Synthesis of Highly Expandable Poly(methacrylimide) (PMI) Precursor Beads Through Optimized Suspension Polymerization of MAA-MAN-tBMA Copolymers

Bead-foaming technology effectively addresses production cycles, polymerization control, and cellular structure defects in conventional bulk foaming, especially in high-performance PMI foams. In this work, highly expandable PMI beads were synthesized based on the aqueous suspension polymerization of methacrylic acid-methacrylonitrile-tert-butyl methacrylate (MAA-MAN-tBMA) copolymers. The suspension polymerization was stabilized by reducing the solubility of MAA by the salting-out effect and replacing formamide (a common PMI foaming agent) with tBMA. The polymerization process was optimized by varying salting-out agents, dispersants, water-to-oil ratio (WOR), and stirring speed to achieve uniform bead sizes (0.2-0.4 mm) and high bead yields (>70%). The expansion ratio of the beads can be easily tuned by adjusting tBMA content and foaming time and temperature. Beads with 10%tBMA can reach up to 64 times under a free-forming process at 240 °C, which serves as an excellent precursor toward high-performance in-mold foaming PMI. The beads exhibit excellent in-mold foaming capabilities, thermal stability (Td = 392 °C), and mechanical properties. This work provides a technical foundation for the bead-foaming technology of PMI foams, reducing the cost of PMI foam production and providing the possibility to expand the application of PMI foam in civilian use.