Poly mers
Many
Smallest repeated
units
Intramolecular =
covalent bond =
1ry bond
Intermolecular =
2ry bond
Definitions:
Polymer: A long chain organic molecule
consisting of many repeating units
called monomers or mers.
Monomer (mer): Smallest repeating unit
in a polymer. Mono = Single
Mer term is derived from the
Greek word meros (parts).
Oligomer: Is a short polymer
composed of 10 mers.
Polymerization reaction: The reaction by which the
monomer units become chemically linked together
to form high molecular weight molecule (polymer).
Homopolymer: One type of monomer
Copolymer: More than on type
Classification of Polymers
1. According to Origin:
• Natural polymers: the basic constituents of any
living matter, whether plant or animal, e.g.
a) proteins
b) polysaccharides (starch)
c) polynucleic acids (DNA)
• Synthetic polymers: They are produced in the
laboratory by chemical reactions, e.g.
• Acrylic resin and elastic impression materials
• vinyl polymers which are derivatives of C2H4
2. According to Spatial Structure
1. Linear:
In linear polymers: the structural units are connected to one another
in linear sequence.
2. Branched:
In which side branch chains are connected to the main ones.
3. Cross-linked polymers:
In which the adjacent linear chains are joined one to another at
various positions by covalent bonds.
Linear homopolymer
Branched homopolymer
Cross-linked
3. According to Thermal Behavior
Thermoplastic polymers
*They are polymers which
can be shaped by heat
and after cooling they
maintain their shape (i.e.
they soften by heat and
harden by cooling).
*In thermoplastics the
polymer chains are
bonded to each other by
secondary bonds.
*They are relatively soft
and their mechanical
properties are sensitive
to heat.
*They dissolve in organic
solvents. e.g. non crosslinked polymethyl
methacrylate.
Thermosetting polymers
* They are formed into
a permanent shape and
set by a chemical
reaction.
*They can not be
remelted and reformed
into other shape but
degrade or decompose
upon being heated to
high temperature.
*Thus thermosets can
not be fused.
The polymer chain in
the thermoset polymers
forms network, with
cross-links between
them (primary covalent
bonds).
Preparation of polymers
by a process called polymerization which
is the chemical reaction by which monomer
units become chemically linked and form
polymer.
Setting reactions
I- Types of polymerization reactions:
A- Condensation
polymerization
B- Addition
polymerization
1. Condensation polymerization:
Definition: reaction between two molecules to form a larger
molecule, with the elimination of a smaller molecule (by-product)).
Characterized by:
1- Stepwise reaction.
2- Usually consists of more than one monomer species.
3- Small molecular weight due to by – product elimination.
4- Slow reaction and short polymer chains.
Examples of condensation polymerization in dentistry are:
Polysulphides and Condensation Silicones : The low molecular weight
paste is converted to a high molecular weight material.
***Water and lead sulfide are
by-products of this reaction.
2. Addition polymerization:
Characterized by:
1- Faster reaction.
2- No by - product.
3- High molecular weight.
Addition polymerization results in the formation of
large molecules without the formation of byproducts.
No change in composition takes place.
The structure of the monomer is repeated many
times in the polymer,
e.g. poly (ethylene), poly (acrylic acid), poly
(methacrylic acid) and poly (methyl methacrylate).
Stages of addition polymerization:
i- Initiation.
ii- Propagation.
iii- Termination.
a. Activation and initiation :
***Requires the presence of free radicals.
A free radical is a compound with an unpaired (unshared)
electron. This unpaired electron makes the radical very reactive.
When a free radical (R•) encounters a double bond →
it pairs with one of the electrons
leaving the other member of the pair free.
Dot = single electron Now the monomer is a free radical
By initiator like benzoyl peroxide
to release free radical.
*** Initiator must be activated to release the
free radicals.
Methods of Activation:
*Heat
*Light
*Chemical cpd. (dimethyl paratoluidine)
→ Dissociation of peroxide initiator
to give free radicals
Addition polymerization could be inhibited by any
material that reacts with the free radical
b. Propagation:
Linear growth of molecules →
***Shifting of free radical to the end of the growing
chain.
*** ↑ Viscosity
Chain reactions should continue,
with the evolution of heat,
until all the monomer has been changed to a polymer.
And so on until R1Mn•. Where n is any integral number.
c. Termination:
The chain reactions can be terminated either by
1) Direct coupling or by the.
2) Exchange of a hydrogen atom
from one growing chain to another
Factors associated with
polymerization:
a. Evolution of heat as the reaction is strongly
exothermic due to breakage of bonds.
b. Reduction in volume on polymerization
(polymerization shrinkage)
c. Residual monomer
Inhibition of polymerization:
Any impurity in the monomer which can react
with free radicals will inhibit or retard the
polymerization reaction
It can react either with the activated initiator or any
activated nucleus, or with an activated growing
chain to prevent further growth.
Such inhibitors influences the length of the initiation
period markedly, as well as the degree of
polymerization. .
Hydroquinone, Eugenol,
Or large amounts of Oxygen
Factors affecting properties of polymers
*1**Generally the polymers are molecular solids where: •
Strong primary covalent bonds does not exist between the
mers along the whole length of the polymer chain
(interatomic)
Weak secondary Van der Waal forces exist between the
chains of the polymer (intermolecular).
Secondary Van der Waal forces (weak, polar bond) are
responsible for;
- Reduced strength, hardness and rigidity.
- Higher water sorption.
