Polymer Chemistry Sem-6, Dse-B3 Part-12, Ppt-12

POLYMER CHEMISTRY

SEM-6, DSE-B3 PART-12, PPT-12

Dr. Kalyan Kumar Mandal Associate Professor St. Paul’s C. M. College Kolkata Polymer Chemistry Part-12

Contents

• Isomerism in Polymers • Sequence Isomerism • Constitutional Isomerism • Stereoisomerism Isomerism in Polymers • Two molecules are said to be isomers if they are made up of the same number and types of atoms, but differ in the arrangement of these atoms. Though a polymer is synthesized from a pure monomer where all the molecules are initially identical, it does not mean that the final product consists of chains of regularly arranged units. However branching can occur in a polymer such as polyethylene and it turns out that from an asymmetric monomer a variety of microstructures is obtained consisting of various geometric- and stereo-isomers.

• It is a general rule that for a polymer to crystallize, it must have highly regular polymer chains. Highly irregular polymers are almost inevitably amorphous. Polymer chains can have isomeric forms that decrease the regularity of the chains. There are three important forms of isomerism in polymers. The most important types are sequence isomerism, structural isomerism and stereoisomerism. Sequence Isomerism (Head-to-Tail or Head-to-Head) • In chain polymerization, monomers with pendant groups can attach in two ways. The usual arrangement is head-to-tail with the pendant groups on every other carbon atom in the chain.

Except in monomers like ethylene (H2C=CH2) and tetrafluoroethylene (F2C=CF2) where the substituents on the two carbons are identical, the two carbons of the double bond in a vinyl

(H2C=CH-X, where X is an atom/group other than hydrogen) type monomer are distinguishable. One of them can be arbitrarily labeled the head and the other the tail of the monomer, as shown for vinyl chloride (A; Figure 1). Sequence Isomerism (Head-to-Tail or Head-to-Head) • When a monomer unit adds to a growing chain it usually does so in a preferred direction. Polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) are only a few examples of common polymers where addition is almost exclusively head-to-tail (the arrangement is shown in Figure 2). • However, for several specific polymers there are significant numbers of structural units that are incorporated “backwards” into the chain leading to head-to-head and tail-to-tail placements. This is referred to as sequence isomerism. Classic examples include poly(vinyl fluoride), poly(vinylidene fluoride), polyisoprene, polychloroprene (the arrangement is shown in Figure 3). Problems on Isomerism • Problem 1: Explain the fact that in all polymerizations the head-to-tail addition is usually the predominant mode of propagation.

• Answer: Vinyl monomers polymerize by attack of an active centre (A; Figure 4) on the double bond. Equation I (Figure 4) shows the propagation step in head-to-tail enchainment and Equation 2 that in head-to-head or tail-to-tail enchainment. Problems on Isomerism • The active centre involved in the propagation reaction may be a free-radical, ion, or metal- carbon bond. A propagating species will be more stable if the unpaired electron or ionic charge at the end of the chain can be delocalized across either or both substituents X and Y. Such resonance stabilization is possible in species (B) but not in (C) shown in Figure 4.

• Moreover when X and/or Y is bulky there will be more steric hindrance in reaction of Equation 1 than in the reaction of Equation 2. So, in general, head-to-tail addition as in Equation 1 is considered to be the predominant mode of propagation in all polymerizations.

• Answer the following questions: 1. Polyethylenes are often characterized as being of high density (HDPE) or low density (LDPE). What causes the difference in density between these polyethylenes? 2. What is “linear low density” polyethylene (LLDPE) and how does it get its name? 3. Different polypropylenes may be described as atactic or isotactic. What does this mean ?

