Branching (polymer chemistry)

[3][4] In crosslinking rubber by vulcanization, short sulfur branches link polyisoprene chains (or a synthetic variant) into a multiple-branched thermosetting elastomer.

Polycarbonate chains can be crosslinked to form the hardest, most impact-resistant thermosetting plastic, used in safety glasses.

Branching sometimes occurs spontaneously during synthesis of polymers; e.g., by free-radical polymerization of ethylene to form polyethylene.

But "star" branched nylon can be produced by the condensation of dicarboxylic acids with polyamines having three or more amino groups.

The ultimate in branching is a completely crosslinked network such as found in Bakelite, a phenol-formaldehyde thermoset resin.

When this curl breaks, it leaves small chains sprouting from the main carbon backbone.

Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.

Branching makes the polymers less dense and results in low tensile strength and melting points.

Developed by Karl Ziegler and Giulio Natta in the 1950s, Ziegler–Natta catalysts (triethylaluminium in the presence of a metal(IV) chloride) largely solved this problem.

Instead of a free radical reaction, the initial ethene monomer inserts between the aluminium atom and one of the ethyl groups in the catalyst.