Polymers in Dentistry – Introduction

WHAT IS POLYMER?

Polymer is described as high molecular weight, chain-like molecule consist of distinct repeating groups of atoms, derived from small molecules (monomers) from which the chain is built up.

 

Monomer

The molecule from which the polymer chain is constructed.

Monomers are generally liquids or gases and during the process of polymerisation they become converted to crystalline or amorphous solid polymers. These may vary from being very rigid at one extreme to being soft and rubbery at the other.

 

Homopolymer

Homopolymer: a polymer formed from a single species of monomer.

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Copolymer

Copolymer (heteropolymer): a polymer formed from different species of monomer.

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According to the monomers sequence there are different types of copolymers such as:

– Random copolymers: where the sequence of the monomers in the copolymer is random such as -A-A-A-A-B-B-A-B-A-B-B-B-A-A-B-B-A-B-.

– Regular copolymers: where the sequence of the monomers in the copolymer is alternating such as -A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-.

– Block copolymer: where segments of each homoploymer are linked if the monomers self polymerise more readily than they copolymerise, -A-A-A-B-B-B-B-A-A-A-B-B-B-.

 

Polymers Dental Applications

  1. Tooth restoratives, Sealants
  2. Cements
  3. Orthodontic  space maintainers
  4. Elastics Obturators for cleft palates
  5. Impressions
  6. Provisional restorations
  7. Root canal filling materials
  8. Denture bases
  9. Athletic mouth protectors
  10. Polymers are one of the components of composites

Polymerisation

Polymerisation is a process by which monomers are converted into polymers.

  • Condensation reaction
  • Addition reaction

 


Condensation Polymerisation

Condensation polymerisation Involves two molecules reacting together to form a third, larger molecule with the elimination of a smaller by-product molecule (often, but not always water).

Each reacting molecule should have at least two reactive groups so that the reaction product is capable of undergoing further condensation reactions.

By using a monomer M which carries two reactive groups X and Y it is possible to produce a polymer as follows:

X−M−Y + X−M−Y → X−M−M−Y + XY

X−M−M−Y + X−M−Y → X−M−M−M−Y + XY

X−M−M−M−Y + X−M−Y → X−M−M−M−M−Y + XY etc.

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A simple generalized reaction sequence for condensation polymerisation for two monomers, X−M1−X and Y−M2−Y, with reactive groups X and Y can be written as follows:

X−M1−X + Y−M2−Y → X−M1−M2−Y + XY

X−M1−M2−Y + X− M1−X → X−M1−M2−M1−X + XY

X−M1−M2−M1−X + Y−M2−Y → X−M1−M2−M1−M2−Y + XY etc.

It can be seen that at each stage of the reaction the chain grows by one monomer unit and there is one molecule of byproduct XY evolved. In addition, the growing polymer chain retains two reactive groups at each stage.


Addition Polymerisation

Addition polymerisations Involves the joining together of two molecules to form a third, larger molecule.

It involves the addition of a reactive species with a monomer to form a larger reactive species which is capable of further addition with monomer.

In simplified terms the reaction may be visualized as follows:

R* + M → R − M*

R − M* + M → R − M − M*

R − M − M* + M → R − M − M − M* etc.

The initial reactive species is represented by R* and the monomer molecules by M. The reactive species which is involved in the addition reaction may be ionic in nature or it may be a free radical.

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A) Free Radical Addition Polymerisation

1. Activation: This involves decomposition of the peroxide initiator using either thermal activation (heat), chemical activators or radiation of a suitable wavelength if a radiation-activated initiator is present.

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In simplified, general terms it may be expressed as follows:

R − O − O − R → 2RO ·

where R represents any organic molecular grouping.

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2. Initiation: The polymerisation reaction is initiated when the radical, formed on activation, reacts with a monomer molecule.

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The reaction may be given in simplified general terms as follows:

RO · + M → RO − M ·

where the symbol M represents one molecule of monomer.

It can be seen from the above equation that the initiation reaction is an addition reaction producing another active free radical species which is capable of further reaction.

 

3. Propagation: Following initiation, the new free radical is capable of reacting with further monomer molecules. Each stage of the reaction produces a new reactive species capable of further reaction, as illustrated in the following equations:

RO − M · + M → RO − M − M ·

RO − M − M · + M → RO − M − M − M ·

RO − M − M − M · + M → RO − M − M − M −M ·

A general equation for the propagation reaction may be written as follows:

RO − M · + nM → RO − (M)n − M ·

where the value of n defines the number of monomer molecules added and hence the length of the chain and the molecular weight.

 

4. Termination: It is possible for the propagation reaction to continue until the supply of monomer molecules is exhausted. In practice however, other reactions, which may result in the termination of a polymer chain, compete with the propagation reaction. These reactions produce dead polymer chains which are not capable of further additions.

One example of termination is the combination of two growing chains to form one dead chain as follows:

RO−(M)n−M· + RO−(M)x−M· → RO−(M)n−M−M−(M)x−OR

Other examples of termination involve the reactions of growing chains with molecules of initiator, dead polymer, impurity or solvent if present.

 

Example of the Polymerisation of Polymethylmethacrylate

1. Activation

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2. Initiation

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3. Propagation

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4. Termination

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B) Ionic Addition Polymerisation

Although most addition polymerisation processes in dental materials may be characterized as free radical processes, other mechanisms involving the growth of chains through ionic species such as anions and cations are also used.

Cationic ring opening polymerisation of imines is employed in the setting of certain impression materials. Cationic ring opening polymerisation of oxiranes and siloranes is being used in newly developed resin matrix composite materials.

 

Example of Polymerisation of Silorane

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Spatial Structure

Addition polymerisation reactions generally lead to the production of linear polymers. This does not imply that the chains form straight lines but simply that there are no branches off the main polymer chain and that the chains are not linked together.

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Chain Branching

Chain branching may result if a growing chain undergoes chain transfer with a polymer molecule.

This involves termination of the growing chain, but a new reactive radical is formed along the side of a polymer molecule. Growth of a fresh chain from this site produces a branched polymer.

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Cross-linking

When polymer chains are joined together by chemical bonds, the polymer is said to be cross-linked. Cross-linking is accomplished by adding cross-linking agents to the polymerizing monomer.

In the case of free radical addition polymerisations these agents are invariably difunctional alkenes in which each of the two double bonds present is able to become polymerised into a separate chain, thus effectively linking two chains together.

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In the case of condensation polymerisations, chain-branching and cross-linking can be produced by introducing trifunctional monomer into the reaction.

  • X
  • |
  • X − M − X

 

– end –

 

 

References:
McCabe JF, Walls AWG; Applied Dental Materials, Ninth Edition, 2008.
Sakaguchi RL, Powers JM; Craig’s Restorative Dental Materials, Thirteenth Edition, 2012.
Noort RV; Introduction to Dental Materials, Third Edition, 2007.
Powers JM, Wataha JC; Dental Materials Properties and Manipulation, Ninth Edition, 2008.

 

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