Chapter 10
Sometimes C chains become extremely long containing
thousands of C atoms. When the number of C atoms in a
molecule reaches these large numbers, a giant molecule is
created, which we call a polymer.
• Polymer means many parts, because usually these
molecules are formed by linking many smaller molecules
(called monomers) together, like an elephant train.
•There are 2 sources of polymers.
•There are many natural polymers found in Nature,
some of which we will study in later chapters.
•The first synthetic polymer was derived from a natural
polymer, cellulose, and was called celluloid and was first
used as a replacement for ivory in billiard balls.
Then beginning in 1909, the first truly synthetic polymer was
produced in a laboratory and this has spawned a multibillion
dollar industry. Everything that consumers call plastics are
synthetic polymers. We will now look briefly at some of these.
2 Types:
1) Addition
2) Condensation
Addition polymers – Large molecule composed of one
or sometimes 2 smaller molecules (called monomers),
linked together end to end, basically (sort of like a train
of elephants).
Requirements of monomers
For addition, at least one double bond – Linking
takes place by breaking the double bonds and adding
each monomer at that point.
•A few common addition polymers are described in your
book on pages 268 – 271 and 272 - 273. You should read
these.
•Note that in each case, the monomer has at least one
double bond.
•Table 10.1 on page 275 has a more detailed list of
common addition polymer plastics and their uses.
•Generally, plastics are insulators (don’t conduct heat or
electricity.
•But there is even a plastic called polyacetylene which does
conduct electricity and is sometimes used as a metal
substitute.
•The key idea to remember is that polymers can be
designed and made to meet almost any need.
By adjusting the reaction conditions, chemists can
adjust the exact structure of these compounds. The
same general polymer, such as polyethylene can be
made elastic or rigid, soft or hard, clear or opaque.
We won’t go into these specifics, except in the
following situation:
Diene Polymerization - C=C-C=C. Usually only one C=C
is utilized, leaving
...-C-C=C-C... , leaving one C=C in the middle.
Natural rubber is a polymer of a diene: The repeating unit is
found to be:
... CH2
C
CH3
CH
CH2
...
This can be made from the monomer isoprene:
C
C
C
C
C
•Natural rubber was not a very useful product until
Charles Goodyear in 1844, discovered that if you heat
natural rubber in the presence of S, cross links are
made between different strands of the polymer
utilizing S atoms as the bridge.
•This process is called vulcanization. It produced a
much tougher & less gummy product.
When this was done for the first time in a laboratory, the
product had some rubber-like properties, but not all & it
was basically not usable (sticky & tacky)
We need to digress for a few minutes here.
.
The 4 C alkene has a new type of isomerism:
The 4 C alkene has a new type of isomerism:
CH3 CH2 CH CH2
1-butene
CH3 CH CH CH3
2-butene
Note that the position of the double bond creates isomers
with alkenes of 4 C's or more. In addition, 2-butene has
another type of isomerism:
CH3
H
H
C
C
H
CH3
CH3
C
C
CH3
H
cis-2-butene
trans-2-butene
Cis and Trans isomerism is one type of a general form of
isomerism, called steroisomerism, which is only different
because of the orientation in space of the atoms. All
bonds are the same.
Further study showed that natural rubber was almost all cis
around C=C, while the synthetic was a mixture of cis &
trans. Using special catalysts the synthetic polymer with
almost all cis was made & it was just as good as natural
rubber. Thus after World War II, synthetic rubber
compounds became a major industry.
Various variations on monomers have yielded
various different synthetic rubbers.
The second type of polymer :
Condensation Polymers – Polymer formed
when 2 or more monomer molecules linked
together via a condensation reaction, each
reaction eliminating a small molecule, usually
H2O or HCl.
•The monomer requirement for condensation
polymerization is that it must contain 2 functional groups
• One functional group on the first monomer reacts with
the opposite functional group on the second monomer
producing a larger molecule, that still has 2 functional
groups.
•This process can continue on and on.
Condensation Polymers:
1. polyamide - Nylon (different types) & protein.
Fibers of nylon are formed by the following:
Melt spinning - Heat to just above M.P., force at high
pressure through tiny holes into stream of N2 gas to cool.
Solidifies quickly. Then drawn into 4 times original length,
which orients polymer chains into regular side by side
position --> increases MP - harder, denser, tougher.
(extensive H bonding)
2. Polyesters – polymer formed when a molecule
containing 2 COOH groups (a di-acid) and a molecule
containing 2 OH groups, a di-alcohol or glycol
•Another way we classify plastics is whether they can be
remolded or not.
•A thermoplastic polymer can be softened by heat and
pressure and then reshaped. This can be done repeated
times.
•These can be recycled with some effort. Some are
currently being recycled by many communities.
•A thermosetting plastic cannot be softened by heat and
remolded. Heating causes discoloration and
decomposition.