4. What configurational isomers are possible following the polymerization of butadiene, CH2=CH-CH=CH2 ? Problems on Isomerism • Problem 2: A chemical method of determining head-to-head structures in poly(vinyl alcohol) is by means of the following difference in diol reactions:

• Poly(vinyl acetate) of number-average molecular weight 250,000 is hydrolyzed by base- catalyzed transesterification with methanol to yield poly(vinyl alcohol). Oxidation of the latter with periodic acid yields the resulting poly(vinyl alcohol) with number-average degree of polymerization 485. Calculate the percentages of head-to-tail and head-to-head linkages in poly(vinyl acetate). Problems on Isomerism

Answer: Repeat unit of poly(vinyl acetate): –[–CH2–CH(OCOCH3)–]– Molar mass of repeat unit = 86 g mol-1

-1 -1 푋n= 250,000 g mol /86 g mol = 2907

let us assume that poly(vinyl acetate) is completely hydrolyzed, so that 푋n of the resulting poly (vinyl alcohol) is also 2907. The fact that 푋n is reduced from 2907 to 485 upon treatment with periodic acid means that each poly(vinyl alcohol) molecule is, on the average, cleaved to yield

2907/485 or 6 smaller molecules each of 푋n = 485 and so the number of cleavages 6 - 1 = 5. Therefore, the average polymer molecule has 5 head-to-head linkages out of a total of 2907 - 1 = 2906 linkages in the polymer chain. % head-to-head = (5 x 100) / 2906 = 0.172 % % head-to-tail = 100 - 0.172 = 99.828 %. Constitutional (or Structural) Isomerism • The type of isomerism, which involves constitutional variations of a molecule is referred to as constitutional isomerism. Polymers of dienes, such as buta-1,3-diene, isoprene and chloroprene, etc., have the potential for head-to-tail and head-to-head (or tail-to-tail) isomerism and variations in double bond position as well. Many constitutional isomers can thus be resulted from the polymerization of these dienes.

• The conjugated diene, e.g., buta-1,3-diene can polymerize to produce 1,4 and 1,2 polymers. The so-called 1,4 polymers are formed by linking carbon atom 4 of one monomer to carbon atom 1 of the next, and so on. The arrangement of the substituent carbon atoms relative to the double bond in 1,4 polymers can be in either the cis or trans configuration. The 1,2 or 3,4 polymers are formed when incorporation into the polymer chain occurs through either the first or second double bond, respectively. Constitutional (or Structural) Isomerism • There are three possible constitutional isomers viz., 1,2 polymer (head-to-tail linkage), 1,2- polymer (head-to-head or tail-to-tail linkage) and 1,4 polymer from buta-1,3-diene. In addition, there is the possibility of mixed structures. The constitutional isomers are shown in Figure 5.

• Isoprene may be polymerized to polyisoprene by a variety of techniques including free radical, ionic and Ziegler-Natta catalysis. Mother Nature also synthesizes “natural” polyisoprenes, Heavea (Natural Rubber) and Gutta Percha, but uses trees instead of round bottom flasks as polymerization vessels. Constitutional (or Structural) Isomerism

• The number of constitutional isomers with 2-substituted buta- 1,3-dienes like isoprene and chloroprene are different from that of butadiene. In this case, there are six possible constitutional isomers of isoprene or chloroprene as shown in Figure 6.

• Elastomeric behavior is shown by 1,4-polymer, particularly if the polymer structure is cis about the residual double bond. The corresponding trans isomer is much more rigid. Stereoisomerism in Polymer • When a chiral centre is present in a polymer molecule, different configurations or optical isomers are possible. Stereochemistry can have an important effect on chain packing. Isotactic polypropylene (PP), for instance, is highly crystalline because the regular chains can pack closely together. Isotactic PP has a melting point of 160 oC. Atactic PP, on the other hand is a soft noncrystalline polymer with a melting point of only 75 oC.

