Metalation, a novel route for the functionalization of reactive elastomers. 1. Superbases in the metalation of poly(isobutylene-co-p-methylstyrene)

A variety of strong bases and hydrocarbon-soluble superbases (SB) have been used for the selective metalation of poly(isobutylene-co-p-methylstyrene) at the benzylic p-methyl position. The reaction was monitored by quenching the metalated polymer with chlorotrimethylsilane and measuring the resulting functionalization by ¹H NMR spectrometry. A study of different SB's prepared from butyllithium and a heavier alkali metal alkoxide showed the importance of the alkali metal used in the alkoxide. Both the degree of metalation and the selectivity increase with the size of the metal cation (Li⁺ < Na⁺ < K⁺ < Cs⁺). Best results for the quantitative metalation of all of the benzylic p-methyl groups of the copolymer were obtained using a 2-fold molar excess of the SB derived from s-BuLi and cesium l-(-)-menthoxide in the ratio of 1:3. Ring metalation is estimated to less than 2%, and the polymer backbone is unreactive. Time-dependent metalation reactions carried out between -78 and +65°C reveal that, regardless of the reaction temperature, maximum metalation takes place in less than 2 min. Other superbases such as [(trimethylsilyl)methyl]potassium or alkali metal amides exhibit negligible reactivities under the reaction conditions used.