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Polymers

• Polymers are long chain molecules composed of many repeating units.
• The monomer is the smallest repeating unit in the polymer chain.
• The entire long chain is the polymer.

Nomenclature

• Monomer: The smallest repeating unit in the polymer.
• Polymer: Macromolecules formed by linking repeating units through covalent bonds in the main backbone.
  • A monomer is a single molecule.
  • A polymer is a chain of monomers.
• Homopolymer: A polymer made of one type of monomer repeat unit (e.g., poly(methyl methacrylate)).
• Copolymer: A polymer formed of two or more different monomers.

Oligomer

• A short polymer chain composed of 2-10 mers.

Polymerization

• The chemical reaction by which polymers are formed from monomers.
• Most polymerization reactions are of two types: condensation and addition polymerization.

Classification of Polymers

• I. According to Origin:
  • Natural Polymers: Proteins, polyisoprenes (gutta percha), polysaccharides (agar, alginate), polynucleic acids (DNA).
  • Synthetic Polymers: Acrylic resin, elastic impression materials, vinyl polymers (polymers of C2H4).
• II. According to Spatial Structure:
  • Linear: Structural units are connected in a linear sequence. Bonds between polymer chains are weak physical bonds (Van der Waals forces). Low mechanical properties, high water sorption, and low thermal and mechanical resistance.
  • Branched: Side branch chains connected to the main chains.
  • Cross-linked: Adjacent linear chains joined by covalent bonds in various positions in a cross-linked structure. High mechanical properties, high glass transition temperature (Tg), and decreased water sorption.
• III. According to Thermal Behavior:
  • Thermoplastic Polymers: Shaped by heating and maintain their shape after cooling. Physical change, reversible reaction. Polymer chains bonded by secondary bonds. Relatively soft and their mechanical properties are sensitive to heat. Dissolve in organic solvents (e.g., non-cross-linked PMMA).
  • Thermosetting Polymers: Formed into a permanent shape by a chemical reaction. Irreversible reaction. Polymer chains form a network with cross-links (primary covalent bonds). Cannot be re-melted and cannot be reformed. Degrade or decompose upon heating to high temperatures.

Polymer Synthesis

• Polymerization is the chemical reaction by which monomer units are linked together to form a polymer.
• Most polymerization reactions are condensation or addition polymerizations.

Condensation Polymerization

• Reaction between two molecules to form a larger molecule, with a by-product (e.g., H2O), of low molecular weight (e.g., polysulfides & silicones).
• Change in composition.
• Slow process.
• Early termination (low molecular weight).

Addition Polymerization

• Reaction between two molecules that forms larger molecules without the formation of byproducts.
• No change in composition takes place.
• Examples: Poly(ethylene), poly(acrylic acid), poly(methacrylic acid), poly(methyl methacrylate).
• Two sub-types of addition polymerization: free radical and ring opening.

Free Radical

• A compound with an unpaired electron. This unpaired electron makes the radical highly reactive.
• Used as initiator in addition polymerizations.
• Examples of initiators: benzoyl peroxide, diketone.

Propagation

• Chain reactions continue with heat evolution until all monomer units are changed into a polymer.
• Propagation continues as chain grows.

Termination

• Chain reactions can be terminated by direct coupling or transfer of a hydrogen atom from one growing chain to another.

Glass Transition Temperature (Tg)

• The temperature at which the polymer starts to soften.
• Below Tg: The polymer is very rigid, hard, and glass-like.
• Above Tg: The polymer changes rapidly to a viscous, soft, and rubber-like state.

Factors Affecting Properties of Polymers

• Molecular weight and degree of polymerization
• Cross-linking
• Co-polymerization
• Plasticizers
• Spatial structure
• Rate of loading
• Temperature
• Addition of fillers

Co-polymerization

• Polymer chains made of two or more different types of monomer units.
• Enables chemists to "tailor-make" molecules of predicted properties for special applications.

Cross-linking

• Adjacent linear chains are joined together at various positions by chemical bonds (covalent bonds), creating a network of chains instead of independent chains.

Plasticizers

• Compounds added to polymers to partially neutralize secondary bonds, allowing resin molecules to slip past one another when stressed.
• Reduce Tg and increase flexibility.
  • External: low volatile substances that interact with polymer chains but are not chemically attached.
  • Internal: incorporated within the polymer main chain.

Addition of inorganic filler

• Adding inorganic filler to polymer to create composite structure, improving properties (strength, hardness, rigidity).

Rate of loading

• Polymers are viscoelastic materials sensitive to the rate of loading.
• Slow rate: ductile behavior, more permanent deformation.
• High rate: brittle behavior.

Physical State of Polymers

• At room temperature (RT), dental polymers exist as rubbers (elastomers) or hard and amorphous (organic glasses).
  • Elastomers (rubbers): consist of long chain molecules coiled and in random thermal motion. Their defining feature is uncoiling upon stretching, which facilitates deformation.
  • Hard and amorphous (organic glasses): rigid polymers with a Tg above room temperature.

Inhibition of Polymerization

• Impurities in the monomer can react with free radicals, hindering or retarding the polymerization reaction, preventing further chain growth.
• Examples of inhibitors: hydroquinone, eugenol, oxygen.
• Shelf life: hydroquinone added to inhibit polymerization in the absence of an initiator.
• Working time: hydroquinone retards polymerization in the presence of an initiator, influencing working time.

General properties of Polymers

• Polymers are molecular solids, where Strong primary covalent bonds exist between the mers along the whole length of the polymer chain (intramolecular) and Weak secondary Van der Waals forces exist between the chains (intermolecular).

Molecular weight Distribution

• The fraction of low, medium, and high molecular weight molecules in a polymer.
• Two polymers with the same chemistry and average molecular weight may have different properties when their molecular weight distribution differs, resulting from different chain lengths.