Chapter 3

Molecular Structure of Polymers

This chapter explores the types of bonds formed in polymers, and focuses on how weak bonds can impact material properties. The arrangement of these bonds and side groups along the polymer backbone also lead to a wide range of possible isomers (same chemical formula, different arrangement of atoms) that are also important in determining polymer properties.

3.1 Types of Bonds

Various types of bonds hold together the atoms in polymeric materials, unlike in metals, for example, where only one type of bond (metallic) exists. These types are: (1) primary covalent, (2) hydrogen bond, (3) dipole interaction, (4) van der Waals, and (5) ionic. Examples of each are shown in Figure 3.1. Hydrogen bonds, dipole interactions, van der Waals bonds, and ionic bonds are known collectively as secondary (or weak) bonds. The distinctions are not always clear-cut, that is, hydrogen bonds may be considered as the extreme of dipole interactions. The secondary bonds are generally weaker bonds and are responsible for many of the bonds between different polymer chains (intermolecular bonds).

Figure 3.1 Bonding in polymer systems.

3.2 Bond Distances and Strengths

Regardless of the type of bond, the potential energy of the interacting atoms as a function of the separation between them is represented qualitatively by the potential function sketched in Figure 3.2. As the interacting centers are brought together from large separation, an increasingly great attraction tends to draw them together (negative potential energy). Beyond the separation r_m, as the atoms are brought closer together, their electronic “atmospheres” begin to interact and a powerful repulsion is set up. At r_m, the system is at a minimum potential energy, its most probable or equilibrium separation, r_m being the equilibrium bond distance. The “depth” of the potential well ε is the energy required to break the bond, separating the atoms completely.

Figure 3.2 Interatomic potential energy and force.

Table 3.1 lists the approximate bond strengths and interatomic distances of the bonds encountered in polymeric materials. The important fact to notice here is how much stronger the primary covalent bonds are than the others. As the material's temperature is raised and its thermal energy (kT)¹ is thereby increased, the primary covalent bonds will be the last to dissociate when the available thermal energy exceeds their dissociation energy.

Table 3.1 Bond Parameters 1, 2.

Bond Type	Interatomic Distance, r_m, nm	Dissociation Energy, ε, kcal/mol
Primary covalent	0.1–0.2	50–200
Hydrogen bond	0.2–0.3	3–7
Dipole interaction	0.2–0.3	1.5–3
van der Waals bond	0.3–0.5	0.5–2
Ionic bond	0.2–0.3	10–20

3.3 Bonding and Response to Temperature

In linear and branched polymers, only the secondary bonds hold the individual polymer chains together (neglecting temporary mechanical entanglements). Thus, as the temperature is raised, a point will be reached where the forces (the weak bonds of types 2–5) holding the chains together become insignificant, and the chains are then free to slide past one another, that is, to flow upon the application of stress. Therefore, linear and branched polymers are generally thermoplastic. The crosslinks in a network polymer, on the other hand, are held together by the same primary covalent bonds as are the main chains. When the thermal energy exceeds the dissociation energy of the primary covalent bonds, both main-chain and crosslink bonds randomly fail, and the polymer degrades. Hence, crosslinked polymers are thermosetting.

There are some exceptions to these generalizations. It is occasionally possible for secondary bonds to make up for in quantity what they lack in the quality (or strength) of a single bond. For example, polyacrylonitrile is capable of strong dipole interactions between the pendent nitrile groups on every other carbon atom along the polymer backbone. If these secondary bonds could be broken one by one (i.e., “unzippped”), polyacrylonitrile would behave as a typical thermoplastic. This is impossible, of course, due to the random nature of polymer configurations. By the time enough of the secondary bonds have been dissociated to free the chains and allow flow, the dissociation energy of some carbon–carbon backbone covalent bonds will have been exceeded and the materials will degrade. Extreme stiffness of the polymer chain also contributes to this sort of behavior. Cellulose has a bulky, complex repeat unit that contains three hydroxyl groups. Though linear, its chains are therefore stiff and strongly hydrogen bonded, and, thus, it is not thermoplastic. If the hydroxyls are reacted with acids, such as nitric, acetic, or butyric, the resulting derivative of cellulose (a cellulose ester) behaves as a typical thermoplastic largely because of the reduced hydrogen bonding:

