An example of
alkene polymerization, in which each
Styrene monomer unit's double bond reforms as a single bond with another styrene monomer and forms
polystyrene.
In
polymer chemistry,
polymerization is a process of reacting
monomer molecules together in a
chemical reaction to form three-dimensional networks or
polymer chains. There are many forms of polymerization and different systems exist to categorize them.
Introduction
In chemical compounds, polymerization occurs via a variety of reaction mechanisms that vary in complexity due to
functional groups present in reacting compounds
[Clayden, J., Greeves, N. et al. (2000), p1450-1466] and their inherent
steric effects explained by
VSEPR Theory. In more straightforward polymerization,
alkenes, which are relatively stable due to
bonding between carbon atoms form polymers through relatively simple radical reactions; in contrast, more complex reactions such as those that involve substitution at the carbonyl group require more complex synthesis due to the way in which reacting molecules polymerize.
As alkenes can be formed in somewhat straightforward reaction mechanisms, they form useful compounds such as
polyethylene and
polyvinyl chloride (PVC) when undergoing radical reactions,
which are produced in high tonnages each year
due to their usefulness in manufacturing processes of commercial products, such as piping, insulation and packaging. Polymers such as PVC are generally referred to as "
homopolymers" as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one molecule are referred to as "
co-polymers".
Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures (>-80
oC) to form
trimers;
molecules consisting of 3 monomer units which can cyclize to form ring cyclic structures, or undergo further reactions to form
tetramers,
or 4 monomer-unit compounds. Further compounds either being referred to as
oligomers
in smaller molecules. Generally, because formaldehyde is an exceptionally reactive electrophile it allows
nucleophillic addition of hemiacetal intermediates, which are generally short lived and relatively unstable "mid stage" compounds which react with other molecules present to form more stable polymeric compounds.
Polymerization that is not sufficiently moderated and proceeds at a fast rate can be very hazardous. This phenomenon is known as
Hazardous polymerization and can cause fires and explosions.
Step-growth
Step-growth polymers are defined as polymers formed by the stepwise reaction between functional groups of monomers. Most step-growth polymers are also classified as condensation polymers, but not all step-growth polymers (like
polyurethanes formed from
isocyanate and alcohol bifunctional monomers) release condensates. Step-growth polymers increase in molecular weight at a very slow rate at lower conversions and reach moderately high molecular weights only at very high conversion (
i.e. >95%).
To alleviate inconsistencies in these naming methods, adjusted definitions for condensation and addition polymers have been developed. A condensation polymer is defined as a polymer that involves
elimination of small molecules during its synthesis, or contains functional groups as part of its
backbone chain, or its
repeat unit does not contain all the atoms present in the hypothetical monomer to which it can be degraded.
Chain-growth
Chain-growth polymerization (or addition polymerization) involves the linking together of molecules incorporating double or triple
chemical bonds. These unsaturated
monomers (the identical molecules that make up the polymers) have extra internal bonds that are able to break and link up with other monomers to form the repeating chain. Chain-growth polymerization is involved in the manufacture of polymers such as
polyethylene,
polypropylene, and
polyvinyl chloride (PVC). A special case of chain-growth polymerization leads to
living polymerization.
In the
radical polymerization of
ethylene, its pi bond is broken, and the two electrons rearrange to create a new propagating center like the one that attacked it. The form this propagating center takes depends on the specific type of addition mechanism. There are several mechanisms through which this can be initiated. The
free radical mechanism was one of the first methods to be used. Free radicals are very reactive atoms or molecules that have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided in to three stages:
chain initiation,
chain propagation, and
chain termination.
Free radical addition polymerization of ethylene must take place at high temperatures and pressures, approximately 300°C and 2000 atm. While most other free radical polymerizations do not require such extreme temperatures and pressures, they do tend to lack control. One effect of this lack of control is a high degree of branching. Also, as termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains.
A newer method of polymerization similar to free radical, but allowing more control involves the
Ziegler-Natta catalyst, especially with respect to
polymer branching.
Other forms of chain growth polymerization include
cationic addition polymerization and
anionic addition polymerization. While not used to a large extent in industry yet due to stringent reaction conditions such as lack of water and oxygen, these methods provide ways to polymerize some monomers that cannot be polymerized by free radical methods such as
polypropylene. Cationic and anionic mechanisms are also more ideally suited for
living polymerizations, although free radical living polymerizations have also been developed.
See also
