Universal enveloping associative Rota-Baxter algebras of preassociative and postassociative algebra

Universal enveloping Rota-Baxter algebras of preassociative and postassociative algebras are constructed. The question of Li Guo is answered: the pair of varieties (RBλAs,postAs) is a PBW-pair and the pair (RBAs,preAs) is not.