Addition,Condensation,and Natural Polymers 11 In a cross-linked polymer,the junction units are different kinds of monomers than the chain repeat units,so these molecules might be considered to be still another comonomer.While the chemical reactions that yield such cross-linked substances are technically copolymerizations,the products are described as cross-linked rather than as copolymers.In this instance,the behavior due to cross-linking takes precedence over the presence of an additional type of monomer in the structure. It is apparent from items 1-3 above that linear copolymers-even those with the same proportions of different kinds of repeat units-can be very different in structure and properties. In classifying a copolymer as random,alternating,or block,it should be realized that we are describing the average character of the molecule;accidental variations from the basic patterns may be present.Furthermore,in some circumstances,nominally "random"copolymers can have substantial sequences of one monomer or the other.In Chapter 5,we shall see how an experimental investigation of the sequence of repeat units in a copolymer is a valuable tool for understanding copolymerization reactions. 1.4 Addition,Condensation,and Natural Polymers In the last section,we examined some of the categories into which polymers can be classified. Various aspects of molecular structure were used as the basis for classification in that section.Next we shall consider the chemical reactions that produce the molecules as a basis for classification. The objective of this discussion is simply to provide some orientation and to introduce some typical polymers.For this purpose,many polymers may be classified as being either addition or condensation polymers;both of these classes are discussed in detail in Chapter 2 and Chapter 3, respectively.Even though these categories are based on the reactions which produce the polymers, it should not be inferred that only two types of polymerization reactions exist.We have to start somewhere,and these two important categories are the usual places to begin. 1.4.1 Addition and Condensation Polymers These two categories of polymers can be developed along several lines.For example,in addition- type polymers the following statements apply: 1.The repeat unit in the polymer and the monomer has the same composition,although,of course,the bonding is different in each. 2.The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions,with either free radicals or ionic groups responsible for propagating the chain reaction. 3. The product molecules often have an all-carbon chain backbone,with pendant substituent groups. In contrast,for condensation polymers: 4.The polymer repeat unit arises from reacting together two different functional groups,which usually originate on different monomers.In this case,the repeat unit is different from either of the monomers.In addition,small molecules are often eliminated during the condensation reaction.Note the words usual and often in the previous statements;exceptions to both statements are easily found. 5.The mechanistic aspect of these reactions can be summarized by saying that the reactions occur in steps.Thus,the formation of an ester linkage between two small molecules is not essentially different from that between a polyester and a monomer. 6.The product molecules have the functional groups formed by the condensation reactions interspersed regularly along the backbone of the polymer molecule: -C-C-Y-C-C-Y-
In a cross-linked polymer, the junction units are different kinds of monomers than the chain repeat units, so these molecules might be considered to be still another comonomer. While the chemical reactions that yield such cross-linked substances are technically copolymerizations, the products are described as cross-linked rather than as copolymers. In this instance, the behavior due to cross-linking takes precedence over the presence of an additional type of monomer in the structure. It is apparent from items 1–3 above that linear copolymers—even those with the same proportions of different kinds of repeat units—can be very different in structure and properties. In classifying a copolymer as random, alternating, or block, it should be realized that we are describing the average character of the molecule; accidental variations from the basic patterns may be present. Furthermore, in some circumstances, nominally ‘‘random’’ copolymers can have substantial sequences of one monomer or the other. In Chapter 5, we shall see how an experimental investigation of the sequence of repeat units in a copolymer is a valuable tool for understanding copolymerization reactions. 