Polymer Synthesis

Polymers

• Polymers are large molecules (macromolecules) composed of repeating structural units connected by covalent bonds.

Polymer Synthesis Methods

• Polymers can be synthesized through addition, condensation, cationic, or anionic reactions.

Addition Reaction

• An addition reaction involves two molecules joining to form a single, larger molecule.

Free Radical Initiators

• Free radical initiators are molecules with a weak bond that breaks down to form two reactive species, each carrying an unpaired electron.

Propagation

• Propagation is when a monomer attaches to a chain, with each new monomer unit creating an active site for the next attachment.

Termination

• Termination occurs through the combination of two free radicals or disproportionation.

Activation of Free Radical Formation

• Free radical formation can be activated chemically, thermally, photochemically, by microwave, or through combined heat and light.
• Any activation method must be safe for the patient.

Chemical Activation

• Chemical activation uses a peroxide (e.g., dibenzoyl peroxide) and an activator (e.g., tertiary amine like N,N dimethyl p toluidine).
• An initiator-activator complex forms (amine + peroxide = free radicals).
• Denture bases can be repaired using this process.
• Benzoyl peroxide is NOT a catalyst.

Thermal Initiation

• Thermal initiation also uses dibenzoyl peroxide.
• The peroxide decomposes when heated, generating free radicals.
• Heat-cured denture bases are cured via this method.

Photoinitiation

• Photoinitiation uses visible light and an alpha-diketone (camphorquinone) initiator, along with a tertiary amine activator (e.g., dimethylaminoethyl methacrylate).
• Dental composites are generally cured this way.

Incremental Curing in Resin Composites

• Incremental curing is required due to limited cure depth.
• It reduces the consequences of shrinkage stress and addresses the "depth of cure."

Depth of Cure

• Depth of cure is the thickness of the composite that is adequately cured.

Factors Limiting Depth of Cure

• Light absorption and scatter within the material limit the depth of cure.
• This depends on the amount, size, and type of fillers, shade, photoinitiator type and concentration, refractive index mismatch, light irradiation source, and irradiation duration.

Composite Curing in Dentistry

• Composites are typically cured using a combination of heat and light.

Cationic Polymerization

• Cationic polymerization usually involves a reaction between an epimine and a benzene sulfonate ester to supply cations.
• Used in the polymerization of polyether impression materials such as Impregum.

Anionic Polymerization

• Anionic polymerization is initiated by an anion (e.g., OH-).
• An example is the polymerization of cyanoacrylates (e.g., superglue).

Polymer Classification Methods

• Polymers can be classified by polymerization method (free-radical, step-growth, ionic, etc.).
• They can be classified by structure (elastomers, glasses).
• Elastomers are uncrosslinked or lightly crosslinked systems.
• Glasses are amorphous systems
• Polymers be classified by thermal behavior (thermoset, thermoplastic).
• Thermosets are cross-linked systems that decompose irreversibly when heated.
• Thermoplastics melt when heated and resolidify when cooled.

Homopolymer

• A homopolymer is a chain of the same monomer (same repeating unit).

Copolymer

• A copolymer is a polymer made up of two or more monomers.

Linear Polymer

• A linear polymer consists of a long, continuous chain of carbon-carbon bonds.
• The remaining valence bonds are primarily attached to hydrogen or a small hydrocarbon moiety.

Factors Controlling Material Properties

• The architecture of the polymer chains controls the properties.
• Properties depend on the flexibility of the backbone, the length of the polymer chain, intermolecular forces, and stereoregularity.

Glass Transition Temperature (Tg)

• Tg is the temperature at which a material's properties change.
• Below the Tg, a polymer behaves as a hard, rigid solid, similar to glass.
• Above the Tg, the polymer exhibits significant elastic behavior.

Plasticizers

• Plasticizers are small molecules added to polymers.
• They get in between the polymer chains, spacing them out and increasing free volume.

Effects of Plasticizers

• Plasticizers allow polymer chains to slide past each other more easily and move at lower temperatures.
• Adding plasticizers lowers the Tg of a polymer, making it more pliable and easier to work with.
• Plasticizers can leach out of polymers.

Increasing Tg

• Cross-linking chemically joins polymer chains together, enhancing physical properties and Tg.
• Heavily cross-linked systems change from a thermoplastic (with a Tg) to a thermoset (without a Tg).

Polydimethylsiloxane Backbone

• The polydimethylsiloxane backbone is very flexible.
• Polydimethylsiloxane has a Tg of -127°C.
• Due to its flexibility, it is a liquid at room temperature.

Bis-GMA Backbone

• The Bis-GMA backbone, used in dental composites, is rigid and has a high Tg.

Isotactic Arrangement

• Isotactic arrangement means all substituents are on the same side of the polymer chain.

Syndiotactic Polymer

• A syndiotactic polymer chain is composed of alternating groups.

Atactic Arrangement

• Atactic arrangement is a random arrangement.

Viscoelastic Materials

• Viscoelastic materials exhibit both viscous and elastic characteristics when undergoing deformation.

Stress Relaxation and Creep

• Stress relaxation is the reduction in stress in a material subjected to constant strain.
• Creep is the increase in strain in a material under constant stress.