21.5 : 聚合物分类：结构（architecture）

Polymers can be classified as linear or branched according to the unique architecture of the polymer chain.

Linear polymers have minimal to no branching at all. Importantly, in the monomer, substituent groups are not considered to be branches.

In branched polymers, secondary polymer chain branches are connected to the main polymer chain. They can have different structural variations.

During polymerization, if the site of polymer growth shifts from the edge of the polymer chain to other areas, branching occurs.

To note the impact of branching, consider polyethylene. The high-density polyethylene variant is linear, while the low-density polyethylene variant is heavily branched.

Consequently, the high-density variant exhibits a high melting point and rigidity due to the efficient close-packing and increased dispersion forces.

In polymers, additional structural variation is achieved by introducing covalent cross-links between polymer chains. For example, the vulcanization of rubber leads to disulfide cross-links between the polymer chains.