Definitions and architectures of polymers

Definitions and architectures of polymers

Polymers are high molecular weight natural or synthetic molecules made up of small repeating units, the connected molecular structure that repeats over and over again in a polymer. The structures of common polymers, their repeating units, and their monomers are shown in Figure 11.1. The structural unit enclosed in brackets or parentheses is referred to as the repeating unit. To indicate the repetition, a subscript n is frequently placed after the closing bracket, for example, -[-CH₂CH₂-]_n-. For polymers of a well-defined and known number of repeating units, the number of mono-meric units constituting a polymer replaces the subscript “n.”

Polymers are synthesized from simple molecules called monomers by a process called polymerization. The structure and molecular formula of the monomer and the repeating unit are very similar but not exactly the same.

If only a few monomer units are joined together, the resulting low molec-ular weight polymer is called an oligomer. For example, dimer, trimer, and tetramer are structures formed with two, three, or four monomer units, respectively.

End groups: There are the structural units that terminate polymer chains. Where end groups are specified, they are shown outside the brackets, for example:

CH ₃CH ₂ − [ − CH₂CH₂ −]_n − CH ₂CH₃

Homopolymers are composed of single, identical repeating units forming the polymer chain or backbone. Heteropolymers or heterochain polymers contain more than one type of repeating unit in their backbone. When two or more monomers combine in specific repeating pattern to make a hetero-polymer, the polymer is called a copolymer. In copolymers, the monomeric units may be distributed randomly (random copolymer), in an alternating fashion (alternating copolymer), or in blocks (block copolymer). A graft copolymer consists of one polymer branching from the backbone of the other. Polymer molecules may be linear or branched, and separate linear or branched chains may be joined by cross-links.

Figure 11.1 Structures of commonly used polymers and their monomers

Figure 11.2 Polymer architectures: (a) linear homopolymer, (b) random copolymer, (c) alternating copolymer, (d) block copolymer, and (e) graft copolymer.

Figure 11.2 shows various arrangements of the hypothetical monomers A and B in the copolymer. Where blocks of A (○ ) and B (● ) alternate in the backbone, the polymer is designated an -[-AB-]- multiblock copolymer. If the backbone consists of a single block of each, it is an AB (○●) diblock copolymer. Other possibilities include ABA (○●○) or BAB (●○●) triblock copolymers. For example, when vinyl pyrrolidone, a monomer, is polymer-ized, it forms the linear polymer PVP, also known as povidone. Polyvinyl pyrrolidone is a commonly used polymer in pharmaceutical processing and products, such as artificial tears. It is a protective colloid capable of forming complex with molecular iodine and is thus used in iodine tincture. Polypropylene sulfone is an alternating copoplymer synthesized by copoly-merization of propylene and sulfur dioxide.

Polymers can be linear, star-shaped, or branched, including the so-called star block copolymers. A branched polymer is not necessarily a graft poly-mer. Star polymers contain three or more polymer chains emanating from a core structural unit. 

Figure 11.3 Structure of a typical dendrimer polymer.

Comb polymers contain pendant chains (which may or may not be of equal length) and are related structurally to graft copoly-mers. Dendrimers, also known as starburst or cascade polymers, resemble star polymers, except that each leg of the star exhibits repetitive branching in the manner of a tree. Dendrimers are highly branched polymer con-structs formed from a central core, which defines their initial geometry (Figure 11.3). Their branch-like structure leads to a spherical shape, which can become as large as the size of micelles or nanospheres, depending on the size of the polymer.