1.12.2 Stereoisomerism
Stereoisomerism occurs in vinyl polymers when one of the carbon atoms of the monomer double bond carries two different substituents. It is formally similar to the optical isomerism of organic chemistry in which the presence of an asymmet- ric carbon atom produces two isomers which are not superimposable. Thus glyc- eraldehyde exists as two stereoisomers with configurations shown in 1-66. (The dotted lines denote bonds below and the wedge signifies bonds above the plane of the page.) Similarly, polymerization of a monomer with structure
1-66
H CHO
C C
HOCH2 HO
OHC H
CH2OH OH
1-67 (where X andY are any substituents that are not identical) yields poly- mers in which every other carbon atom in the chain is a site of steric isomerism.
1-67 CH2 C
X Y
Such a site, labeled Cxin 1-68, is termed apseudoasymmetricorchiralcarbon atom.
1-68
CH2 Cx CH2 Cx CH2 Cx X
Y
X Y
X Y
The two glyceraldehyde isomers of 1-66 are identical in all physical properties except that they rotate the plane of polarized light in opposite directions and form enantiomorphous crystals. When more than one asymmetric center is present in a low-molecular-weight species, however, stereoisomers are formed which are not mirror images of each other and which may differ in many physical properties.
An example of a compound with two asymmetric carbons (a diastereomer) is tartaric acid, 1-69, which can exist in two optically active forms (D and L,
39 1.12 Configurational Isomerism
mp 170C), an optically inactive form (meso, mp 140C), and as an optically inactive mixture (DL racemic, mp 206C).
1-69 COOH
COOH HOC
C H
H OH
Vinyl polymers contain many pseudoasymmetric sites, and their properties are related to those of micromolecular compounds that contain more than one asym- metric carbon. Most polymers of this type are not optically active. The reason for this can be seen from structure 1-68. Any Cxhas four different substituents: X, Y, and two sections of the main polymer chain that differ in length. Optical activity is influenced, however, only by the first few atoms about such a center, and these will be identical regardless of the length of the whole polymer chain. This is why the carbons marked Cx are not true asymmetric centers. Only those Cx centers near the ends of macromolecules will be truly asymmetric, and there are too few chain ends in a high polymer to confer any significant optical activity on the mol- ecule as a whole.
Each pseudoasymmetric carbon can exist in two distinguishable configura- tions. To understand this, visualize Maxwell’s demon walking along the polymer backbone. When the demon comes to a particular carbon Cx she will see three substituents: the polymer chain, X, and Y. If these occur in a given clockwise order (say, chain, X, and Y), Cx has a particular configuration. The substituents could also lie in the clockwise order: chain, Y, andX, however, and this is a dif- ferent configuration. Thus, every Cxmay have one or another configuration. This configuration is fixed when the polymer molecule is formed and is independent of any rotations of the main chain carbons about the single bonds that connect them.
The configurational nature of a vinyl polymer has profound effects on its physical properties when the configurations of the pseudoasymmetric carbons are regular and the polymer is crystallizable. The usual way to picture this phenome- non involves consideration of the polymer backbone stretched out so that the bonds between the main chain carbons form a planar zigzag pattern. In this case the X andYsubstituents must lie above and below the plane of the backbone, as shown in Fig. 1.5. If the configurations of successive pseudoasymmetric carbons are regular, the polymer is said to bestereoregular or tactic. If all the configura- tions are the same, the substituents X (andY) will all lie either above or below the plane when the polymer backbone is in a planar zigzag shape. Such a polymer is termed isotactic. This configuration is depicted inFig. 1.5a. Note that it is not
possible to distinguish between all-D and all-L configurations in polymers because the two ends of the polymer chain cannot be identified. The structure in Fig. 1.5ais thus identical to its mirror image in which all the Y substituents are above the plane.
If the configurations of successive pseudoasymmetric carbons differ, a given substituent will appear alternatively above and below the reference plane in this planar zigzag conformation (Fig. 1.5b). Such polymers are calledsyndiotactic.
When the configurations at the Cx centers are more or less random, the poly- mer is not stereoregular and is said to beatactic. Polymerizations that yield tactic polymers are called stereospecific. Some of the more important stereospecific polymerizations of vinyl polymers are described briefly in Chapter 11.
The reader should note that stereoisomerism does not exist if the substituents XandYin the monomer 1-67 are identical. Thus there are no configurational iso- mers of polyethylene, polyisobutene, or poly(vinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic, and atactic configurational iso- mers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isom- erism is also taken into account.