Here, three –OH groups in the basic repeat unit for cellulose, glucose or C₆H₁₂O₆, are converted to acetate esters. Since cellulose is already a polymer, the acetate esters simply modify side groups, making the structure of these cellulose derivatives more “workable.” They are commonly used in pharmaceutical coatings and a number of foodstuffs, even being a component in some soft-serve ice creams.

Polytetrafluoroethylene (Teflon, TFE) with the repeat unit –(CF₂–CF₂)– is another example of a thermosetting material, as the close packing and extensive secondary bonding of the main chains prevents flow when the polymer is heated.

3.4 Action of Solvents

The action of solvents on polymers is in many ways similar to that of heat. Appropriate solvents, that is, those that can form strong secondary bonds with the polymer chains, can penetrate, replace the interchain secondary bonds, and thereby pull apart and dissolve linear and branched polymers. The polymer–solvent secondary bonds cannot overcome primary covalent crosslinks, however, so crosslinked polymers are not soluble, although they may swell considerably. (Try soaking a rubber band in toluene overnight or take apart a diaper and add your favorite beverage.) The amount of swelling is, in fact, a convenient measure of the extent of crosslinking. A lightly crosslinked polymer, such as the rubber band or superabsorbent polymers in diapers, will swell tremendously, while one with extensive crosslinking, for example, an ebonite (“hard rubber”) bowling ball, will not swell noticeably at all.

3.5 Bonding and Molecular Structure

It is obvious that the chemical nature of a polymer is of considerable importance in determining the polymer's properties. Of comparable significance is the way the atoms are arranged geometrically within the individual polymer chains.

A look into protein structure illustrates the different types of bonds and their effects on the three-dimensional organization of a polymer. The bonds that are found in proteins (polypeptides) are the same as those listed here for polymers, yet weak bonds play a very important role in determining the three-dimensional organization and biological activity of proteins. These bonds are divided into interactions at four levels (and demonstrated for a protein in Figure 3.3).

Figure 3.3 Demonstration of the different levels of bonding and structural arrangement in polymers (here, for a polypeptide). The primary structure links amino acids (monomers) and starts to form shapes at the secondary level through weak bonds (such as the hydrogen bonds and cysteine disulfide bonds visible in the alpha helix). Higher levels of structure are quite regular in proteins, allowing for complex shapes that are unique for each protein. For synthetic polymers, the shapes are much more random.

1. Primary Bonds: These are covalent bonds that link monomers and make up the backbone of the polymer. This is also termed the one-dimensional structure, as it can be represented by laying out repeat units in a straight line (which is handy for demonstrations in a book, but hardly representative of the complex three-dimensional structure of polymers, as depicted as a bowl of spaghetti).

2. Secondary Bonds: These are weak bonds between near-neighbor repeat units, covering a range of perhaps 10–20 repeat units. Hydrogen bonds are common here and cause an impact on the formation of coils or zig-zag structures.

3. Tertiary Bonds: These are weak intramolecular bonds (those between a polymer and itself) between repeat units that are widely separated in the one-dimensional structure (i.e., perhaps an alcohol in the side group of the third repeat unit hydrogen bonds to a carboxylic acid in the 8000th repeat unit). These bonds hold the three-dimensional structure of an individual polymer in place.

4. Quaternary Bonds: These are weak intermolecular bonds (those between two different polymers). They stabilize the three-dimensional structure of a polymeric material, linking the different chains without forming covalent bonds (which would cause a network to form). This breakdown of bonding in proteins is essentially the same for all polymers, although commercial homopolymers do not display the same structural complexity of proteins.

