Disclosed are crystalline polyphosphonates and a method to induce crystallization. These crystalline polyphosphonates exhibit a unique and advantageous combination of properties, allowing for solid state post polymerization reactions and solid state dry mixing with other polymers. Also disclosed are polymer compositions that comprise these polyphosphonates and at least one other polymer, wherein the resulting polymer compositions exhibit flame retardant properties. Further disclosed are articles of manufacture produced from these crystalline polyphosphonates and polymer compositions, such as fibers, films, coated substrates, moldings, foams, fiber-reinforced articles, or any combination thereof.