<p>We disclose a new route for the preparation of linear polyethylene (PE)–poly(methyl acrylate) (PMA) diblock copolymers. The PE segment is synthesized by coordination–insertion polymerization (CIP), and the PMA segment is polymerized through radical polymerization (RP). Notably, no solvent is used during the process. Linear alkyl–nickel species synthesized by the CIP of ethylene under a solid−gas two-phase reaction are thermally activated at room temperature in neat methyl acrylate (MA), which triggers RP. Diblock copolymer formation and its composition are supported by a unimodal SEC shift and ¹H/¹³C NMR analyses. The presence of small PE crystals in amorphous PMA is implied by powder XRD analysis. The difference in the reactivities of MA and methyl methacrylate (MMA) suggests that RP initiates through the thermal homolytic scission of the C−Ni bond of α-carbonyl Ni species.</p>