Polymers and Polymerization

Chapter 32

 Macromolecules. Polymers and Polymerization

Macromolecules

• So far, our study of organic chemistry has dealt mainly with rather small molecules, containing perhaps as many as 50 to 75 atoms. But there also exist enormous molecules called macromolecules, which contain hundreds of thousands of atoms. Some of these are naturally occurring, and make up classes of compounds that are, quite literally, vital: the polysaccharides starch and cellulose, which provide us with food, clothing, and shelter; proteins, which constitute much of the animal body, hold it together, and run it; and nucleic acids, which control heredity on the molecular level.

• Macromolecules can be man-made, too. The first syntheses were aimed at making substitutes for the natural macromolecules, rubber and silk; but a vast technology has grown up that now produces hundreds of substances that have no natural counterparts. Synthetic macromolecular compounds include elastomers, which have the particular kind of elasticity characteristic of rubber; fibers, long, thin, and threadlike, with the great strength along the fiber that characterizes cotton, wool, and silk; and plastics, which can be extruded as sheets or pipes, painted on surfaces, or molded to form countless objects. We wear these man-made materials, eat and drink from them, sleep between them, sit and stand on them; turn knobs, pull switches, and grasp handles made of them; with their help we hear sounds and see sights remote from us in time and space; we live in houses and move about in vehicles that are increasingly made of them.

• We sometimes deplore the resistance to the elements of these seemingly all too immortal materials, and fear that civilization may someday be buried beneath a pile of plastic debris plastic cigar tips have been found floating in the Sargasso Sea but with them we can do things never before possible. By use of plastics, blind people can be made to see, and cripples to walk; heart valves can be repaired, and arteries patched; damaged tracheas, larynxes, and ureters can be replaced, and some day, perhaps, entire hearts. These materials protect us against heat and cold, electric shock and fire, rust and decay. As tailor-made solvents, they may soon be used to extract fresh water from the sea. Surely the ingenuity that has produced these substances can devise ways of disposing of the waste they create: the problem is not one of technology, but of sociology and, ultimately, of politics.

• In this chapter, we shall be first and chiefly concerned with the chemical reactions by which macromolecules are formed, and the structures that these reactions produce. Then, we shall see how these structures lead to the properties on which the use of the macromolecules depends: why rubber is elastic, for example, and why nylon is a strong fiber. In later chapters, we shall take up the natural macromolecule's polysaccharides, proteins, and nucleic acids and study them in much the same way.

• In all this, we must remember that what makes macromolecules special is, of course, their great size. This great size permits a certain complexity of structure, not just on the molecular level, but on a secondary level that involves the disposition of molecules with respect to each other. Are the molecules stretched out neatly alongside one another, or coiled up independently? What forces act between different molecules? What happens to a collection of giant molecules when it is heated or cooled. As we shall see, the answers to questions like these are found ultimately in structure as we have known it: the nature of functional groups and substituents, their sequence in the molecule, and their arrangement in space. 

Polymers and polymerization

• Macromolecules, both natural and man-made, owe their great size to the fact that they are polymers (Greek: many parts); that is, each one is made up of a great many simpler units identical to each other or at least chemically similar joined together in a regular way. They are formed by a process we touched on earlier: polymerization, the joining together of many small molecules to form very large molecules. The Simnel compounds from which polymers are made are called monomers.

Polymers are formed in two general ways

• In chain-reaction polymerization, there is a series of reactions each of which consumes a reactive particle and produces another, similar particle; each individual reaction thus depends upon the previous one. The reactive particles can be free radicals, cations, or anions. A typical example is the polymerization of ethylene.

• Here the chain-carrying particles are free radicals, each of which adds to a monomer molecule to form a new, bigger free radical.

• In step-reaction polymerization, there is a series of reactions each of which is essentially independent of the preceding one; a polymer is formed simply because the monomer happens to undergo reaction at more than one functional group. A glycol, for example, reacts with a dicarboxylic acid to form an ester; but each moiety of the simple ester still contains a group that can react to generate another ester linkage and hence a larger molecule, which itself can react further, and so on.

• There is an alternative, somewhat less meaningful system of classification: addition polymerization, in which molecules of monomer are simply added together; and condensations polymerization, in which monomer molecules combine with loss of some simple molecules like water. As it happens, the two systems almost exactly coincide; nearly all: aces of chain-reaction polymeric/action involve addition polymerization; nearly all cases Df step-reaction polymerization involve condensation polymerization. Indeed, some: chemists use the term "addition polymerization'* to mean polymerization via chain reactions.

