For

Graft block copolymers Multiblock copolymers

Figure 8.2 The range of structures of homopolymers and star block, graft block and multiblock copolymers.

reactions (generations); such chemical architecture has virtually no bounds. Dendrons are partial dendrimers.

8.1.2 Polydispersity

Nearly all synthetic polymers and naturally occurring macromolecular substances exist with a range of molecular weights; exceptions to this are proteins and natural polypeptides, each of which occurs as a substance with a single well-defined molecular weight. The molecular weight of a polymer or macromol-ecular subtance is thus an average molecular weight, which may be determined by chemical analysis or by osmotic pressure or light-scattering measurements. When determined by chemical analysis or osmotic pressure measurement a number average molecular weight, Mn, is found, which in a mixture containing nv n2, n3, ... moles of polymer with molecular weights Mj, M2, M3, ..., respectively, is defined by

The individual molecular weights M:, M2, ... cannot be determined separately - the equation merely explains the meaning of the value Mn. In light-scattering techniques, larger molecules produce greater scattering; thus the weight (or more strictly the mass) rather than the number of the molecules is important, giving a weight average molecular weight, Mw:

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