Thermoset resin compositions with increased toughness

A block copolymer (M) suitable for toughening a thermoset resin (R), said block copolymer (M) having one or more segments of a thermoplastic aromatic polymer (A) chemically linked to one or more segments of a low Tg polymer (B) wherein:

• (i) the low Tg polymer (B) exhibits a Tg in the range of from about −130° C. to about +40° C.;
• (ii) the one or more segments of aromatic polymer (A) is soluble in uncured thermoset resin precursor(s) (P) of said thermoset resin (R),
• (iii) the one or more segments of low Tg polymer (B) is insoluble in the uncured thermoset resin precursor(s) (P),
• (iv) the aromatic polymer (A) is a polyarylsulphone comprising ether-linked repeating units, the units being selected from:
• —[ArSO2Ar]n—
• and optionally from:
• —[Ar]a—
• wherein:
• Ar is phenylene;
• n=1 to 2 and can be fractional;
• a=1 to 3 and can be fractional and when a exceeds 1, said phenylene groups are linked linearly through a single chemical bond or a divalent group other than —SO2—, or are fused together,
• provided that the repeating unit —[ArSO2Ar]n— is always present in the polyarylsulphone in such a proportion that on average at least two of said —[ArSO2Ar]n— units are in sequence in each polymer chain present,
• and wherein the polyarylsulphone has one or more reactive pendant or end group(s),
• (v) the low Tg polymer (B) is selected from saturated aliphatic polyesters derived from at least divalent linear, branched or cyclic aliphatic alcohols having from 2 to 60 carbon atoms, and at least divalent linear, branched or cyclic aliphatic carboxylic acids having from 3 to 60 carbon atoms, with the proviso that at least one of the alcohol or the acid components has at least 4 carbon atoms.