To obtain the subject polymer which can produce plastic cases for carriers with extremely reduced occurrence of foreign substances, even the plastic undergoes the treatment with chemical liquid or the ultrasonic cleaning by reducing the contents of foreign substances lower than specific values.

This polymer has a recurring unit of the formula (R1-R4 are independently from each other H, a hydrocarbon, a halogen, hydroxyl group, ester, cyano;... is a C-C single bond or double bound) and an intrinsic viscosity [η]of 0.01-20dl/g in decalin at 80°C., the content of foreign substances of ≥0.5μm particle size of ≥3× 104/g. When a variety of molded products are produced by using this polymer, the molded products have excellent dimensional stability and accuracy and very smooth surfaces.