Abstract: The present invention relates to a process for manufacturing a new elastomer having high elasticity, mechanical strength and processability from epoxy compounds, trimellitoyl-functional polymers and a third element. The third element is any of acid anhydride compounds having a molecular weight lower than 700, and having at least one acid anhydride group, such as maleic, phthalic, succinic, adipic, trimellitic, pyromellitic, and benzophenone tetracarboxylic anhydrides. It is, according to another aspect, an improvement of physical properties, especially tensile and tear strengths of the elastomer manufactured by reacting said first two elements according to the copending application Ser. No. 143,643, filed May 14, 1971, while keeping desirably low exothermicity and thermal contraction during the curing.