Polymerization
Forming large molecules from small molecules - Polymerization
Just as a house can be constructed from building blocks, so too large
molecules can be constructed from small ones. The building
blocks, monomers are connected to form polymers.
Just as houses can have many shapes depending on the shapes of the blocks
used and how they are arranged, so too polymers can have many shapes depending
on how the monomers are connected. The properties of the polymer
depend on how the kinds of monomers and how they are connected together.
There are two basic kinds of polymerization reactions:
Condensation Polymerization
In condensation reactions, covalent bonds are rearranged in such
a way that two monomers are connected and water is "condensed" out. 
In
the illustration, there are two molecules . "X" and "Y" are atoms
of unspecified elements, "O" and "H" are symbols for oxygen and hydrogen.
Each stick connecting atoms represents a covalent bond (consisting of shared
electrons). Notice that both molecules have OH connected to some
other element. It this reaction, the bond between X and OH
is broken in one molecule and the bond between O and H is broken in the
other. Two new molecules are formed. One
is water formed by the OH from XOH and the H from HOY. The
other new molecule is a "dimer" (two monomers) with the remaining O atom
forming a bridge between X and Y. The diagram shows water being
"condensed" out of the two reacting molecules.
The chemical reactions that occur in the curing of mortar or of concrete
are complex, but condensation is an important feature. 
If
you follow the links about concrete, you will discover that Portland cement
is the material that, when mixed with water, holds aggregate (sand and
pieces of rock or gravel) together. Portland cement contains,
among other things, silicates. Silicates are minerals that
contain some form of the silicate anion and various cations (e.g., calcium,
sodium, magnesium, iron, aluminum ions). A simple silicate
anion is represented here. Each of the four oxygens forms a
covalent bond to an atom of silicon. The structure is like
a triangular pyramid with silicon in the very center of the pyramid and
with an oxygen is at each vertex. 
Each
oxygen has a single negative charge (an extra electron).
In the presence of water, silicates become hydrated. Some of
the oxygens form a covalent bond with with a hydrogen nucleus from the
water. One possible product of hydration is illustrated. The
small black spheres represent hydrogens. There are now only
two charged oxygens. Other possibilities for hydrated silicates
would have just one or three or even four hydrogens attached.
Since the hydrated silicates contain Si-O-H bonds, condensation is possible:
form
The resulting structure is a silicate dimer. Note that the
dimer contains OH groups. Further reactions are possible.
The two monomers are connected by an oxygen bridge. In effect
there are now two pyramids sharing one corner. Diagrammatically,
the dimerization can be represented by pyramids being connected:
More complex structures can be formed in one, two, or three dimensions:
A trimer might look like this:
A more extended two dimensional structure might look like:
One can imagine a huge structure extending not only in a sheet (as illustrated
above) but extending up from the top vertices of the pyramids.
The essential ingredients of concrete are cement, water, and aggregate.
When all are mixed together, the silicate polymers start to form.
As water evaporates, the polymers can get quite large and can bind together
and enclose the aggregate (gravel and sand) together. The polymerized
silicate is the framework for this composite material. It is ironic
that a process that starts by hydrating the silicates continues by condensing
water out of the structure. This view of the chemistry involved
is very simplistic but gives some idea of why concrete is very strong.
Free Radical Polymerization
The polymerization of vinyl 
chloride
is typical of free radical polymerizations. Vinyl chloride
molecules contain two carbon atoms, three hydrogens, and one chlorine.
One of the chemical characteristics of carbon is that each carbon atom
usually forms four covalent bonds. Remember that a covalent
bond is a shared pair of electrons. In the structure above, each "stick"
represents one bond. " _ is the same
as .." Notice that in vinyl chloride,
the bond between the two carbons is a double bond consisting of
two shared pairs or four electrons.
During free radical polymerization, one
of the two bonds between the two carbons ruptures, leaving one unshared
electron on each carbon atom. An atom with an unpaired electron is very
reactive. It is called a free radical. The
vinyl chloride free radical is illustrated to the right. If
two such free radicals meet, they can form a dimer with a new covalent
bond linking the two vinyl chlorides:
This dimer can react with another vinyl chloride to form a trimer:
By repeating this process many times, a long polymer, hundreds or even
thousands of monomer units long, can be formed. This polymer
is then called polyvinyl chloride or simply, PVC.