Replacing the direct or indirect heating for polymerizing heat drying can avoid the formation of oligomer and reduce the oligomer content in the ultimate product. 本發(fā)明通過(guò)采用直接加熱或間接加熱干燥的方式代替聚合熱干燥方式,避免了聚合蒸發(fā)干燥階段聚合生成低聚物的可能,減少了最終產(chǎn)品中的低聚物含量。