 Free-radical vinyl polymerization

• we discussed briefly the polymerization of ethylene and sub - tuted ethylene under conditions where free radicals are generated typically in the presence of small amounts of an initiator, such as a peroxide. Reaction.

• occurs at the doubly bonded carbons the vinyl groups -and is called vinyl polymerization. A wide variety of unsaturated monomers may be used, to yield polymers with different pendant group* (G) attached to the polymer backbone. For example:

• Polymerization involves addition of free radicals to the double bond of the monomer: addition, first, of the free radical generated from the initiator, and then of the growing polymer molecule. This is, of course, an example of chain-reaction polymerization.

Copolymerization

• So far, we have discussed only polymerization of a single monomeric compound to form a homopolymer, a polymer made up except, of course, at the two ends of the long molecule of identical units. Now, if a mixture of two (or more) monomers is allowed to undergo polymerization, there is obtained a copolymer: a polymer that contains two (or more) kinds of monomeric units in the same molecule. For example:

• Through copolymerization there can be made materials with different properties than those of either homopolymer, or thus another dimension is added to the technology. Consider, for example, styrene. Polymerized alone, it gives a good electric insulator that is molded into parts for radios, television sets, and automobiles. Copolymerization with butadiene (30%) adds toughness; with acrylonitrile (20-30%) increases resistance to impact and to hydrocarbons; with maleic anhydride yields a material that, on hydrolysis, is water-soluble, and is used as a dispersant and sizing agent. The copolymer in which butadiene predominates (75% butadiene, 25% styrene) is an elastomer, and since World War II has been the principal rubber substitute manufactured in the United State.

• Let us look more closely at the copolymerization process. Consider free radical vinyl polymerization of two monomers, Mj and M2. In each step the growing free

• First, of course, there are the relative concentrations of the two monomers; the higher the concentration of a particular monomer, the greater its chance of being incorporated into the chain, and the more abundant its units are in the final product.

• Next, there are the relative reactivities' of the monomers toward free radical addition; in general, the more reactive the monomer, the greater its chance of being incorporated into the polymer. We know that the reactivity of a carbon Carbon double bond toward free radical addition is affected by the stability of the new free radical being formed: factors that tend to stabilize the free radical product tend to stabilize the incipient free radical in the transition state, so that the more stable free radical tends to be formed faster. Now, stability of a free radical depends upon accommodation of the odd electron.

• The group stabilizes the radical by delocalization: the phenyl group in styrene, through formation of a benzylic radical; the vinyl group of 1,3-butadicne, through formation of an allylic radical; the -COOCH3 group of methyl methacrylate, through formation of a radical in which acyl oxygen helps carry the odd electron. (Problem: Draw resonance structures to show how this last effect could arise.

• Now, let us see what kind of copolymer we would expect to get on the basis of what v\e have said so far. In the copolymerization of styrene (Mi) and butadiene (M 2), for example, reaction can proceed via either of two growing radicals: one ending in a styrene unit (Mj), or one ending in a butadiene unit(M2 -)- Either radical can add to either monomer, to form a copolymer with styrene and butadiene units distributed randomly along the molecule:

• With these particular monomers, copolymerization is in fact random. Now, toward free radical type, it happens, butadiene is about 1.4 times as reactive as styrene, so that, if monomer concentrations were equal, butadiene units would tend to predominate in the product. Furthermore, since butadiene is consumed faster, the relative concentrations of monomers would change as reaction goes on, and so would the composition of the polymer be being produced. These effects can be compensated for by adjusting the ratio of monomers fed into the reaction vessel; indeed, by control of the feed ratio, copolymers of any desired composition can be made.

• The relative reactivity of a monomer does depend upon the nature of the radical that is attacking it. Maleic anhydride is much more reactive than stilbene toward radicals ending in a stilbene unit, and stilbene is much more reactive than maleic anhydride toward the other kind of radical. (Indeed, these two compounds, individually, undergo self-polymerization only with extreme difficulty.) A more modest and more typical- tendency toward alternation is shown by styrene and methyl methacrylate. Here, toward either radical (M,) the %s opposite" monomer (M2) is about twice as reactive as the "same" monomer (M!).

• The alternating tendency in copolymeri/action was established on a quantitative basis by Frank R. Mayo (of the Stanford Research Institute) and Cheves Walling (of the University of Utah) while working in the laboratories of the U.S. Rubber Company. Their work was fundamental to the development of free radical chemistry: it showed clearly for the first time the dependence of reactivity on the nature of the attacking free radical, and led directly to the concept of polar factors, working not only in copolymerization and other additions of free radicals, but in free radical reactions of all kinds.

