Optical activity in polymers

Optical Activity in Polymers Stereoisomerism in polymers is formally similar to the optical isomerism of organic chemistry. In a vinyl polymer with the general structure shown in (XIH) every other carbon atom in the chain, labeled C, is a site of steric isomerism, because it has four different substituents, namely, X, Y, and two sections of the main chain that differ in length (Rudin, 1982). 

Problem 2.10 Explain why polypropylene of relatively high molecular weight is optically inactive despite having an asymmetric center at every other carbon, while, on the other hand, polyfpropylene oxide) is optically active. 

Answer Optical activity is influenced only by the first few atoms around an asymmetric carbon (C ). For the two sections of the main chain, these will be identical regardless of the length of the whole polymer chain. The carbons marked C in (Xlll) are thus not truly asymmetric and are termed pseudoasymmetric or pseudochiral carbons. Only those C centers near the ends of a polymer molecule will be truly asymmetric, but since there are too few chain ends in a high molecular-weight polymer such centers do not confer any significant optical activity on the molecule as a whole. Polypropylene is thus optically inactive. 

In poly(propylene oxide), —t-CH2C (H)(CH3)0-3-, the C is true asymmetric center, as it is surrounded by -H, -CH3, -CH2, and -0-. The polymer is therefore optically active. 

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (the role of the end groups is generally negligible). Polymers can be 

An example of an asymmetric induction from optically inactive monomers in an anionic polymerization of esters of butadiene carboxylic acids with (/ )-2-methylbutyllithium or with butyllithium complexed with (-)-menthyl ethyl ether as the catalyst. The products, tritactic polymers, exhibit small, but measurable, optical rotations. Also, when benzofiiran, that exhibits no optical activity, is polymerized by cationic catalysts like aluminum chloride complexed with an optically active cocatalyst, like phenylalanine, an optically active polymer is obtained.  

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents on the originally inducing center on the center portion must differ considerably in size. (3) The location 

Optically active polymers are rarely encountered. Most syndiotactic polymers are optically inactive since they are achiral. Most isotactic polymers, such as polypropene and poly(methyl methacrylate), are also inactive (Sec. 8-la-l). Optically active polymers have been obtained in some situations and these are discussed below. 

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Arcus, C. L. Stereoisomerism of Addition Polymers. Part I. The Stereochemistry of Addition and Configurations of Maximum Order. J. chem. Soc. [London] 1955, 2801. The Stereoisomerism of Addition Polymers. Part II. Configurations of Maximum Order from Altering Copolymerisation. The Requirements for Optical Activity in Polymers. J. chem. Soc. [London] 1957, 1189. 

Some progress has also been achieved in the use of chiral polymer films at electrodes. Conductive polythiophenes containing optically active substituents in the 3-positions were prepared by electropolymerization of suitable monomers without apparent lc s of optical activity The polymer of exhibits distinct... 

Additionally, copolymers of 30-37 containing 20% enantiopure chiral silane units (see Chart 4.6) are optically active helical polymers which obey the sergeants-and-soldiers principle, as shown in Figure 4.18.29g Interestingly, from the observation of the CD sign in the phenyl region, the arrangement of phenyl... 

The polymerization of monosubstituted vinyl compounds that give polymers like PS and PP produces polymer chains that possess chiral sites on every other carbon in the polymer backbone. Thus, the number of possible arrangements within a polymer chain is staggering since the number of possible isomers is 2" where n is the number of chiral sites. For a relatively short chain containing 50 propylene units the number of isomers is about 1 x lO. While the presence of such sites in smaller molecules can be the cause of optical activity, these polymers are not optically active since the combined interactions with light are negated by other similar, but not identical, sites contained on that particular and other polymer chains. Further, it is quite possible that no two polymer chains produced during a polymerization will be exactly identical because of chiral differences. 

Isoselective polymerization of one enantiomer or the other of a pair of enantiomers results in an optically active polymer [Ciardelli, 1987 Delfini et al., 1985 Pino et al., 1963]. For example, polymerization of (5)-3-methyl-l-pentene yields the all-.S polymer. The optical activity of the polymer would be maximum for the 100% isotactic polymer. Each racemic placement of the S-monomer decreases the observed optical activity in the polymer. 

Farina, M. and G. Ressan, Optically Active Stereoregular Polymers, Chap. 4 in The Stereochemistry of Macromolecules, Vol. 3, A. D. Ketley, ed., Marcel Dekker, New York, 1968. 

Recently, we found that the polymerization of TrMA with chiral anionic catalysts gave an optically active polymer the chirality of which is caused by helicity (12). This is the first example of optically active vinyl polymer the activity of which arises only from the helicity. This article describes the detailed results of the polymerization of TrMA by chiral anionic catalysts in addition to a brief review on our earlier studies described above. 

Several attempts were made to prepare optically active vinyl polymers, - CH2-CXY-)-g, which have chirality in the polymer chain (17-23). However, all the polymers obtained showed no or only small rotation which arose from the chiral terminal groups. It has been pointed out that even if an isotactic polymer composed of units of a single configuration is formed in the process of the polymerization, the polymer can not be optically active because the polymer becomes pseudo asymmetric (24, 25). 

Polymers, Theoretical Aspects of Optical Activity [in] (Tinoco). 4 113... 

In the next year, Price and his coworkers (6,7) found that the crystalline polymer obtained by Pruitt and Baggett was isotactie. The fact that the crystalline polymer obtained from racemic monomer with the iron catalyst had the same X-ray pattern as the optically active crystalline polymer obtained from the optically active monomer under the same condition showed that these polymers were isotactic, and that the asymmetric carbon atoms in this polymer had the same configuration as in the monomer from which it was derived, i.e., propylene oxide polymerized with retention of configuration of its asymmetric carbon atom. 

Certain chiral organic compounds create crystalline environments and act as enantio-controlling media (7) even though they do not function as true catalysts. Natta s asymmetric reaction of prochiral trans-1,3-pentadiene, which was included in the crystal lattice of chiral perhydro-triphenylene as a host compound, to form an optically active, isotactic polymer on 7-ray irradiation, is a classic example of such a chiral molecular lattice (Scheme 1) (2). Weak van der Waals forces cause a geometric arrangement of the diene monomer that favors one of the possible enantiomeric sequences. 

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