Chemistry Reference
In-Depth Information
This is the structure of alkyd polymers, which are the reaction products of
acids with di- and polyhydric alcohols. Such polymers are used primarily for sur-
face coatings, which can be caused to react further in situ through residual
hydroxyl, acid, or olefin groups.
The major example of the second branched polymer type is the polyethylene that
is made by free-radical polymerization at temperatures between about 100
Cand
300
C and pressures of 1000-3000 atm (100-300 MPa). Depending on reaction con-
ditions, these polymers will contain some 20 to 30 ethyl and butyl branches per 1000
carbon atoms and one or a few much longer branches per molecule. They differ suffi-
ciently from linear polyethylene such that the two materials are generally not used
for the same applications. Poly(vinyl acetate) polymers resemble polyethylene in that
the conventional polymerization process yields branched macromolecules.
By convention, the term branched implies that the polymer molecules are dis-
crete, which is to say that their sizes can be measured by at least some of the
usual analytical methods described in Chapter 3. A network polymer is an inter-
connected branch polymer. The molecular weight of such polymers is infinite, in
the sense that it is too high to be measured by standard techniques. If the average
functionality of a mixture of monomers is greater than 2, reaction to sufficiently
high conversion yields network structures (Chapter 7).
Network polymers can also be made by chemically linking linear or branched
polymers. The process whereby such a preformed polymer is converted to a net-
work structure is called cross-linking. Vulcanization is an equivalent term that is
used mainly for rubbers. The rubber in a tire is cross-linked to form a network. The
molecular weight of the polymer is not really infinite even if all the rubber in the
tire is part of a single molecule (this is possible, at least in theory), since the size of
the tire is finite. Its molecular weight is infinite, however, on the scale applied in
polymer measurements, which require the sample to be soluble in a solvent.
The structure of a ladder polymer comprises two parallel strands with regular
cross-links, as in polyimidazopyrrolone (1-37), which is made from pyromellitic
dianhydride (1-38) and 1,2,4,5-tetraminobenzene (1-39).
N
C
N
C
N
N
C
C
n
O
O
1-37
O
O
C
C
O
O
C
C
O
O
1-38
