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.
![Monomer in Plastic Industry Monomer in Injection Molding](https://www.plexpert.ca/wp-content/uploads/2019/07/Monomer.jpg)
Plastic: Single Ethylen-Monomers
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This glossary of plastic industry is provided by PLEXPERT Canada Inc.