Repeat unit sequence distribution structural properties

Effects of Repeat Unit Sequence Distribution and Specific Interactions. Most theories and quantitative structure-property relationships for Tg only consider the case of a random distribution of repeat units along the polymer chains in treating copolymers. They give equations which predict a monotonic change of Tg between the Tg values of the homopolymers of the constituent repeat... 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

According to Kravelen,( l the fundamental characteristics of a polymer are the chemical structure and the molecular mass distribution pattern. The former includes the nature of the repeating units, end groups, composition of possible branches and cross-links, and defects in the structural sequence. The molecular mass distribution, which depends upon the synthesis method, provides information about the average molecular size and its irregularities. These characteristics are responsible, directly or indirectly, forthe polymer properties. They are directly responsible forthe cohesive force, packing density and potential crystallinity, and molecular mobility (with phase transitions). Indirectly, these properties control the morphology and relaxation phenomena (behavior of the polymer). 

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