The α‐Methyl Effect on the Synthesis of Ethynyl‐Functionalized Polymethacrylates by Free Radical Polymerization

Ethynyl‐functionalized polymethacrylates are promising precursors for diverse click reactions, yet their direct preparation from corresponding monomers is often hindered by gelation, low conversion, and low molecular weight. This work reports a novel and straightforward strategy to prepare soluble, high‐molecular‐weight ethynyl‐functionalized polymers via conventional free radical polymerization of disubstituted ethynyl methacrylates. We reveal the α‐methyl group selectively suppress 1,2‐ and 1,3‐migrations and prevent the formation of stable low‐reactivity allylic radicals, increasing both the monomer conversion and molecular weight. Specifically, with the α‐methyl group, the monomer conversion reaches up to 86.2%, and number‐average molecular weight ( M n ) exceeds 14,000 Da, whereas the corresponding ethynyl acrylates yielded only 14.8%–17.8% conversion and M n = 3,900–4,200 Da. The α‐methyl effect mechanism is further supported by ab initio calculations, 13 C‐NMR, and ESR spectroscopy. The resulting polymers retain reactive ethynyl groups and undergo efficient thiol‐yne click reactions with approximately 80% conversion of C≡C bonds within 6 s under UV irradiation. This work establishes the α‐methyl effect in controlling polymerization pathways of acetylenic (meth)acrylates and offers a new routine toward polymerization of functional (meth)acrylate monomers with high conversion and molecular weight.