• Basically, Mayo and Walling's interpretation was the following. Although free radicals are neutral, they have certain tendencies to gain or lose electrons, and hence they partake of the character of electrophilic or nucleophilic reagents. The transition states for their reactions can be polar, with the radical moiety acquiring a partial negative or positive charge at the expense of the substrate the alkene, in the case of addition. In copolymerization, a substituent generally exerts the same polar effect electron-withdrawing or electron-releasing on a free radical as on the alkene (monomer) from which the free radical was derived. Electron-withdrawal makes a free radical electrophilic but makes an alkene less able to supply the electrons which that radical is seeking. An electrophilic radical will, then, preferentially add to a monomer containing an electron-releasing group. In a similar way, a nucleophilic radical, containing an electron-releasing substituent, will seek out a monomer containing an electron-withdrawing substituent.

• Styrene and methyl methacrylate tend to alternate because their substituents are of opposite polarity: in methacrylate the COOCH3 group tends to withdraw electrons; in styrene the phenyl group tends (via resonance) to release electrons. The transition states for addition to the opposite monomers are thus stabilized:

• Perhaps the most convincing evidence for the play of polar forces comes from copolymerization of a series of ring-substituted styrenes; here relative reactivities toward a variety of monomers not only fall into a pattern consistent with the familiar electronic effects of the substituents but show the same quantitative relationships (the Hammett sigma-rho relationship, as do ionic reactions: dissociation of carboxylic acids, for example, or hydrolysis of esters.

• The concept of polar transition states in free-radical reactions has recently been questioned, at least for reactions in which hydrogen is abstracted halogenation, for example. Here, it has been suggested, electron-withdrawing or electron-releasing groups affect reactivity simply by strengthening or weakening the bonds holding hydrogen in the substrate.

• We must realize that polar effects are superimposed on effects due to delocalization of the odd electron. Styrene and butadiene, for example, are highly reactive toward any radical since the transition state contains an incipient benzylic or allylic free radical. This high reactivity is modified enhanced or lowered by the demands of the particular attacking radical.

Ionic polymerization. Living polymers

• Chain-reaction polymerization can proceed with ions instead of free radicals as the chain-carrying particles: either cations or anions, depending on the kind of initiator that is used.

• Cationic polymerization is initiated by acids. Isobutylene, for example, undergoes cationic polymerization to a tacky material used in adhesives. Copolymerization with a little isoprene gives butyl rubber, used to make automobile innertubes and tire liners. A variety of acids can be used; sulfuric acid; A1C13 or BF3 plus a trace of water. We recognize this process as an extension of the dimerization discussed.

• Anionic polymerization, as we might expect, is initiated by bases: Li + NH2 ~, for example, or organometallic compounds like w-butyllithium. For example:

Coordination polymerization

• When we speak of organic ions as chain-carriers, we realize, of course, that each of these must be balanced by an ion of opposite charge, a counterion. A growing carbanion, for example, has more or less closely associated with it a metallic cation like Li + or Na + . Ion pairs or even higher aggregates can play important parts in polymerization. If the bonding between the reactive center and the metal is appreciably covalent, the process is called coordination polymerization. The growing organic chain is not a full-fledged anion, but its reactivity is due to its anion-like character.

• Until 1953, almost all vinyl polymerization of commercial importance was of the free-radical type. Since that time, however, ionic polymerization, chiefly in the form of coordination polymerization, has revolutionized the field. Following discoveries by Karl Ziegler (of the Max Planck Institute for Coal Research) and by Guilio Natta (of the Polytechnic Institute of Milan) who jointly received the Nobel Prize in 1963 for this work catalysts have been developed that permit control of the polymerization process to a degree never before possible.

