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.
occur because of rotations about single bonds. Molecules with different conforma- tions are called conformational isomers, rotamers, or conformers.
Macromolecules in solution, melt, or amorphous solid states do not have regular conformations, except for certain very rigid polymers described inSection 4.6and certain polyolefin melts mentioned in Section 1.14.2. The rate and ease of change of conformation in amorphous zones are important in determining solution and melt viscosities, mechanical properties, rates of crystallization, and the effect of tempera- ture on mechanical properties.
Polymers in crystalline regions have preferred conformations which represent the lowest free-energy balance resulting from the interplay of intramolecular and intermolecular space requirements. The configuration of a macromolecular spe- cies affects the intramolecular steric requirements. A regular configuration is required if the polymer is to crystallize at all, and the nature of the configuration determines the lowest energy conformation and hence the structure of the crystal unit cell.
Considerations of minimum overlap of radii of nonbonded substituents on the polymer chain are useful in understanding the preferred conformations of macro- molecules in crystallites. The simplest example for our purposes is the polyethyl- ene (1-3) chain in which the energy barriers to rotation can be expected to be similar to those inn-butane.Figure 1.6shows sawhorse projections of the confor- mational isomers of two adjacent carbon atoms in the polyethylene chain and the corresponding rotational energy barriers (not to scale). The angle of rotation is
Δε skew −
0 60 120 180 Rotational angle (°)
trans trans
240 300 cis skew +
gauche + gauche −
Energy
ΔE
FIGURE 1.6
Torsional potentials about adjacent carbon atoms in the polyethylene chain. The white circles represent H atoms and the black circles represent segments of the polymer chain.
that between the polymer chain substituents and is taken here to be zero when the two chain segments are as far as possible from each other.
When the two chain segments would be visually one behind the other if viewed along the polymer backbone, the conformations are said to beeclipsed.
The other extreme conformations shown are ones in which the chain substitu- ents are staggered. The latter are lower energy conformations than eclipsed forms because the substituents on adjacent main chain carbons are further removed from each other. The lowest energy form in polyethylene is the staggeredtransconfor- mation. This corresponds to the planar zigzag form shown in another projection in Fig. 1.6. It is also called an all-trans conformation. This is the shape of the macromolecule in crystalline regions of polyethylene.
The conformation of a polymer in its crystals will generally be that with the lowest energy consistent with a regular placement of structural units in the unit cell. It can be predicted from a knowledge of the polymer configuration and the van der Waals radii of the chain substituents. (These radii are deduced from the distances observed between different molecules in crystal lattices.) Thus, the radius of fluorine atoms is slightly greater than that of hydrogen, and the all-trans crystal conformation of polyethylene is too crowded for poly(tetrafluoroethylene) which crystallizes instead in a very extended 131helix form. Helices are character- ized by a number of fj, where f is the number of monomer units per j complete turns of the helix. Polyethylene could be characterized as a 11helix in its unit cell.
Helical conformations occur frequently in macromolecular crystals. Isotactic polypropylene crystallizes as a 31helix because the bulky methyl substituents on every second carbon atom in the polymer backbone force the molecule from a trans/trans/trans . . . conformation into atrans/gauche/trans/gauche . . . sequence with angles of rotation of 0 (trans) followed by a 120(gauche) twist.
In syndiotactic polymers, the substituents are farther apart because the config- urations of successive chiral carbons alternate (cf.Fig. 1.5). Thetrans/trans/trans
. . . planar zigzag conformation is generally the lowest energy form and is
observed in crystals of syndiotactic 1,2-poly(butadiene) and poly(vinyl chloride).
Syndiotactic polypropylene can also crystallize in this conformation but a trans/
trans/gauche/gauche. . .sequence is slightly favored energetically.
Polyamides are an important example of polymers that do not contain pseudo- asymmetric atoms in their main chains. The chain conformation and crystal struc- ture of such polymers is influenced by the hydrogen bonds between the carbonyls and NH groups of neighboring chains. Polyamides crystallize in the form of sheets, with the macromolecules themselves packed in planar zigzag conformations.
The difference between the energy minima in thetrans andgauche staggered conformations is labeledΔeinFig. 1.6. When this energy is less than the thermal energyRT/Lprovided by collisions of segments, none of the three possible stag- gered forms will be preferred. If this occurs, the overall conformation of an iso- lated macromolecule will be a random coil. When Δe.RT/L, there will be a preference for thetransstate. We have seen that this is the only form in the poly- ethylene crystallite.
45 1.13 Polymer Conformation
The time required for the transition between trans and gauche states will depend on the height of the energy barrierΔEinFig. 1.6. IfΔE,RT/L, the bar- rier height is not significant and trans/gauche isomerizations will take place in times of the order of 10211sec. When a macromolecule with small ΔE is stretched into an extended form, the majority of successive carboncarbon links will be trans, butgauche conformations will be formed rapidly when the mole- cule is permitted to relax again. As a result, the overall molecular shape will change rapidly from an extended form to a coiled, ball shape. This is the basis of the ideal elastic behavior outlined in more detail in Section 4.5. Note that a stretched polymer molecule will recoil rapidly to a random coil shape only if (1) there is no strong preference for any staggered conformation over another (Δeis small; there is little difference between the energy minima) and (2) if the rotation about carboncarbon bonds in the main chain is rapid (ΔE is small; the energy barriers between staggered forms are small). If condition (1) holds but (2) does not, the polymer sample will respond sluggishly when the force holding it in an extended conformation is removed.
The trans staggered conformation is a lower energy form than either of the gauche staggered forms of polyethylene. The difference is much less for polyiso- butene, however, as illustrated in Fig. 1.7. Here the chain substituent on the rear carbon shown is either between a methyl and polymer chain or between two methyl groups on the other chain carbon atom. Since no conformation is favored, this polymer tends to assume a random coil conformation. The polymer is elasto- meric and can be caused to crystallize only by stretching. However, rotations between staggered conformations require sufficient energy for the chain to over- come the high barrier represented by crowded eclipsed forms, and polyisobutene does not retain its elastic character at temperatures as low as those at which more resilient rubbers can be used.
CH2
CH2 CH2 CH2
CH2 CH3 CH2 H
CH3
H H H H
H
gauche − trans gauche +
CH3
H3C H3C H3C
FIGURE 1.7
Newman projections of staggered conformations of adjacent carbons in the main chain of polyisobutene.
The preference for trans conformations in hydrocarbon polymer chains may be affected by the polymer constitution. Gauche conformations become more energetically attractive when atoms with lone electron pairs (like O) are present in the polymer backbone, and polyformaldehyde (or polyoxymethylene), 1-12, crystallizes in the all-gaucheform.