1.4 Addition, Condensation, and Natural Polymers In the last section, we examined some of the categories into which polymers can be classified. Various aspects of molecular structure were used as the basis for classification in that section. Next we shall consider the chemical reactions that produce the molecules as a basis for classification. The objective of this discussion is simply to provide some orientation and to introduce some typical polymers. For this purpose, many polymers may be classified as being either addition or condensation polymers; both of these classes are discussed in detail in Chapter 2 and Chapter 3, respectively. Even though these categories are based on the reactions which produce the polymers, it should not be inferred that only two types of polymerization reactions exist. We have to start somewhere, and these two important categories are the usual places to begin. 1.4.1 Addition and Condensation Polymers These two categories of polymers can be developed along several lines. For example, in additiontype polymers the following statements apply: 1. The repeat unit in the polymer and the monomer has the same composition, although, of course, the bonding is different in each. 2. The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions, with either free radicals or ionic groups responsible for propagating the chain reaction. 3. The product molecules often have an all-carbon chain backbone, with pendant substituent groups. In contrast, for condensation polymers: 4. The polymer repeat unit arises from reacting together two different functional groups, which usually originate on different monomers. In this case, the repeat unit is different from either of the monomers. In addition, small molecules are often eliminated during the condensation reaction. Note the words usual and often in the previous statements; exceptions to both statements are easily found. 5. The mechanistic aspect of these reactions can be summarized by saying that the reactions occur in steps. Thus, the formation of an ester linkage between two small molecules is not essentially different from that between a polyester and a monomer. 6. The product molecules have the functional groups formed by the condensation reactions interspersed regularly along the backbone of the polymer molecule: ---C---C---Y---C---C---Y--- Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 11 5.11.2007 8:21pm Compositor Name: JGanesan Addition, Condensation, and Natural Polymers 11
12 Introduction to Chain Molecules Next let us consider a few specific examples of these classes of polymers.The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First,the reactive center must be initiated by a suitable reaction to produce a free radical, anionic,or cationic reaction site.Next,this reactive entity adds consecutive monomer units to propagate the polymer chain.Finally,the active site is capped off,terminating the polymer formation.If one assumes that the polymer produced is truly a high molecular weight sub- stance,the lack of uniformity at the two ends of the chain-arising in one case from the initiation and in the other from the termination-can be neglected.Accordingly,the overall reaction can be written (1.A) Again,we emphasize that end effects are ignored in writing Reaction (1.A).These effects as well as the conditions of the reaction and other pertinent information will be discussed when these reactions are considered in Chapter 3 and Chapter 4.Table 1.1 lists several important addition polymers,showing each monomer and polymer structure in the manner of Reaction (1.A).Also included in Table 1.1 are the molecular weights of the repeat units and the common names of the polymers.The former will prove helpful in many of the problems in this book;the latter will be discussed in the next section.Poly(ethylene oxide)and poly(g-caprolactam)have been included in this list as examples of the hazards associated with classification schemes.They resemble addition polymers because the molecular weight of the repeat unit and that of the monomer are the same;they resemble condensation polymers because of the heteroatom chain backbone.The reaction mechanism,which might serve as arbiter in this case,can be either of the chain or the step type,depending on the reaction conditions.These last reactions are examples of ring-opening polymerizations,yet another possible category of classification. The requirements for formation of condensation polymers are twofold:the monomers must possess functional groups capable of reacting to form the linkage,and they ordinarily require more than one reactive group to generate a chain structure.The functional groups can be distributed such that two difunctional monomers with different functional groups react or a single monomer reacts,which is difunctional with one group of each kind.In the latter case especially, but also with condensation polymerization in general,the tendency to form cyclic products from intramolecular reactions may compete with the formation of polymers.Condensation polymeriza- tions are especially sensitive to impurities.The presence of monofunctional reagents introduces the possibility of a reaction product forming which would not be capable of further growth.If the functionality is greater than 2,on the