What is the degree of polymerization?
The degree of polymerization (abbreviated as DP) is a measure for the chain length of a polymer. It gives the number of monomeric units that are linked together to form the polymer. In chitin, the degree of polymerization is equal to the number of N-acetylglucosamine units in the chain. In chitosans, the degree of polymerization is equal to sum of the numbers of glucosamine and N-acetylglucosamine units. The maximum degree of polymerization of natural chitin is believed to be around 3,000-5,000 but the real value is unknown, as extraction procedures are likely to lead to partial depolymerization. The degree of polymerization of natural chitosan is unknown. Commercially available chitin and chitosan polymers range in degree of polymerization from ca. 100 to ca. 3,500. The degree of polymerization of chitin oligomers ranges from 2 to ca. 10, that of chitosan oligomers from 2 to ca. 20, but there is no clearly defined border between oligomers and polymers. There are no commonly accepted definitions for the terms "low DP", "medium DP" or "high DP" chitosans. It is important to note that like other polymers and biopolymers, chitins and chitosans are always mixtures of molecules differing in their chain lengths so that the degree of polymerization of a chitosan is always an average value. The distribution of degrees of polymerization in a chitin or chitosan sample is described as its polydispersity index. In contrast to polymers, samples of chitin oligomers or chitosan oligomers can have a defined degree of polymerization (though they are often also mixtures). The degree of polymerization strongly determines the molecular weight of chitins/chitosans, but DP and MW are not the same (as MW is also influenced by the degree of acetylation). The degree of polymerization (and, hence, molecular weight) is one of the most important parameters influencing the material properties of chitosan polymers, and it also influences their biological activities.