Free radical polymerization is a method of polymerization by which a polymer
forms by the successive addition of free radical building blocks. Free radicals
can be formed via a number of different mechanisms usually involving separate
initiator molecules. Following its generation, the initiating free radical
adds monomer units, thereby growing the polymer chain.
Free radical polymerization is a key synthesis route for obtaining a wide
variety of different polymers and material composites. The relatively
non-specific nature of free radical chemical interactions makes this one of
the most versatile forms of polymerization available and allows
facile reactions of polymeric free radical chain ends and other chemicals
or substrates. In 2001, 40 billion of the 110 billion pounds of polymers
produced in the United States were produced by free radical polymerization.
Free radical polymerization is a type of chain growth polymerization, along with
anionic, cationic and coordination polymerization.
Initiation Initiation is the first step of the
polymerization process. During initiation, an active center is created
from which a polymer chain is generated. Not all monomers are susceptible to all
types of initiators. Radical initiation works best on the carbon-carbon double
bond of vinyl monomers and the carbon-oxygen double bond in aldehydes
and ketones. Initiation has two steps. In the first step, one or two radicals
are created from the initiating molecules. In the second step, radicals
are transferred from the initiator molecules to the monomer units present.
Several choices are available for these initiators.
= Types of initiation and the initiators Thermal decomposition: The initiator is
heated until a bond is homolytically cleaved, producing two radicals. This
method is used most often with organic peroxides or azo compounds.
Photolysis: Radiation cleaves a bond homolytically, producing two radicals.
This method is used most often with metal iodides, metal alkyls, and azo
compounds. Photoinitiation can also occur by
bi-molecular H-abstraction when the radical is in its lowest triplet excited
state. An acceptable photoinitiator system should fulfill the following
requirements:High absorptivity in the 300-400 nm range.
Efficient generation of radicals capable of attacking the olefinic double bond of
vinyl monomers. Adequate solubility in the binder
system. Should not impart yellowing or
unpleasant odors to the cured material. The photoinitiator and any byproducts
resulting from its use should be non-toxic.
Redox reactions: Reduction of hydrogen peroxide or an alkyl hydrogen peroxide
by iron. Other reductants such as Cr2+, V2+, Ti3+, Co2+, and Cu+ can be employed
in place of ferrous ion in many instances.
Persulfates: The dissociation of a persulfate in the aqueous phase. This
method is useful in emulsion polymerizations in which the radical
diffuses into a hydrophobic monomer-containing droplet.
Ionizing radiation: α-, β-, γ-, or x-rays cause ejection of an electron
from the initiating species, followed by dissociation and electron capture to
produce a radical. Electrochemical: Electrolysis of a
solution containing both monomer and electrolyte. A monomer molecule will
receive an electron at the cathode to become a radical anion, and a monomer
molecule will give up an electron at the anode to form a radical cation. The
radical ions then initiate free radical polymerization. This type of initiation
of especially useful for coating metal surfaces with polymer films.
Plasma: A gaseous monomer is placed in an electric discharge at low pressures
under conditions where a plasma is created. In some cases, the system is
heated and/or placed in a radiofrequency field to assist in creating the plasma.
Sonication: High-intensity ultrasound at frequencies beyond the range of human
hearing can be applied to a monomer. Initiation results from the effects of
cavitation. The collapse of the cavities generates very high local temperatures
and pressures. This results in the formation of excited electronic states
which in turn lead to bond breakage and radical formation.
Ternary Initiators: A ternary initiator is the combination of several types of
initiators into one initiating system. The types of initiators are chosen based
on the properties they are known to induce in the polymers they produce. For
example, poly(methyl methacrylate) has been synthesized by the ternary system
benzoyl sopropylcarbazole-di-η5-indenylzicronium
dichloride. This type of initiating system contains
a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid.
Metallocenes in combination with initiators accelerate polymerization of
poly(methyl methacrylate) and produce a polymer with a narrower molecular weight
distribution. The example shown here consists of indenylzirconium and benzoyl
peroxide. Also, initiating systems containing heteroaromatic diketo
carboxylic acids, such as s(o-carboxybenzoyl)-N-isopropylcarbazole
in this example, are known to catalyze the decomposition of benzoyl peroxide.
Initiating systems with this particular heteroaromatic diket carboxylic acid are
also known to have effects on the microstructure of the polymer. The
combination of all of these components—a metallocene, an initiator, and a
heteroaromatic diketo carboxylic acid—yields a ternary initiating system
that was shown to accelerate the polymerization and produce polymers with
enhanced heat resistance and regular microstructure.
= Initiator Efficiency = Due to side reactions and inefficient
synthesis of the radical species, chain initiation is not 100%. The efficiency
factor, f, is used to describe the effective radical concentration. The
maximum value of f is 1.0, but values typically range from 0.3-0.8. The
following is a list of reactions that decrease the efficiency of the
initiator. Primary recombination: Two radicals
re-combine before initiating a chain. This occurs within the solvent cage,
meaning that no solvent has yet come between the new radicals.
Other recombination pathways: Two radical initiators re-combine before
initiating a chain but not in the solvent cage.
Side reactions: One radical is produced instead of the three radicals that could
be produced. Propagation
During polymerization, a polymer spends most of its time in increasing its chain
length, or propagating. After the radical initiator is formed, it attacks
a monomer. In an ethene monomer, one electron pair is held securely between
the two carbons in a sigma bond. The other is more loosely held in a pi bond.
The free radical uses one electron from the pi bond to form a more stable bond
with the carbon atom. The other electron returns to the second carbon atom,
turning the whole molecule into another radical. This begins the polymer chain.