The carbon atom is normally (exclusively, for our purposes) tetravalent. In compounds such as methane (CH₄) and carbon tetrachloride (CCl₄), the four identical substituents surround the carbon in a symmetrical tetrahedral geometry. If the substituent atoms are not identical, the symmetry is destroyed, but the general tetrahedral pattern is maintained. This is still true for each carbon atom in the interior of a linear polymer chain, where two of the substituents are the extensions of the polymer backbone. If a polyethylene chain (in normal spaghetti-type coil) were to be stretched out, for example, the carbon atoms in the chain backbone would lie in a zigzag fashion in a plane, with the hydrogen substituents on either side of the plane (Figure 3.4). In the case of polyethylene, in which all the substituents are the same, this is the only arrangement possible. With vinyl polymers, however, there are several possible ways of arranging the side groups.

Figure 3.4 The geometry of a polyethylene chain. The carbon backbone is shown in black, with C–C–C bond angle of 109.5°, and the gray hydrogen atoms alternating positions along the backbone to minimize repulsive forces between consecutive side groups.

3.6 Stereoisomerism in Vinyl Polymers

Before beginning a discussion of the isomerism in vinyl polymers, it must be pointed out that the monomers (H₂C=CHX, where X can be Cl, –OH, an organic group, etc.) polymerize almost exclusively in a head-to-tail fashion, placing the X groups on every other carbon atom along the chain. Although head-to-head (or tail-to-tail) connections are possible, steric hindrances between successive X groups (particularly if the X group is bulky) and electrostatic repulsion between groups with similar polarities generally keeps most of the linkages as head-to-tail. Ignoring head-to-head connections, there are then three possible ways in which the X side group may be arranged with respect to the carbon backbone plane (see Figure 3.5). These arrangements represent three types of stereoisomers:

1. Atactic: A random arrangement of the X groups. Its lack of regularity has important consequences.

2. Isotactic: The structure in which all the X groups are lined up on the same side of the backbone plane.

3. Syndiotactic: Alternating placement of the X group on either side of the plane.

These three terms were coined by Dr. Giulio Natta, who shared the 1964 Nobel Chemistry Prize for his work in this area. Although atactic polymers are certainly most common, the arrangement and packing (and thus the properties) of stereoregular (syndiotactic and isotactic) polymers makes them important for certain applications. Methods to synthesize the stereoregular isomers will be discussed in Part II of this book.

Figure 3.5 Stereoisomers of polystyrene showing isotactic, syndiotactic, and atactic structures.

Although useful for descriptive purposes, the planar zigzag arrangement of the main-chain carbon atoms is not always the one preferred by nature, that is, it is not necessarily the minimum free-energy configuration. In the case of polyethylene, it is, but for isotactic and syndiotactic polypropylene (which has a pendent methyl, –CH₃, group), the preferred (minimum-energy) configurations are quite regular, with the backbone forming helical twists to maximize the distance between consecutive –CH₃ groups. The atactic polymer, however, lacks regular twists and has an irregular shape.

Atactic polypropylene has a consistency somewhat like used chewing gum, whereas the stereoregular forms are hard, rigid plastics. The reason why this regularity (or lack of it) has a profound effect on mechanical properties is discussed in the next chapter.

The type of stereoregularity described above is a direct result of the dissymmetry of vinyl monomers. It is established in the polymerization reaction and no amount of twisting and turning the chain about its bonds can convert the three-dimensional geometry of one stereoisomer into another (molecular models are a real help, here).

The situation is even more complex for monomers of the form HXC=CHX′, where X and X′ are different substituent groups. This is discussed by Natta 3, but is currently of no commercial importance.

Example 3.1 Both isotactic and atactic polymers of propylene oxide

have been prepared by ring scission polymerization.

a. Write the general structural formula of the polymer.

b. Indicate how the atactic, isotactic, and syndiotactic structures differ.