• Polymerization of a vinyl monomer, H2C=CHX, X may be a halogen, alkyl or other chemical moiety except hydrogen, leads to polymers with microstructures that are described in terms of tacticity. The substituent placed on every other carbon atom has two possible arrangements relative to the chain and the next X group along the chain. At one extreme, the monomer unit, which contains an asymmetric carbon atom, may be incorporated into the polymer chain in a manner such that each X group is meso to the preceding X group (i.e., on the same side of the extended chain; Figure 7A). This polymer is described as isotactic. Stereoisomerism in Polymer • Conversely, at the other extreme, addition of a monomer unit to the growing polymer chain may be directed to yield a polymer in which each successive X group is racemic to the preceding one (i.e., on the opposite side of the stretched out chain; Figure 7B). This is called a syndiotactic polymer.

• Polypropylene (X=CH3), for example, may be synthesized to yield an essentially isotactic or syndiotactic polypropylene by varying the polymerization conditions. These structures are illustrated in Figure 8 for an extended polypropylene chain. Because of steric repulsions the isotactic chain does not like this zig-zag shape, but would prefer to fold into a helix.

• Between these two extremes there is an enormous number of polymer chain microstructures based on different distributions of meso and racemic placements of structural repeating units. Stereoisomerism in Polymer • If there is no preferred direction to the addition of a monomer to the growing chain, i.e., the monomers are incorporated randomly with respect to the stereochemistry of the preceding unit, an atactic polymer is formed. Geometrical Isomerism • Since rotation cannot take place between two carbon atoms about a double bond, two nonsuperimposable configurations (geometrical isomers) are possible in polymeric chain if the two substituents on each carbon of a double bond differ from each other. • In solid, the molecules of trans isomers pack more closely and crystallize more readily than those of cis isomers. Hence, the differences in the properties of cis and trans isomers of polymers are also significant. • Natural rubber is 1,4-polyisoprene and the polymer configuration is cis at each double bond in the chain, as shown in (A; Figure 9). Consequently, the polymer molecule has a bent and less symmetrical structure. Natural rubber does not crystallize at room temperature and is amorphous and elastomeric. • Balata (gutta-percha; A tree) is also 1,4-polyisoprene, but the polymer configuration is trans at the double bond (B). The molecule is more extended and has symmetrical structure. The trans isomer is thus a nonelastic, hard, and crystalline polymer. It is used as a thermoplastic. Optical Activity in Polymers • In a vinyl polymer with the general structure shown in (A; Figure 10) every other carbon atom in the chain, labeled C*, is a site of optical isomerism, because it has four different substituents, namely, X, Y, and two sections of the main chain that differ in length. • There are two distinct configurational arrangements of the repeat unit of (A), viz., (B) and (C), where the solid and the dotted lines denote bonds which are extending above and below the plane of the paper, respectively. • These two stereoisomers of the repeat unit cannot be interchanged by bond rotation and they exist because the substituted carbon atom, labeled C*, is attached to four different groups. Thus, every C* may have one or other of the two configurations. One of the configurations is designated as D (or d) and the other as L (or l). The configuration is fixed when the polymer molecule is formed and is independent of any rotations of the main chain carbons about the single bonds connecting them. Problem on Isomerism • Problem 3: Explain why polypropylene of relatively high molecular weight is optically inactive despite having an asymmetric centre at every other carbon, while, on the other hand, poly(propylene oxide) is optically active. • Answer: Optical activity of polypropene is influenced only by the first few asymmetric carbon (C*) in the chain. For the two sections of the main chain, there is similarity regardless of the length of the whole polymer chain. The carbons marked C* in A (Figure 10) are thus not truly asymmetric, though the syndiotactic polypropylene is optically active. Only the C* centres 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 centres do not confer any significant optical activity on the molecule as a whole. This type of chirality is termed as cryptochirality (a special case of chirality in which a molecule is chiral but its specific rotation is non-measurable). Polypropylene is, thus, considered as optically inactive. * * • In poly(propylene oxide), -[-CH2C (H)(CH3)O-]n-, the C is an asymmetric centre, as it is surrounded by H, CH3, CH2-, and O-. The polymer is therefore optically active.