Introduction
Polymers are high-molecular compounds built from many repeating small units, called monomers, joined by covalent bonds. A compound counts as a polymer above a molecular weight of about 5000. Dental resins have low density, insulate against heat and resist chemicals.
Polymerization
Monomers with double bonds, triple bonds or rings can polymerize. Heat, ultraviolet light, radiation or chemical initiators break these bonds. The usual initiator is benzoyl peroxide, which heat or an activator splits into free radicals.
The number of monomers in a chain is the degree of polymerization. Multiplying it by the molecular weight of the monomer gives the molecular weight of the polymer. A higher degree gives better mechanical and biological properties. If polymerization runs too fast, the chains stay short and more residual monomer remains.
Structure of polymers
- Monopolymer: built from one type of monomer.
- Copolymer: built from two or more monomers, and usually has better properties.
- Oligomer (prepolymer): a short chain, with a molecular weight under 5000. With more than one double bond, it acts as a cross-linking agent.
Larger monomers, comonomers, longer chains and more cross-linking make a resin harder, stronger and more rigid. Less cross-linking or a spiral arrangement of chains gives an elastic plastic.