Page 49 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN008C-602 July 25, 2001 20:31

864 Macromolecules, Structure

In step reactions, for example, in the reaction of adipic
acid and hexamethylenediamine,
O
O
OH H 2 N CO
HO NH 2
O (CH 2 ) 2
CO
O
O
H
N OCO(CH 2 ) 2 CO 2 CH 2 CHCH 2 OCO(CH 2 ) 2 CO
HO NH 2
O
This predictability stems from the fact that the reactions
the growing molecules remain reactive and continue to involved are straightforward and well understood.
grow by condensation through out the reaction period, On the other hand, in the chain propagation reaction in
which may be several hours. Although the “monomers” vinyl monomer polymerization, many options are possible
remain, their proportion on a weight basis rapidly becomes and their occurrence and structure are not so readily pre-
negligible (although on a molar basis they remain the most dictable.Hence,suchisomericspeciesmustbedetermined
probable species). by spectroscopic or other means. One such option is the
In living polymerization, the chains continue to grow formation of inverted or head-to-head:tail-to-tail units, as
without chain transfer or termination. During the transfer- we have seen. Actually, in polystyrene formed by the usual
less polymerization, the number of polymer molecules re- chain reactions of styrene, the occurrence of such units is
mains constant. There is no termination, so the chain ends negligible, but in the polymerization of such monomers
remain active when all of the monomer has been polymer-
as vinylidene fluoride and vinyl fluoride they occur with
ized, and when fresh monomer is added, polymerization
a substantial probability (see Section III).
resumes. Because the chains are all growing at the same
Vinyl polymers may also exhibit marked differences
rate, they are more uniform than those prepared by other
in stereochemical configuration (i.e., the relative handed-
methods. This makes living polymerizations an invaluable ness of successive monomer units may vary). The simplest
method for preparing block, graft, and star copolymers. regular arrangements of successive units are the isotactic
The polymer properties are often dependent both on structure (Fig. 4a), in which all R substituents are located
the chain length (molecular weight) and the distribution on the same side ofthe zigzag plane, representingthechain
in chain lengths. In chain reactions there is a large distri- stretched out in an all-trans (see Section IV) conforma-
bution in molecular weights because of the random nature tion, and the syndiotactic arrangement, in which R groups
of the initiation and termination processes. In step reac- alternate regularly from side to side (Fig. 4b). In the atac-
tions, it results from random interactions of all species, tic arrangement, the R groups appear at random on either
the rates of which are independent of molecular weight. side of the zigzag plane (Fig. 4c). These isomeric forms
By comparison, the chains from the living polymerization are determined by the covalent bonding of each monomer
are more uniform in molecular weight. The methods for unit and cannot be interconverted by rotations about the
measuring molecular weight distributions and averages main chain bonds.
are discussed in Section II. Another type of isomerism occurs in polymers having
unsaturation in the main chain. Since carbon atoms linked
D. Isomerism in Macromolecular Chains together by double bonds are not free to rotate about the
chain axis, repeating units in polymers such as polyiso-
In polymers produced by step reactions there is generally
prene can exist as two different geometrical isomers:
no formation of isomeric species in the usual sense except
for those that may be readily predicted from the known
CH 3 H CH 3 CH 2
composition of the starting mixture. Thus, for example,
C C C C
in the reaction of ethylene glycol and succinic acid to H
produce the polyester poly(ethylene succinate), CH 2 CH 2 CH 2
cis trans
HOOC(CH 2 ) 2 COOCH 2 CH 2 OCO(CH 2 ) 2 CO NATURAL RUBBER GUTTA PERCHA
··· OCH 2 CH 2 OH
The trans polymer is a semicrystalline plastic whereas the
the introduction of a measured quantity of glycerol will cis form is normally a rubber at room temperature.
produce a predictable proportion and distribution of tri- Yet another option is the production of branching, both
functional branch units: short and long. An example of branching in polyethylene