• These Ziegler-Natta catalysts are complexes of transition metal halides with organometallic compounds: typically, triethylaluminium-titanium trichloride Reaction involves nucleophilic addition to the carbon-carbon double bond in the monomer, with the carbanion-like organic group of the growing organometallic compound as nucleophile. The transition metal may play a further role in complexing with the n electrons of the monomer and thus holding it at the reaction site. Polymerization thus amounts to insertion of alkene molecules into the bond between metal and the growing alkyl group. For example, in the formation of polyethylene:

• Polymerization with Ziegler-Natta catalysts has two important advantages over free-radical polymerization: it gives linear polymer molecules; and it permits stereochemical control

• Polyethylene made by the free-radical process has a highly branched structure due to chain-transfer of a special kind, in which the transfer agent is a polymer nw/ecu/e: a hydrogen atom is abstracted from somewhere along the polymer chain,

• A branch grows at the point of attack. In contrast, polyethylene made by the coordination process is virtually unbranched. These unbranched molecules fit together well, and the polymer is said to have a high degree of crystallinity; as a result, it has a higher melting point and higher density than the older (low density) polyethylene and is mechanically much stronger.

• A second, far-reaching development in coordination polymerization is stereochemical control. Propylene. for example, could polymerize to any of three different arrangements: isotactic, with all methyl groups on one side of an extended chain; syndiotactic, with methyl groups alternating regularly from side to side; and atactic, with methyl groups distributed at random. By proper choice of experimental conditions catalyst, temperature, solvent each of these stereoisomeric polymers has been made. Atactic polypropylene is a soft, elastic, rubbery material. Both isotactic and syndiotactic polypropylenes are highly crystalline: regularity of structure permits their molecules to fit together well. Over a billion pounds of isotactic polypropylene is produced every year, to be molded or extruded as sheets, pipes, and filaments; it is on its way to becoming one of the principal synthetic fibers.

• Coordination catalysts also permit stereochemical control about the carboncarbon double bond. By their use, isoprene has been polymerized to a material virtually identical with natural rubber: m-l,4-polyisoprene.

• The Ziegler-Natta polymerization of ethylene can be adapted to make molecules of only modest size (C6-C20) and containing certain functional groups. If, for example, the metal-alkyls initially obtained are heated (in the presence of Ethylene and a nickel catalyst), the hydrocarbon groups are displaced as straight-chain 1-alkenes of even carbon number. Large quantities of such alkenes in the Ci 2-C2 o range are consumed in the manufacture of detergents Alternatively, the Metal alkyls can be oxidized by air to give straight-chain primary alcohols.

Step-reaction polymerization

• Carboxylic acids react with amines to yield amides, and with alcohols to form esters. When an acid that contains more than one COOH group reacts with an amine that contains more than one NH2 group, or with an alcohol that contains more than one OH group, then the products are polyamides and polyesters. For example:

• These are examples of step-reaction polymerization. Here, reaction does not depend on chain-carrying free radicals or ions. Instead, the steps are essentially independent of each other; they just happen to involve more than one functional group in a monomer.

• f each monomer molecule contains just two functional groups, growth can occur in only two directions, and a linear polymer is obtained, as in nylon 66 or Dacron. But if reaction can occur at more than two positions in a monomer, there is formed a highly cross-linked space network polymer, as in Glyptal, an alkyd resin. Dacron and Glyptal are both polyesters, but their structures are quite different and, as we shall see, so are their uses.

• Step-reaction polymerization can involve a wide variety of functional groups and a wide variety of reaction types. Among the oldest of the synthetic polymers, and still extremely important, are those resulting from reaction between phenols and formaldehyde: the phenol-formaldehyde resins (Bakelite and related polymers). When phenol is treated with formaldehyde in the presence of alkali or acid, there is obtained a high molecular weight substance in which many phenol rings are held together by ~ CH2 groups:

• The stages involved in the formation of the polymer seem to be the following. First, phenol reacts with formaldehyde to form o- or /7-hydroxymethylphenol. Hydroxymethylphenol then reacts with another molecule of phenol, with the loss of water, to form a compound in which two rings are joined by a -CH2 link. This process then continues, to yield a product of high molecular weight. Since three positions in each phenol molecule are susceptible to attack, the final product contains many cross-links and hence has a rigid structure.

• The first stage can be viewed as both electrophilic substitution on the ring by the electron-deficient carbon of formaldehyde, and nucleophilic addition of the aromatic ring to the carbonyl group(Jgase catalyzes reaction by converting phenol into the more reactive (more nucleophilic) phenoxide ion.^Acid catalyzes reaction by protonating formaldehyde and increasing the electron deficiency of the carbonyl carbon.

