In general, thermoplastic resins are widely used in a variety of fields. Single thermoplastic resins alone do not always exhibit sufficient properties, and attempts have been made to combine thermoplastic resins with other resins or rubbers. However, it has been difficult to produce thermoplastic resin compositions which are excellent in oil resistance, heat resistance, weatherability, impact resistance, transparency, and moldability.
In order to solve the problems described above, there are methods in which methacrylic ester-acrylic ester block copolymers having excellent oil resistance, heat resistance, and weatherability are combined with thermoplastic resins. However, a technique has not been known yet in which such methacrylic ester-acrylic ester block copolymers are economically synthesized. Examples of methods for synthesizing such block copolymers include atom transfer radical polymerization (ATRP) methods disclosed in Japanese Unexamined Patent Application Publication No. 10-509475, Japanese Unexamined Patent Application Publication No. 10-81706, Japanese Patent No. 2946497, Japanese Patent No. 2866635, and Japanese Patent No. 2975967, etc. In these methods, since metal compounds are used as catalysts, the resultant polymers may be contaminated with the metal compounds. In order to remove the metal compounds, complicated purification steps are required, resulting in a decrease in productivity and an increase in equipment cost, and thus the methods are not economical.
Examples of known thermoplastic elastomers having excellent weatherability and oil resistance include an acrylic block copolymer including a methacrylic ester polymer block, such as a poly(methyl methacrylate) block, serving as a hard segment, and an acrylic ester polymer block, such as a poly(butyl acrylate) block, serving as a soft segment. However, it is difficult to industrially produce such an acrylic block copolymer, and in particular, hardly any processes for producing an acrylic block copolymer in which the structures of the hard segment and the soft segment are controlled are known.
Examples of known methods for producing acrylic block copolymers in which the structures are controlled without using metal catalysts include reversible addition-fragmentation chain transfer (RAFT) polymerization methods disclosed in PCT Publication No. WO98/01478; PCT Publication No. WO99/05099; PCT Publication No. WO99/31144; Macromolecules, 1998, 31, page 5559; Macromolecules, 1999, 32, page 2071; Macromolecules, 1999, 32, page 6977; Macromolecules, 2000, 33, page 243; etc. According to these methods, it is possible to produce acrylic block copolymers in which the structures are controlled. With respect to the acrylic block copolymers synthesized by such methods, a process for improving properties by incorporation of a thermoplastic resin or rubber component has not been known, and development thereof has been in strong demand. Moreover, when a methacrylic ester-acrylic ester-methacrylic ester triblock copolymer is synthesized, trithiocarbonate groups originating from the chain transfer agent used remain in the main chain, resulting in poor heat resistance and weatherability.