Reactive, chemical connections, which exist separately are called monomers.
Monomers are low-molecular and will be combined by a chemical reaction to branched or unbranched polymers.
Most monomers do have a C=C double bond.
Plastic melt might contain residual monomer. At higher temperatures, they will evaporate, which can be realized in many cases by its smell.
This glossary of plastic industry is provided by PLEXPERT Canada Inc.