Lecture # 6
Polymerization conditions and polymer reactions
1. Polymerization in homogeneous systems
2. Polymerization in heterogeneous systems
1. Polymerization in homogeneous systems
The homogeneous polymerization techniques involve pure
monomer or homogeneous solutions of monomer and polymer is
a solvent.
These techniques can be divided into 2 methods: the bulk and the
solution polymerizations.
1. Bulk polymerization
The conversion of a monomer into a polymer in the
absence of diluents or dispersing agent known as bulk
polymerization. This process requires simply heating the
monomer in the presence of small amount of initiator
under a suitable condensing or pressure system.
Advantages:
• Bulk polymerizations are the simplest technique and
produce the highest-purity polymers.
• Only monomer, a monomer-soluble initiator (& chain
transfer agent to control the molecular weight) are used.
• This method is practiced widely in the manufacture of
condensation polymers.
 Easy polymer recovery and easy for cast polymerization
into final product forms.
• The viscosity of the mixture is still low to allow ready
mixing, heat transfer, and bubble elimination.
Disadvantages:
• Free-radical polymerizations are typically highly
exothermic.
• An increase temperature will increase the polymerization
rate, generate heat dissipation and a tendency to develop
of localized “hot spots”.
• Near the end of pol’n, the viscosity is very high and
difficult to control.
This method is seldom used in commercial manufacture.
E.g. PS and PMM
2. Solution polymerization
It requires the correct selection of the solvents. Both the
initiator and monomer be soluble in each other and that
the solvent are suitable for chain-transfer characteristics
and melting and boiling points, regarding the solventremoval steps.
Advantages:
• Heat is removed during pol’n via solvent.
• “Cheap” materials for the reactors (stainless steel or glass
lined).
Disadvantages:
• Small production per reactor volume.
• Not suitable for dry polymers.
• Difficult of complete solvent removal.
E.g. PVA, poly (acrylic acid), polyacrylamide.
2. Polymerization in heterogeneous systems
2.1. Suspension polymerization
Suspension pol’n consists of an aqueous system with
monomer as a dispersed phase and results in polymer as a
dispersed solid phase.
Advantages:
• Excellent heat transfer because of the presence of the
solvent.
• Solvent cost and recovery operation are cheap.
Disadvantages:
• Contamination by the presence of suspension and other
additives low polymer purity.
• Reactor cost may higher than the solution cost.
E.g. PVC, PSAN, Poly(vinylidene chloride –VC)
2. 2 Emulsion polymerization
An emulsion pol’n consists of water (as the heat-transfer
agent), monomer, water-soluble initiator, a chain-transfer
agent, a surfactant (such as sodium salt of a long-chain fatty
acid fatty-acid soap).
Method:
The hydrophobic monomer molecules form droplets (Å0.5 –
10 μm).
The fatty-acid soap forms aggregates of 50 – 100 soap
molecules with a layered structure. This structure is called
micelles
The hydrophobic monomer molecules form droplets (Å0.5 –
10 μm), which are surrounded by the surfactant molecules.
The surfactant molecules arrange themselves with
hydrophilic ends point outward and hydrophobic (aliphatic)
ends point inward toward the monomer droplets. This
process generates free radicals in aqueous phase.
The size of monomer droplets depends on the temperature
pol’n and the agitation rate.
As the polymer particles grow much larger than the original
micelles and absorbs all the soap from the aqueous phase.
The monomer droplets are unstable at the beginning. If the
agitation stopped, the oil contains no polymer.
When the polymer contains of 50% monomers (60 – 80%
pol’n), both the monomer droplets and the left micelles
disappear. The suspension of polymer particles in water is
called latex. Then the rate of pol’n is constant over the
reaction.
PHYSICAL STATE
1-Crystalline and Amorphous Behavior
Solid polymers differ from ordinary, low-molecular-weight
compounds in the nature of their physical state or morphology.
Most polymers show simultaneously the characteristics of both
crystalline and amorphous solids
The terms crystalline and amorphous are used to indicate
the ordered and unordered polymer regions, respectively.
Different polymers show different degrees of crystalline
behavior. The known polymers constitute a spectrum of
materials from those that are completely amorphous to others
that possess low to moderate to high crystallinity. The term
semicrystalline is used to refer to polymers that are partially
crystalline. Completely crystalline polymers are rarely
encountered.
Thermal Transitions
Polymeric materials are characterized by two major types of
transition temperatures—the crystalline melting temperature Tm
and the glass transition temperature Tg. The crystalline melting
temperature is the melting temperature of the crystalline
domains of a polymer sample. The glass transition temperature
is the temperature at which the amorphous domains of a
polymer take on the characteristic properties of the glassy
state—brittleness, stiffness, and rigidity. The difference between
the two thermal transitions can be understood more clearly
by considering the changes that occur in a liquid polymer as it is
cooled. The translational, rotational, and vibrational energies of
the polymer molecules decrease on cooling. When the total
energies of the molecules have fallen to the point where the
translational and rotational energies are essentially zero,
crystallization is possible. If certain symmetry requirements are
met, the molecules are able to pack into an ordered, lattice
arrangement and crystallization occurs. The temperature at
which this occurs in Tm. However, not all polymers meet the
necessary symmetry requirements for crystallization. If the
symmetry requirements are not met, crystallization does not
take place, but the energies of the molecules continue to
decrease as the temperature decreases. A temperature is
finally reached—the Tg—at which long-range motions of the
polymer chains stop. Long-range motion, also referred to as
segmental motion, refers to the motion of a segment of a
polymer chain by the concerted rotation of bonds at the ends of
the segment. _CH3 and C_COOCH3 bonds in poly(methyl
methacrylate), do not cease at Tg.]
Whether a polymer sample exhibits both thermal
transitions or only one depends on its morphology. Completely
amorphous polymers show only a Tg. A completely crystalline
polymer shows only a Tm. Semicrystalline polymers exhibit both
the crystalline melting and glass transition temperatures.
Changes in properties such as specific volume and heat
capacity occur as a polymer undergoes each of the thermal
transitions.
A variety of methods have been used to determine Tg and
Tm, including dilatometry (specific volume), thermal analysis,
dynamic mechanical behavior, dielectric loss, and broad-line
NMR
The values of Tg and Tm for a polymer affect its mechanical
properties at any particular temperature and determine the
temperature range in which that polymer can be employed.
The Tg and Tm values for some of the common polymers
are shown in Table 1–3