC diols in enamels offers vast possibilities for manipulation and improvement of film properties. 

Urea-formaldehyde resins are generally prepared by condensation in aqueous basic medium. Depending on the intended application, a 50-100% excess of formaldehyde is used. All bases are suitable as catalysts provided they are partially soluble in water. The most commonly used catalysts are the alkali hydroxides. The pH value of the alkaline solution should not exceed 8-9, on account of the possible Cannizzaro reaction of formaldehyde. Since the alkalinity of the solution drops in the course of the reaction, it is necessary either to use a buffer solution or to keep the pH constant by repeated additions of aqueous alkali hydroxide. Under these conditions the reaction time is about 10-20 min at 50-60 C. The course of the condensation can be monitored by titration of the unused formaldehyde with sodium hydrogen sulfite or hydroxylamine hydrochloride. These determinations must, however, be carried out quickly and at as low temperature as possible (10-15 °C), otherwise elimination of formaldehyde from the hydroxymethyl compounds already formed can falsify the analysis. The isolation of the soluble condensation products is not possible without special precautions, on account of the facile back-reaction it can be done by pumping off the water in vacuum below 60 °C imder weakly alkaline conditions, or better by careful freeze-drying. However, the further condensation to crosslinked products is nearly always performed with the original aqueous solution. 

The statistical treatment of random stepwise crosslinking reactions (e.g. polycondensation) neglecting ring formation originates from Stock-mayer and Flory and is explained in Flory s book (55) on a number of examples. Using simple probability statistics, it is possible to calculate the molecular size distribution in the sol and in the gel, fractions of sol and gel, the crosslinking density and the fraction of free functionalities in... 

A precise characterization of the telechelic precursor is necessary, prior to the crosslinking reaction, since precise stoichiometry must be achieved if the number of defects in the network is to be kept as small as possible. 

The synthesis of PU can be carried out by the reaction described in Eq. (2.24). If the functionality of the hydroxy-containing compounds or the isocyanate is increased beyond 2, branched and possibly crosslinked polymers are produced. Because the nature of the polyol (polyether, polyester, polybutadiene, etc.) and isocyanate components can vary widely, PU are among the most versatile polymers, producing a wide variety of materials such as elastomers, foams, coatings, adhesives, or fibers. 

Another factor has to be considered. When the macromonomer chain segments can give rise to transfer reactions, the probability of such events is enhanced because the number of chain segments per unsaturation is high. Homopolymerization of such macromonomers should be expected to involve additional branching and possibly crosslinking. A typical example is that of PEO macromonomers because oxyethylene units are known to induce transfer reactions U3). 

---