It may also be useful at this point to reiterate that the stereoisomerism, which is the topic of this section, is confined to polymers of substituted ethylenic mono- mers. Polymers with structures like 1-5 or 1-6 do not have pseudoasymmetric car- bons in their backbones.
The importance of stereoregularity in vinyl polymers lies in its effects on the crystallizability of the material. The polymer chains must be able to pack together in a regular array if they are to crystallize. The macromolecules must have fairly
X C CC
CC CC
CC CC
CC Y H Y H Y Y H H Y H Y H Y
(a)
(b) H X H X HX H X H X H X
X HY H X H Y H X H Y HX H
Y C CC
CC CC
CC CC
CC C HX H Y H X H Y H X HY H FIGURE 1.5
(a) Isotactic polymer in a planar zigzag conformation. (b) Syndiotactic polymer in a planar zigzag conformation.
41 1.12 Configurational Isomerism
regular structures for this to occur. Irregularities like inversions in monomer placements (head-to-head instead of head-to-tail), branches, and changes in con- figuration generally inhibit crystallization. Crystalline polymers will be high melting, rigid, and difficultly soluble compared to amorphous species with the same constitution. A spectacular difference is observed between isotactic polypro- pylene, which has a crystal melting point of 176C, and the atactic polymer, which is a rubbery amorphous material. Isotactic polypropylene is widely used in fiber, cordage, and automotive and appliance applications and is one of the world’s major plastics. Atactic polypropylene is used mainly to improve the low- temperature properties of asphalt.
Isotactic and syndiotactic polymers will not have the same mechanical proper- ties, because the different configurations affect the crystal structures of the poly- mers. Most highly stereoregular polymers of current importance are isotactic.
[There are a few exceptions to the general rule that atactic polymers do not crys- tallize. Poly(vinyl alcohol) (1-8) and poly(vinyl fluoride) are examples. Some mono- mers with identical 1,1-substituents like ethylene, vinylidene fluoride, and vinylidene chloride crystallize quite readily, and others like polyisobutene do not. The concepts of configurational isomerism do not apply in these cases for reasons given above.]
Stereoregularity has relatively little effect on the mechanical properties of amorphous vinyl polymers in which the chiral carbons are trisubstituted. Some dif- ferences are noted, however, with polymers in which X andY in 1-67 differ and neither is hydrogen. Poly(methyl methacrylate) (Fig. 1.4) is an example of the lat- ter polymer type. The atactic form, which is the commercially available product, remains rigid at higher temperatures than the amorphous isotactic polymer.
Completely tactic and completely atactic polymers represent extremes of ste- reoisomerism which are rarely encountered in practice. Many polymers exhibit intermediate degrees of tacticity, and their characterization requires specification of the overall type and extent of stereoregularity as well as the lengths of the tac- tic chain sections. The most powerful method for analyzing the stereochemical nature of polymers employs nuclear magnetic resonance (NMR) spectroscopy for which reference should be made to a specialized text [7]. Readers who delve into the NMR literature will be aided by the following brief summary of some of the terminology that is used [8]. It is useful to refer to sequences of two, three, four, or five monomer residues along a polymer chain as a dyad, triad, tetrad, or pen- tad, respectively. A dyad is said to be racemic (r) if the two neighboring mono- mer units have opposite configurations and meso if the configurations are the same. To illustrate, consider a methylene group in a vinyl polymer. In an isotactic molecule the methylene lies in a plane of symmetry. This is a meso structure.
1-70 m R H R
C H
In a syndiotactic region, the methylene group is in a racemic structure
r R H C H R 1-71
In a triad, the focus is on the central methine between two neighboring mono- mer residues. An isotactic triad (mm) is produced by two successive meso placements:
1-72
m m
R R R
H C
A syndiotactic triad (rr) results from two successive racemic additions:
1-73
r r
C R
H R
R
Similarly, an atactic triad is produced by opposite monomer placements, i.e., (mr) or (rm). The two atactic triads are indistinguishable in an NMR analysis.
The dyads in commercial poly(vinyl chloride) (PVC) are about 0.55% race- mic, indicating short runs of syndiotactic monomer placements. The absence of a completely atactic configuration is reflected in the low levels of crystallinity in this polymer, which have a particular influence on the processes used to shape it into useful articles.