Step-by-Step Breakdown of Polymerization in Dental Prosthetics

Polymers In Prosthodontics

Polymerization: The term polymerization refer to a series of chain reaction by which a macromolecule or polymer is formed from a single molecule known as monomer. Polymerization is a repetitive intermolecular reaction that is capable of proceeding indefinitely.

Stages of Polymerization:

• Chemical Stages: Occurs in four stages

1. Induction
2. Propagation
3. Chain transfer
4. Termination.

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1. Induction:

The induction or initiation period is the time during which the molecules of the initiator become energized or activated and start to transfer the energy to the monomer.

There are three induction systems for dental resins:

• Heat activation: Most denture resins are polymerized by this method, e.g. the free radicals liberated by heating benzoyl peroxide will initiate the polymerization of methyl methacrylate monomer.
• Chemical activation: This system consists at least two reactants, when mixed they undergo a chemical reaction and liberate free radicals, e.g. benzoyl peroxide and an aromatic amine (Dimethyl-p-toluidine).
•  Light activation: Photons of light energy activates the inhibitor to generate free radicals. For example, camphor quinone and an amine will react to form free radicals when they are irradiated with visible light.

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2. Propagation:

The resultant free radical monomer complex, then act as a free radical center when it approaches another monomer to form a dimer which also becomes a free radical, this process of polymerization continues by forming new active center.

• 2H₂C = CH₂ +  CH₂ = CH₂ →  2H₂C — CH₂ — CH₂ — CH₂

This process continues very rapidly till all monomer are exhausted.

3. Chain transfer: Also called “Energy transfer”, here the active state in transferred from an activated radical to an inactive molecule, and a new nucleus of growth is created.

4. Termination: The chain reaction can be terminated either by direct coupling or by the exchange of a hydrogen atom from one growing chain to another.

Physical Stages of Polymerization:

• Stage I: Wet Sand Stage: Polymer gradually settles into the monomer forming a fluid, incoherent mass.
• Stage II: Sticky Stage:
  • Monomer attacks the polymer by penetrating into the polymer.
  • The mass is sticky and stringy (cobweb-like) when touched or pulled apart

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• Stage III: Dough or Gel Stage:
  • As the monomer diffuses into the polymer, it becomes smooth and dough-like.
  • It does not adhere to the walls of the jar. It consists of undissolved polymer particles suspended in a plastic matrix of monomer and dissolved polymer.
  • The mass is plastic and homogenous and can be packed into the mold at this stage.
• Stage IV: Rubbery Stage:
  • Monomer disappears by further penetration into the polymer for evaporation.
  • The mass is rubber-like, non-plastic, and can not be moulded.
• Stage V: Stif Stage: Formal mass is totally unworkable and should be discarded.