2** Polymers are characterized by being amorphous and
having glass transition temperature (it is the temperature at
which the polymer start to be soft
i.e. above which the polymer is soft and rubber like material,
and below which the polymer will be very rigid
Factors affecting properties of
polymers
1. Molecular weight:
Molecular weight of the various mers multiplied by the number of
the mers
= M.W of one monomer X No. of monomer units
The higher the molecular weight of the polymer,
→
the higher the degree of polymerization
2- Degree of polymerization:
defined as the total number of mers in a polymer chain.
D.P.= M.W. of a polymer/ M.W. of a mer
Effect on the properties;
Polymers made up of large molecules are
**stronger and
** more resistant to thermal and mechanical stresses
than those composed of small molecules.
The longer the polymer chain →
the greater the number of entanglement that can
be formed among the polymer chain.
i.e. The more difficult to distort the polymer.
***Strength, rigidity, and glass transition
temperature increase with increasing chain length.
3- Cross-linking:
Adjacent linear chains are joined one to another at various
positions by chemical bonds (covalent bonds).
→ 3-dimensional network cross-linked
structure.
The effect of cross-1inking:
A small degree of cross-linking limits the amount of movement of the
polymer chains relative to each other when the material is stressed.
This * increases : - strength, hardness, rigidity,
resistance to the action of solvents
and - glass transition temperature.
* decreases water sorption.
Extensive cross-linking may lead to embrittleness of the material
4. Co-polymerization:
Copolymers are polymer chains containing two or more different
types of monomeric units.
Co-polymerization processes enable chemists to "tailor-make"
molecules of predicted properties for special applications.
Co-polymers are of three different types:
random, block and graft.
Branched co-polymer random
Graft co-polymer
Linear co-polymer random
Block co-polymer
5. Plasticizers:
They are compounds which are added to partially neutralize the
secondary bonds or intermolecular forces that normally prevent the
resin molecules from slipping past one another when the material is
stressed.
There are mainly two types of plasticizers:
a. External plasticizers: in which the plasticizer penetrates between the
polymer chains. The polymer chains become further apart and the
forces between them become less.
b. Internal plasticizers: plasticizer of a resin can also be accomplished
by copolymerization with a suitable co-monomer. In this case the
plasticizer is a part of the polymer main chain.
The effect of plasticizers:
*They usually reduce the strength and hardness of the resin
* Reduce the softening point (glass transition temperature)
***At room temperature the main difference between rigid and
flexible polymers is that the former have their glass transition
temperature above room temperature whereas the latter have
theirs below room temperature.
(the opposite action of cross-linking agent).
6. Addition of inorganic filler
to the polymer forms composite structure
Effect on the properties:
It increases strength, hardness, and rigidity of a
polymer.
Physical state of polymers
Dental polymers exist at room temperature either as
rubbers (elastomers) or hard and amorphous (organic
glasses).
Rubbers (Elastomers) consist of long chain
molecules that are coiled and in random thermal
motion.
The specific feature of a rubbery polymer is that when the
material is stretched the only work done is in uncoiling
the molecules. Thus such materials are easy to deform,
the deformation being largely reversible.
Applications in dentistry
1. Denture base materials.
2. Acrylic artificial teeth
for partial and complete dentures.
3. Composite filling materials.
4. Cements (zinc polycarboxylate and glass ionomer).
5. Impression materials (Agar, Alginate and Rubbers)
N
Chain transfer:
Process differs from the termination reactions described in
that the active state is transferred from an activated radical
to an inactive molecule and
a new nucleus for further growth is created.
An already terminated chain might be reactivated by chain
might be reactivated by chain transfer, and continue to
grow
General Points
Polymers made up of large molecules are stronger and more
resistant to thermal and mechanical stresses than those composed of
small molecules.
Growth of polymer chains is a random process. Some chains grow
faster than others and some are terminated before others. Thus, not all
chains within the polymer will have the same length.
Each chain will have its own molecular weight and degree of
polymerization.
Molecular weight of a polymer is reported as the
average molecular weight because the number of repeating units may
vary greatly from one molecule to another.
The molecular weight distribution (i.e. the fraction of low,
medium, and high molecular weight molecules in a material), has an
important effect on the physical and mechanical properties as the average
M.W. does.
e.g. Two polymers having the same chemistry and average molecular weight
may have different properties. This may be because one is formed of
long large chains while the other is formed mainly of
shorter chains i.e. different molecular weight distribution
Properties of polymers:
Polymer chains are weakly bond by secondary forces while
metals have relatively strong primary (metallic) bonds.
1. The elastic moduli of polymers are much lower than metals.
2. More tendency to creep or flow.
3. Sensitive to the rate of loading (or rate of deformation). i.e. At slow rate of
loading they behave in a ductile manner (more permanent deformation). At
high rates of loading they respond in a brittle manner (fracture after elastic
limit).
4. Sensitive to temperature. They soften as they are heated near to their glass
transition temperature (below Tg the material will be rigid).
5. strength can be increased by:
a. Increasing the degree of polymerization and thus increasing M.W.
b. Cross-linking.
c. Copolymerization.
d. Addition of inorganic fillers to form composite structures. e.g.
composite restorative resins.
The effect of temperature on intermolecular forces
increase as temperature decreases, and at a
certain temperature (the glass transition
temperature Tg) the intermolecular forces
become so large as to inhibit uncoiling.
Below the glass transition temperature the material
will be rigid. Likewise if a hard polymer
(organic glass) is heated (above Tg),
it looses rigidity at a certain temperature above
room temperature and becomes rubbery