Comonomer

A comonomer is a monomer that is polymerized together with another monomer to form a copolymer. In a copolymer, the repeating units originate from more than one distinct monomer, as opposed to a homopolymer, which contains only one type of monomer. The arrangement of comonomer units—random, alternating, block, gradient, or graft—depends on the monomers’ reactivity ratios and the polymerization method. The choice and amount of comonomer influence properties such as crystallinity, glass transition temperature, toughness, clarity, and chemical resistance.

Common comonomers include higher alpha-olefins (for example, butene or hexene) used with ethylene to produce polyethylenes with controlled branching; vinyl acetate or other acrylate monomers used with ethylene to make ethylene–vinyl acetate copolymers (EVA); and styrene or butadiene used in styrene–butadiene copolymers (SBR) for elastomeric materials. More complex systems use several comonomers to form terpolymers, which can offer a balance of properties not achievable with two-monomer systems; examples include acrylonitrile–butadiene–styrene (ABS) polymers.

Comonomer incorporation occurs across various polymerization methods, including radical polymerization for vinyl monomers and coordination or anionic polymerization for dienes and polyolefins. The distribution and extent of comonomer units influence crystallinity, melting behavior, processability, and mechanical performance. In industry, comonomers are selected to tailor properties for applications in packaging, automotive parts, adhesives, and elastomeric products.