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.