Polymers containing -methylenelact(ones)(ams) such as -methylenebutyrolactones
are useful in blends with other polymers. For examples such polymers which have
reactive groups are toughened by mixing with a rubbery material which has complimentary
reactive groups, or polymers containing repeat units derived from -methylenelact(ones)(ams)
are toughened by mixing with polymeric core-shell particles having an elastomeric
core and a specified thermoplastic shell. The properties of thermoplastics are
also improved by blending with -methylenelact(ones)(ams) such as -methylenebutyrolactones
containing (co)polymers.