Solution.

3.7 Stereoisomerism in Diene Polymers

Another type of stereoisomerism arises in the case of poly-1,4-dienes because carbon–carbon double bonds are rigid and do not allow rotation. The substituent groups on the double-bonded carbons may be either on the same side of the chain (cis) or on the opposite sides (trans), as shown in Figure 3.6 for 1,4 polyisoprene.

Figure 3.6 cis and trans isomers result from polymerization of isoprene.

The chains of cis-1,4-polyisoprene assume a tortured, irregular configuration because of the steric interference of the substituents adjacent to the double bonds. This stereoisomer is familiar as natural rubber (made by the rubber tree) and is used in rubber bands. trans-1,4-polyisoprene chains assume a regular structure. This polymer is known as gutta-percha, a tough but not elastic material, long used as a golf-ball cover.

Note that stereoisomerism in poly-1,4-dienes does not depend on the dissymmetry of the repeating unit. Atactic, syndiotactic, and isotactic isomers are possible with butadiene, even though all of the carbon substituents are hydrogen, as shown in Example 3.2.

Example 3.2 Identify all possible structural and stereoisomers that can result from the addition polymerization of butadiene:

Solution. Recall from Chapter 2 that butadiene can undergo 1,4-addition or 1,2-addition (3,4-addition results in the same structure as 1,2-addition). The 1,4-polymer can have cis and trans isomers like the 1,4-polyisoprene in Figure 3.6. The 1,2-polymer can have atactic, isotactic, and syndiotactic stereoisomers like any vinyl polymer. The 1,4-polymer results in a different structure, but cannot have stereoisomers because all of the carbons are part of the backbone of the polymer and both the substituents remaining on the double bond in the backbone are hydrogens.

Example 3.3 Identify all the possible structural and stereoisomers that can result from the addition polymerization of chloroprene (this rubbery polymer is commercially known as Neoprene and is used in wetsuits).

Solution. As in Example 3.2, the 1,4-polymer can have cis and trans stereoisomers (two different possibilities). This unsymmetrical diene monomer can also undergo both 1,2-and 3,4-addition:

Each of these structural isomers may have atactic, isotactic, or syndiotactic stereoisomers (making for six more possible isomers). Thus, in principle at least, there are eight different isomers of polychloroprene possible.

3.8 Summary

Bonding in polymers is significantly more complex than in metals. The variety of bonds and the way side groups are organized in isomers make for the wide range of properties polymers can have, as will be demonstrated in later chapters.

Problems

1. Given the monomers below:

a. Show the repeating units of all the linear homopolymers that could be produced.

b. Which of the polymers, if any, could have cis and trans isomers? Illustrate. How would this differ from the isomerism that arises in 1,4-polydienes?

c. Which of the polymers, if any, could have isotactic and syndiotactic isomers? Illustrate.

2. Draw chemical structures of isotactic and syndiotactic oligomers of vinyl alcohol with five repeat units; include hydrogen atoms for the end groups.

a. Draw a structure to represent the triblock polymer, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide), PEO_m-PPO_n-PEO_m. This polymer is used as a surfactant and has the tradename Pluronic®. For your structure, use Pluronic F127, where m = 99 and n = 65.

b. If you were to stretch out a Pluronic F127 in planar zigzag form, without violating required bond angles, so that all of the atoms in the polymer backbone were in the same plane, estimate the order of magnitude for the polymer chain length, including the end groups (–H's), on the polymer backbone.

Notes

1. Note: kT is a representation of molecular energy from thermodynamics. k is the Boltzmann constant and T is the absolute temperature.

References

1. Platzer, N., Ind. Eng. Chem. 61(5) 10, 1969.

2. Miller, M.L., The Structure of Polymers. Reinhold, New York, 1966.

3. Natta, G., Sci. Am. 205(2), 33 (1961).