284
ORGANIC COMPOUNDS AND POLYMERS
The more complex monomer
H A
-c-cH B
forms the polymer
R- C-C- R
[ : : I .
(11.7)
(11.8)
When A is the methyl radical -CH, and B is the radical -COOCH,, then the
polymer is polymethyl methacrylate, which was mentioned in Chapters 9 and 12. It
generally has a molecular weight between lo5 and lo6 Daltons (Da, g/mol), and
since the molecular weight of the monomer CH,=C(CH,)CO,CH, is 100 Da, there
are between 1000 and 10,000 monomers in this polymer. Polymers are also formed
from a number of other radicals, such as allyl (CH,=CHCH,-)
and vinyl
(CH,=CH-).
Natural rubber is a polymer based on the isoprene molecule:
H H CH,
H- C=C -C=CH,
I
l
l
(1 1.9)
which has a pair of double bonds. The second double bond is used to form
crosslinkages between the linear polymer chains. The methyl group -CH, at the
top of the molecule is an example of a side chain on an otherwise linear molecule.
11.2.2. Sizes of Polymers
Polymers are generally classified by their molecular weight, and to discuss them
from the nanoparticle aspect, we need a convenient way to convert molecular weight
to a measure of the polymer size d. The volume Vin the units cubic nanometers
(nm3) of a substance of molecular weight Mw and density p is given by
M W
V = 0.001661 -
P
(11.10)
where Mw has the unit dalton or g/mol (grams per mole) and p has the conventional
units g/cm3. If the shape of the nanoparticle is fairly uniform, with very little
stretching or flattening in any direction, then a rough measure of its size is the cube
root of the volume (1 1. IO), which we call the size parameter d
d = 0.1184 (5)""
(1 1.11)