Electrophilic Additions of Aldehydes
These are additions to double bonds, like the Prins reaction, and they can be carried out on natural and synthetic rubbers [59, 60]. They take place rapidly in the presence of acid catalysts. Aqueous formaldehyde [61], or paraform in CCl4 [62], can be used. The catalysts are inorganic acids or anhydrous Lewis acids, like boron trifluoride in acetic acid solution [63]:
The reaction takes a different path in the absence of a catalyst [62]:
The products of the Prins reaction with rubbers are thermoplastic polymers that possess fair resistance to acids and bases. Free hydroxyl groups in the products are available for cross-linking with diisocyanates [64] or by other means. The Prins reaction can be carried out directly on rubber latexes [65]. It is also possible to just mill the rubber together with formaldehyde and then heat the resultant mixture in the presence of anhydrous metal chlorides [64] to get similar results [66]. Higher aldehydes also react with natural rubber [67]. The reaction works best with purified rubber. Additions take place without a catalyst at 180C or in the presence of AlCl3–NaCl at 120C. These reactions can be carried out in the solid phase by milling the rubber with an aldehyde, like glyoxal [68]. Heating in a pressure vessel at above 175C is required to complete the reaction. Infra-red spectra of the products from reactions in solution show presence of ether, carbonyl, and hydroxyl groups [69]. Two types of additions appear to take place [69]:
Products from reactions of rubber with glyoxal have a strong tendency to become spontaneously insoluble. This is probably due to a presence of residual aldehyde groups, because a treatment of the product with 2,4-dinitrophenyl-hydrazine eliminates spontaneous gelation. Chloral adds to polyisoprene similarly. The reaction is catalyzed by Lewis acids [70]. Both AlCl3 and BF3 are efficient catalysts. Less cross-linking is encountered with aluminum chloride. Infra-red spectra of the products shows presence of hydroxyl groups, chlorine atoms, and vinylidene unsaturation [70].