Figure 12 shows how the orbitals of an ethylene monomer interact with a radical
initiator. Once a chain has been initiated, the
chain propagates until there is no more monomer or until termination occurs.
There may be anywhere from a few to thousands of propagation steps depending
on several factors such as radical and chain reactivity, the solvent, and
temperature. The mechanism of chain propagation is as follows:
Termination Chain termination will occur unless the
reaction is completely free of contaminants. In this case, the
polymerization is considered to be a living polymerization because
propagation can continue if more monomer is added to the reaction. Living
polymerizations are most common in ionic polymerization, however, due to the high
reactivity of radicals. Termination can occur by several different mechanisms.
If longer chains are desired, the initiator concentration should be kept
low; otherwise, many shorter chains will result.
Combination of two active chain ends: one or both of the following processes
may occur. Combination: two chain ends simply
couple together to form one long chain. One can determine if this mode of
termination is occurring by monitoring the molecular weight of the propagating
species: combination will result in doubling of molecular weight. Also,
combination will result in a polymer that is C2 symmetric about the point of
the combination. Radical disproportionation: a hydrogen
atom from one chain end is abstracted to another, producing a polymer with a
terminal unsaturated group and a polymer with a terminal saturated group.
Combination of an active chain end with an initiator radical.
Interaction with impurities or inhibitors. Oxygen is the common
inhibitor. The growing chain will react with molecular oxygen, producing an
oxygen radical, which is much less reactive. This significantly slows down
the rate of propagation. Nitrobenzene, butylated hydroxyl
toluene, and diphenyl picryl hydrazyl are a few other inhibitors. The latter
is an especially effective inhibitor because of the resonance stabilization
of the radical. Chain transfer
Contrary to the other modes of termination, chain transfer results in
the destruction of only one radical, but also the creation of another radical.
Often, however, this newly created radical is not capable of further
propagation. Similar to disproportionation, all chain transfer
mechanisms also involve the abstraction of a hydrogen atom. There are several
types of chain transfer mechanisms. To solvent: a hydrogen atom is
abstracted from a solvent molecule, resulting in the formation of radical on
the solvent molecules, which will not propagate further.
The effectiveness of chain transfer involving solvent molecules depends on
the amount of solvent present, the strength of the bond involved in the
abstraction step, and the stability of the solvent radical that is formed.
Halogens, except fluorine, are easily transferred.
To monomer: a hydrogen atom is abstracted from a monomer. While this
does create a radical on the affected monomer, resonance stabilization of this
radical discourages further propagation. To initiator: a polymer chain reacts
with an initiator, which terminates that polymer chain, but creates a new radical
initiator. This initiator can then begin new polymer chains. Therefore, contrary
to the other forms of chain transfer, chain transfer to the initiator does
allow for further propagation. Peroxide initiators are especially sensitive to
chain transfer. To polymer: the radical of a polymer
chain abstracts a hydrogen atom from somewhere on another polymer chain. This
terminates one of the polymer chains, but allows the other to branch. When
this occurs, the average molar mass remains relatively unaffected.
Effects of chain transfer: The most obvious effect of chain transfer is a
decrease in the polymer chain length. If the rate of termination is much larger
than the rate of propagation, then very small polymers are formed with chain
lengths of 2-5 repeating units. The Mayo–Lewis equation estimates the
influence of chain transfer on chain length: . Where ktr is the rate constant
for chain transfer and kp is the rate constant for propagation. The Mayo-Lewis
equation assumes that transfer to solvent is the major termination
pathway. Methods
There are four industrial methods of radical polymerization:
Bulk polymerization: reaction mixture contains only initiator and monomer, no
solvent. Solution polymerization: reaction
mixture contains solvent, initiator, and monomer.
Suspension polymerization: reaction mixture contains an aqueous phase,
water-insoluble monomer, and initiator soluble in the monomer droplets.
Emulsion polymerization: similar to suspension polymerization except that
the initiator is soluble in the aqueous phase rather than in the monomer
droplets. An emulsifying agent is also needed.
Other methods of radical polymerization include the following:
Template polymerization: In this process, polymer chains are allowed to
grow along template macromolecules for the greater part of their lifetime. A
well-chosen template can affect the rate of polymerization as well as the molar
mass and microstructure of the daughter polymer. The molar mass of a daughter
polymer can be up to 70 times greater than those of polymers produced in the
absence of the template and can be higher in molar mass than the templates
themselves. This is because of retardation of the termination for
template-associated radicals and by hopping of a radical to the neighboring
template after reaching the end of a template polymer.
Plasma polymerization: The polymerization is initiated with plasma.
A variety of organic molecules including alkenes, alkynes, and alkanes undergo
polymerization to high molecular weight products under these conditions. The
propagation mechanisms appear to involve both ionic and radical species. Plasma
polymerization offers a potentially unique method of forming thin polymer
films for uses such as thin-film capacitors, antireflection coatings, and
various types of thin membranes. Sonication: The polymerization is
initiated by high-intensity ultrasound. Polymerization to high molecular weight
polymer is observed but the conversions are low ( 0, the polymer will undergo
depolymerization. According to the thermodynamic equation ΔG = ΔH - TΔS, a
negative enthalpy and an increasing entropy will shift the equilibrium
towards polymerization. In general, the polymerization is an
exothermic process, i.e. negative enthalpy change, since addition of a
monomer to the growing polymer chain involves the conversion of π bonds into
σ bonds or an ring opening reaction that releases the ring tension in a cyclic
monomer. Meanwhile, during polymerization, a large amount of small
molecules are associated, losing rotation and translational degrees of
freedom. As a result, the entropy decreases in the system, ΔSp