Question

What is branching in chemistry?

Answer 1

Chain branching usually refers to chain polymers such as polyethylene.
If the polyethylene is non-branched then it is referred to as a "linear" polymer, and the polymer molecules are predominantly composed of long linear chains -CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2 etc.

Chain branching occurs when the growing polymer chain reacts with an earlier part of the chain, such that the "loop" splits leaving smaller chains originating from the main one. Like this:

-CH2-CH(C3H7)-CH2-CH2-CH(C4H9)-CH2-CH2...

This irregularity causes the molecules to have less chance of packing together tightly, so the polymer becomes lower in density (e.g LDPE), and the melt point reduces due to intermolecular bonds being weaker.

Standard gas phase high pressure reactions to produce polyolefins have no real way of avoiding the formation of chain branching, but using techniques such as Zieger catalysis, it is possible to avoid the branching and produce much more linear polymers, such as Linear Low Density Polyethylene (LLDPE).