Page 66 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 29

Cl Cl Cl Cl Cl Cl Cl Cl Cl
H H H H H H H H H
R
R
Isotactic
H Cl H Cl H Cl H Cl H
Cl H Cl H Cl H Cl H Cl
R
R
Syndiotactic
H H Cl Cl H Cl Cl H H
Cl Cl H H Cl H H Cl Cl
R
R
Atactic

FIGURE 2.5 Skeletal formulas of isotactic, syndiotactic, and atactic of poly(vinyl chloride), PVC.

R
R R R 1
R 1 R 1 R 1 R 1
R 2 R 2 R 2
R 2
R R 1
R 2
R 2
R R
R 2 R 2 R R R 1 R 1
Erythrodiisotactic Threodiisotactic Erythrodisyndiotactic Threodisyndiotactic
FIGURE 2.6 Simulated formulas of ditactic isomers where R 2 are chain extensions and R and R 1 are not
hydrogen.
possible structures. As shown in Figure 2.5 the isomer corresponding to the arrangement
DDDDDD or LLLLLL is called isotactic (same). The isomer corresponding to the DLDLDLDL
alternating structural arrangement about carbon is called syndiotactic (alternating). The isomer
arrangement that corresponds to some mix of stereo arrangements about the chiral carbons is
called atactic (having nothing to do with). The differences in stereoregularity about the chiral

carbon influence the physical properties of the polymers. Thus, those with isotactic or syn-
diotactic arrangements are more apt to form compact crystalline arrangements and those with
atactic stereoregularity are more apt to form amorphous arrangements. Isotactic PP (iPP) has a
o
o
MP of about 160 C and it is highly crystalline whereas atactic PP (aPP) melts at about 75 C and
is amorphous. The term eutactic is used to describe either an isotactic or syndiotactic polymer
or a mixture of both.
While most polymers contain only one chiral or asymmetrical center in each repeat unit, it is
possible to have diisotacticity where two different substituents are present at chiral centers. These
isomers are labeled erythro- and threodiisotactic and erythro- and threosyndiotactic isomers. This
topic is further described in Appendix J (Figure 2.6).
The many different conformers resulting from rotation about the carbon–carbon bonds in simple
molecules like ethane and n-butane may be shown by Newman projections (Figure 2.7). The most
stable is the anti or trans projection where the steric hindrance is minimized. There are a number
of eclipsed and gauche arrangements of which only one of each is shown in Figure 2.7. The energy
difference between the anti and the eclipsed, the least stable form, is about 12 kJ/mol.
The ease in going from one conformer to the other conformer decreases as the pendant groups
increase in size and in secondary bonding. Thus, poly(methyl methacrylate) (PMMA) is hard at

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