other hand,branching becomes possible.Both of these modifications dramatically alter the product compared to a high molecular weight linear product. When reagents of functionality less than or greater than 2 are added in carefully measured and controlled amounts,the size and geometry of product molecules can be manipulated.When such reactants enter as impurities,the undesired results can be disastrous.Marvel has remarked that more money has been wasted in polymer research by the use of impure monomers than in any other manner [6]. Table 1.2 lists several examples of condensation reactions and products.Since the reacting monomers can contain different numbers of carbon atoms between functional groups,there are quite a lot of variations possible among these basic reaction types.The inclusion of poly(dimethylsiloxane) in Table 1.2 serves as a reminder that polymers need not be organic compounds.The physical properties of inorganic polymers follow from the chain structure of these molecules,and the concepts developed in this volume apply to them and to organic polymers equally well.We shall not examine explicitly the classes and preparations of the various types of inorganic polymers in this text
Next let us consider a few specific examples of these classes of polymers. The addition polymerization of a vinyl monomer CH2 ¼ CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical, anionic, or cationic reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation and in the other from the termination—can be neglected. Accordingly, the overall reaction can be written H H H X n X n (1:A) Again, we emphasize that end effects are ignored in writing Reaction (1.A). These effects as well as the conditions of the reaction and other pertinent information will be discussed when these reactions are considered in Chapter 3 and Chapter 4. Table 1.1 lists several important addition polymers, showing each monomer and polymer structure in the manner of Reaction (1.A). Also included in Table 1.1 are the molecular weights of the repeat units and the common names of the polymers. The former will prove helpful in many of the problems in this book; the latter will be discussed in the next section. Poly(ethylene oxide) and poly(e-caprolactam) have been included in this list as examples of the hazards associated with classification schemes. They resemble addition polymers because the molecular weight of the repeat unit and that of the monomer are the same; they resemble condensation polymers because of the heteroatom chain backbone. The reaction mechanism, which might serve as arbiter in this case, can be either of the chain or the step type, depending on the reaction conditions. These last reactions are examples of ring-opening polymerizations, yet another possible category of classification. The requirements for formation of condensation polymers are twofold: the monomers must possess functional groups capable of reacting to form the linkage, and they ordinarily require more than one reactive group to generate a chain structure. The functional groups can be distributed such that two difunctional monomers with different functional groups react or a single monomer reacts, which is difunctional with one group of each kind. In the latter case especially, but also with condensation polymerization in general, the tendency to form cyclic products from intramolecular reactions may compete with the formation of polymers. Condensation polymerizations are especially sensitive to impurities. The presence of monofunctional reagents introduces the possibility of a reaction product forming which would not be capable of further growth. If the functionality is greater than 2, on the other hand, branching becomes possible. Both of these modifications dramatically alter the product compared to a high molecular weight linear product. When reagents of functionality less than or greater than 2 are added in carefully measured and controlled amounts, the size and geometry of product molecules can be manipulated. When such reactants enter as impurities, the undesired results can be disastrous. Marvel has remarked that more money has been wasted in polymer research by the use of impure monomers than in any other manner [6]. Table 1.2 lists several examples of condensation reactions and products. Since the reacting monomers can contain different numbers of carbon atoms between functional groups, there are quite a lot of variations possible among these basic reaction types. The inclusion of poly(dimethylsiloxane) in Table 1.2 serves as a reminder that polymers need not be organic compounds. The physical properties of inorganic polymers follow from the chain structure of these molecules, and the concepts developed in this volume apply to them and to organic polymers equally well. We shall not examine explicitly the classes and preparations of the various types of inorganic polymers in this text. Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 12 5.11.2007 8:21pm Compositor Name: JGanesan 12 Introduction to Chain Molecules
