Solid state polymerization is polymerization conducted while the reacting monomer or prepolymer remains below its melting temperature and retains a solid, commonly semicrystalline, morphology. In industrial step-growth systems, partially polymerized polyester or polyamide particles are heated above the glass-transition temperature but below the melting point under vacuum or a flowing inert gas. Reactive chain ends diffuse through amorphous regions and undergo condensation, while volatile by-products diffuse to the particle surface and are removed; their removal shifts the equilibrium toward higher molar mass ^[1]. Temperature, particle size, crystallinity, end-group stoichiometry, catalyst, and external mass transfer therefore jointly govern the rate ^[2]. The concept is distinct from melt polymerization because no continuous polymer melt is present, and from solid-state modification because its defining outcome is chain extension or monomer conversion. It also includes topochemical polymerization of crystalline monomers, although prepolymer chain extension is the dominant process for commercial polycondensates.