Positive resist composition and pattern forming process

A positive resist composition comprising a base polymer comprising a repeat unit (a) having two triple bonds and a repeat unit (b) adapted to increase its solubility in an alkaline developer under the action of acid;

• wherein the repeat unit (a) has the formula (a):
• wherein RA is hydrogen or methyl,
• X1 is an ester bond or phenylene group,
• X2 is a single bond, a phenylene group, or a C1-C10 aliphatic hydrocarbylene group in which any constituent —CH2— may be replaced by an ether bond, ester bond or sulfonic ester bond,
• X3 is a single bond, ether bond, ester bond, carbonate bond or urethane bond,
• R1 and R2 are each independently hydrogen, C1-C4 alkyl group or phenyl group.