Polymers are a class of materials comprising small repeating chemical units. The building block or the repeating unit in the polymer is called a monomer. Polymerization, or polymer synthesis, is a chemical reaction in which monomers are joined together by covalent bonding to form polymer structures. The length of the polymer chain is characterized by the number of repeating units in the chain, which is referred to as degree of polymerization (DP). The molecular weight of the polymer is the product of the molecular weight of the repeating unit and the DP. The basic properties inherent to polymers mainly depend on the molecular weight, structure (linear or branched), and DP.
Based on the types of chemical reaction involved, polymerizations are grouped into two types, known as condensation and addition polymerization. In condensation or step-growth polymerization, a condensation reaction takes place between two different bi-functional or tri-functional monomers to produce polymers, whereby small molecules, often water, are eliminated from the reaction. In addition polymerization, or chain-growth polymerization, the growth of the polymer occurs by adding monomers to active sites on the polymer chain with regeneration of the active sites at the end of each growth step. An initiator is required to produce the initiator species with reactive centers. The reactive center can be a free radical, cation, anion, or organometallic complex.
The characteristics of polymers are highly dependent on monomer types and the polymerization method used for the synthesis. Living radical polymerization, also referred to as controlled radical polymerization (CRP), enables better control over the polymer molecular weight, molecular weight distribution, functionality, and composition. CRP can be utilized with a wide range of vinyl monomers to build polymers with diverse properties for different applications.
There are three fundamental CRP techniques:
ATRP is well-suited for surface modification applications which rely on tailored hydrophilicity, adhesive properties, or nanoparticle functionalization. Polymers derived from both RAFT and ATRP can be modified post-polymerization and are used extensively in the biomedical field for drug delivery and tissue engineering applications. Block copolymers for biomedical applications, are commonly developed from RAFT or ATRP. Block copolymers, developed from NMP, are essential for pigment dispersion, memory devices, and composite manufacturing.
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