Which agent is known to be extremely hazardous as a monomer but virtually harmless as a polymer?

Vinyl chloride is recognized for its hazardous properties when in the monomer form, primarily due to its potential to cause serious health issues, including cancer upon prolonged exposure. However, once polymerized into polyvinyl chloride (PVC), the structure changes significantly, transforming it into a stable, chemically inert material that poses minimal risk. This shift in properties showcases the importance of understanding the different risks associated with materials in their monomeric versus polymeric states.

Benzene, acrylonitrile, and formaldehyde, while also important in discussions of safety and toxicity, do not display the same stark contrast in hazard levels when comparing their monomer and polymer forms as vinyl chloride does. Benzene remains a known carcinogen, acrylonitrile has associated health risks even in polymer form, and formaldehyde continues to have safety concerns. Thus, vinyl chloride stands out for its significant change in toxicity profile post-polymerization.