8.2 Polymers

The term polymer is used to describe high-molecular-weight substances. However，this is a very broad definition，and in practice it is convenient to divide polymers into subcategories according to their molecular weight and structure. Although there is no general agreement on this point，in this book we will consider low polymers to have molecular weights below about 10，000 to 20，000 and high polymers to have molecular weights between 20，000 and several million. Obviously，this is a rather arbitrary dividing line，and a better definition might be based on the number of repeating units in the structure. For example，since polymer properties become almost independent of molecular weight when more than 1，000 to 2，000 repeating units are present，this point could also constitute a satisfactory dividing line between low and high polymers.^①