Thermosetting resin composition of semi-IPN composite, and varnish, prepreg and metal clad laminated board using the same

A thermosetting resin composition comprising a polyphenylene ether-modified butadiene prepolymer which includes a polyphenylene ether (A) and a butadiene polymer, characterized in that the thermosetting resin composition is obtained by a process for manufacture of the thermosetting resin composition wherein:

• the butadiene polymer has a crosslinked structure;
• the process has a step (1) in which a butadiene polymer (B) crosslinks with a crosslinking agent (C) (excluding the butadiene polymer (B)) in the presence of the polyphenylene ether (A) in a medium to obtain the polyphenylene ether-modified butadiene prepolymer;
• a number average molecular weight of the polyphenylene ether (A) is in a range of 7,000 to 30,000;
• the butadiene polymer (B) molecule contains 40% or more of a 1,2-butadiene unit having a 1,2-vinyl group in a side chain;
• and
• the butadiene polymer (B) is not a modified polybutadiene in which the 1,2-vinyl group in the side chain, or one or both of the terminals in the molecule, is chemically modified to be converted to epoxy, glycol, phenol, maleic acid, (meth)acryl, or urethane, and
• with the proviso that excluded from the polyphenylene ether (A) is a polyphenylene ether which is a modified phenol product so obtained by redistribution reaction of a polyphenylene ether resin having a number average molecular weight of 10,000-30,000 with a phenolic compound in the presence of a reaction initiator that the number average molecular weight of the product becomes 5 to 70% of that of the polyphenylene ether resin.