Structure and properties of macromolecules

• The characteristic thing about macromolecules, we have said, is their great size. This size has little effect on chemical properties. A functional group reacts much as we would expect, whether it is in a big or little molecule: an ester is hydrolyzed, an epoxide undergoes ring-opening, an allylic hydrogen is susceptible to abstraction by free radical

• It is in their physical properties that macromolecules differ from ordinary molecules, and it is on these that their special functions depend. To begin with, let us look at the property of crystallinity. In a crystalline solid, we know, the structural units molecules, in the case of a non-ionic compound are arranged in a very regular, symmetrical way, with a geometric pattern repeated over and over. If a long molecule is to fit into such a pattern, it cannot be looped and coiled into a random conformation but must be extended in a regular zig-zag. This lack of randomness corresponds to an unfavorable entropy for the system. On the other hand, the regularity and close fitting of the molecules in a crystal permits operation of strong intermolecular forces hydrogen bonding, dipole-dipole attractions, van der Waals forces which result in a favorable enthalpy (heat content). As we shall see, this tug-of-war between entropy and enthalpy is a key factor in determining the use to which a macromolecule can be put.

• Now, in general, a high polymer does not exist entirely in crystalline form not even a polymer whose regularity of molecular structure might be expected to permit this. The problem is the size of the molecule. As solidification begins, the viscosity of the material rises, and the polymer molecules find it difficult to move about and arrange their long chains in the regular pattern needed for crystal formation. Chains become entangled; a change in shape of a chain must involve rotation about single bonds, and this becomes difficult because of hindrance to the swinging about of pendant groups. Polymers, then, form solids made up of regions of crystallinity, called crystallites, embedded in amorphous material. We speak of the degree of crystallinity of a polymer to mean the extent to which ii is composed of crystallites.

• Let us examine the various uses of polymers and see how these depend on their structure molecular and intermolecular. Fibers are long, thin, threadlike bits of material that are characterized by great tensile (pulling) strength in the direction of the fiber. The natural fibers cotton, wool, silk are typical. Fibers are twisted into threads, which can then be woven into cloth, or embedded in plastic material to impart strength. The tensile strength can be enormous, some synthetic fibers rivalling on a weight basis steel.

• The gross characteristics of fibers are reflected on the molecular level the molecules, too, are long, thin, and threadlike. Furthermore, and most essential, they lie stretched out alongside each other, lined up in the direction of the fiber. The strength of the fiber resides, ultimately, in the strength of the chemical bonds of the polymer chains. The lining-up is brought about by drawing stretching the polymeric material. Once lined up, the molecules stay that way; the tendency to return to random looping and coiling is overcome by strong intermolecular attractions. In a fiber, enthalpy wins out over entropy. This high degree of molecular orientation is usually although not always accompanied by appreciable crystallinity.

• The principal synthetic fibers are polyamides (the nylons), polyesters (Dacron, Terylene, Vycron), polyacrylonitrile ("acrylic fibers," Orion, Acrilan), polyurethanes (Spandex, Vycra), and isotactic polypropylene. In nylon and polyurethanes, molecular chains are held to each other by hydrogen bonds  In polyesters and polyacrylonitrile, the polar carbonyl and cyano groups lead to powerful dipole-dipole attractions. The stereoregular chains of isotactic polypropylene fit together so well that van der Waals forces are strong enough to maintain alignment. 

• An elastomer possesses the high degree of elasticity that is characteristic of rubber: it can be greatly deformed stretched to eight times its original length, for example and yet return to its original shape. Here, as in fibers, the molecules are long and thin; as in fibers, they become lined up when the material is stretched. The big difference is this: when the stretching force is removed, the molecular chains of an elastomer do not remain extended and aligned but return to their original random conformations favored by entropy. They do not remain aligned because the intermolecular forces necessary to hold them that way are weaker than in a fiber. In general, elastomers do not contain highly polar groups or sites for hydrogen bonding; the extended chains do not fit together well enough for van der Waals forces to do the job. In an elastomer entropy beats enthalpy.

• One further requirement: the long chains of an elastomer must be connected to each other by occasional cross-links: enough of them to prevent slipping of molecules past one another; not so many as to deprive the chains of the flexibility that is needed for ready extension and return to randomness. Natural rubber illustrates these structural requirements of an elastomer: long, flexible chains; weak intermolecular forces; and occasional cross-linking. Rubber is c/s-l,4-polyisoprene. With no highly polar substituents, intermolecular attraction is largely limited to van der Waals forces. But these are weak because of the all-c/5 configuration about the double bond. Figure 32.4 compares the extended chains of rubber with those of its trans stereoisomer. As we can see, the trans configuration permits highly regular zig-zags that fit together well; the cis configuration does not. The all-trans stereoisomer occurs naturally as gutta percha\ it is highly crystalline and non-elastic.