Homopolymer or Copolymer
Polymers may be either homopolymers or copolymers depending on the composition. Polymers composed of only one repeating unit in the polymer molecules are known as homopolymers. However, chemists have developed techniques to build polymer chains containing more than one repeating unit. Polymers composed of two different repeating units in the polymer molecule are defined as copolymers. An example is the copolymer (1) formed when styrene and acrylonitrile are polymerized in the same reactor. The repeating unit and the structural unit of a polymer are not necessarily the same. As indicated earlier, some polymers such as nylon 6,6 and poly(ethylene terephthalate)  have repeating units composed of more than one structural unit. Such polymers are still considered homopolymers.

(1)

The repeating units on the copolymer chain may be arranged in various degrees of order along the backbone; it is even possible for one type of backbone to have branches of another type. There are several types of copolymer systems:

• Random copolymer — The repeating units are arranged randomly on the chain molecule. It we represent the repeating units by A and B, then the random copolymer might have the structure shown below:

• Alternating copolymer — There is an ordered (alternating) arrangement of the two repeating units along the polymer chain: 

• Block copolymer — The chain consists of relatively long sequences (blocks) of each repeating unit chemically bound together:

• Graft copolymer — Sequences of one monomer (repeating unit) are “grafted” onto a backbone of  the another monomer type: