Idemitsu Kosan Company, Limited

Shigenori Shirouzu

Kazuyoshi Shigematsu

Shuji Sakamoto

Toshiyasu Suzuki

A resin material for optical instruments comprising a polycarbonate having the repeating units represented by the following general formula: ##STR1## wherein X represents ##STR2## has an extremely small birefringence particularly to oblique-incident lights.

Optical disk substrate comprising aromatic polycarbonate

Polyformal resins having a repeating unit represented by the general formula: ##STR1## wherein the symbols are as defined in the appended claims and an intrinsic viscosity at 20.degree. C. as a methylene chloride solution of at least 0.2 dl/g, and a process for efficiently producing the polyformal resins. The present polyformal resins are excellent in heat resistance and optical properties, and thus are useful as materials for production of optical instruments, for example.

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Chiba, Japan

Shigenori Shirouzu