5. Polymers as Biomaterials

The chemical reaction in which high molecular weight molecules are formed from monomers is known as polymerization reaction. Polymerization can proceed according to two different mechanisms, chain growth (or addition) and step growth (or condensation) polymerization. A distinction can be made between condensation and addition mechanisms of polymerization; in condensation polymerization, two functional groups bond with each other, generally by releasing a small molecule such as H₂O, while in addition polymerization, double bonds of monomers react without releasing any molecule. However, there are some condensation reactions where no molecule release occurs such as polyurethane polymerization. Meanwhile, these two methods are not exclusive; both approaches can be used in the polymerization of the same monomer, or the same polymer can be prepared by two or more different monomers through both of these approaches, as long as suitable groups are available for individual polymerizations. Nylon 6 is a polymer with repeating units ~NH(CH₂)₅CO- and can be synthesized either from 6-aminocaproic acid by condensation or from caprolactam by addition polymerization [1]. There are also some new polymerization techniques as click polymerization, atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer polymerization (RAFT) which became popular in the last decades. In these techniques, some special catalysts and agents are used to obtain specially designed polymers with controlled molecular weights (Fig. 5.1).