Kxxxx

Graft copolymer

Random atactic copolymer

Isotactic copolymer

Symmetrical

Figure 8.1 (a) Varieties of copolymer molecular structures attainable though the polymerisation of two different monomers represented by O and (b) Representations of random atactic, isotactic and symmetrical copolymers.

It is apparent that polymer molecules will have a much wider range of physical properties than small chemical entities. Even when considering one chemical type (for example, polyethylene) the properties of the product may be altered by increasing or decreasing the molecular weight (Fig. 8.3). There is a possible degree of control over properties which there is not with simple organic materials and, generally, it is because of this that synthetic polymers have an advantage over many variable natural polymers. Natural materials can be modified chemically, and this approach can lead to useful new products, as with those derived from cellulose or dextran. The structural formulae of some common macromolecules are given in Table 8.1.

Dendrimers are highly branched polymer constructs formed from a central core which defines their initial geometry. 1 Their branchlike structure (Fig. 8.4) leads to spheres which in higher generations appear to be the size of micelles and ultimately nanospheres of small dimensions. They can be functionalised and in this way 'layered' systems can be formed by using different monomers for succeeding

Homopolymer architectures Star block copolymers / Q^y Linear n = 1 (Diblock)

Star

Branched

0 0

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