Addition,Condensation,and Natural Polymers 13 Table 1.1 Reactions by Which Several Important Addition Polymers Can Be Produced Monomer M(g/mol) Repeat unit Chemical name(s) 28.0 n Polyethylene 104 Polystyrene 62.5 Yn Poly(vinyl chloride)."vinyl" 53.0 1n Polyacrylonitrile. CN CN “acrylic'" CI 97.0 Poly(vinylidene chloride) H CI CI CI Me Me 100 入 Poly(methyl methacrylate) Plexiglas,Lucitem Me Me H 00 Me 56.0 Polyisobutylene Me MeMe 100 Poly(tetrafluoroethylene). Teflon 8 44.0 0n Poly(ethylene oxide). poly(ethylene glycol) 113 Poly(E-caprolactam), Nylon-6 1.4.2 Natural Polymers We conclude this section with a short discussion of naturally occurring polymers.Since these are of biological origin,they are also called biopolymers.Although our attention in this volume is primarily directed toward synthetic polymers,it should be recognized that biopolymers,like inorganic polymers,have physical properties which follow directly from the chain structure of
1.4.2 Natural Polymers We conclude this section with a short discussion of naturally occurring polymers. Since these are of biological origin, they are also called biopolymers. Although our attention in this volume is primarily directed toward synthetic polymers, it should be recognized that biopolymers, like inorganic polymers, have physical properties which follow directly from the chain structure of Table 1.1 Reactions by Which Several Important Addition Polymers Can Be Produced H H H H 28.0 n Polyethylene H H H 104 n Polystyrene Cl H H H 62.5 Cl n Poly(vinyl chloride), “vinyl” CN H H H 53.0 CN n Polyacrylonitrile, “acrylic” Cl H Cl H 97.0 Cl Cln Poly(vinylidene chloride) H O H Me O Me 100 O O Me Me n Poly(methyl methacrylate), Plexiglas®, Lucite® Me H Me H 56.0 Me Me n Polyisobutylene F F F F 100 F F F F n Poly(tetrafluoroethylene), Teflon® O 44.0 O n Poly(ethylene oxide), poly(ethylene glycol) N O H 113 H N O n 5 Poly(ε-caprolactam), Nylon-6 Monomer M(g/mol) Repeat unit Chemical name(s) Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 13 5.11.2007 8:21pm Compositor Name: JGanesan Addition, Condensation, and Natural Polymers 13
14 Introduction to Chain Molecules Table 1.2 Reactions by Which Several Important Condensation Polymers Can Be Produced 1.Polyester 2nH20 Poly(ethylene terephthalate).Terylene, Dacron,Mylar Poly(12-hydroxystearic acid) 2.Polyamide 2n HCI Poly(hexamethylene adipamide),Nylon-6,6 3.Polyurethane nO-C=N-N-c-0+nHo H ) Poly(tetramethylenehexamethylene urethane), Spandex,Perlon 4.Polycarbonate 2n HCI Me Poly(4,4-isopropylidenediphenylene carbonate) bisphenol A polycarbonate,Lexan 5.Inorganic Me n Me-Si-Cl n H2O 2n HCI Me Poly(dimethylsiloxane) their molecules.For example,the denaturation of a protein involves an overall conformation change from a "native"state,often a compact globule,to a random coil.As another example, the elasticity and integrity of a cell membrane is often the result of an underlying network of fibrillar proteins,with the origin of the elasticity residing in the same conformational entropy as in a rubber band.Consequently,although we will not discuss the synthesis by,and contribution to the function of,living organisms by such biopolymers,many of the principles we will develop in detail apply equally well to natural polymers. As examples of natural polymers,we consider polysaccharides,proteins,and nucleic acids. Another important natural polymer,polyisoprene,will be considered in Section 1.6.Polysaccharides
their molecules. For example, the denaturation of a protein involves an overall conformation change from a ‘‘native’’ state, often a compact globule, to a random coil. As another example, the elasticity and integrity of a cell membrane is often the result of an underlying network of fibrillar proteins, with the origin of the elasticity residing in the same conformational entropy as in a rubber band. Consequently, although we will not discuss the synthesis by, and contribution to the function of, living organisms by such biopolymers, many of the principles we will develop in detail apply equally well to natural polymers. As examples of natural polymers, we consider polysaccharides, proteins, and nucleic acids. Another important natural polymer, polyisoprene, will be considered in Section 1.6. Polysaccharides Table 1.2 Reactions by Which Several Important Condensation Polymers Can Be Produced Poly(4,4-isopropylidenediphenylene carbonate) bisphenol A polycarbonate, Lexan® 1. Polyester Poly(ethylene terephthalate), Terylene®, Dacron®, Mylar® HO OH + O HO O OH O O O O H2O n n n n HO + 2n OH 10 C6H13 O O C6H13 O + n H2O Poly(12-hydroxystearic acid) n 10 2. Polyamide H2N NH2 6 n n Cl O 4 Cl O H N H N 6 O O 4 n + HCl Poly(hexamethylene adipamide), Nylon-6,6 2n 3. Polyurethane O C N N C O + n HO OH O N H N H O O 6 4 6 O n n n Poly(tetramethylenehexamethylene urethane), Spandex®, Perlon® 4 4. Polycarbonate Me n HO OH Cl Cl O + Me Me O O C n + 2n HCl O 5. Inorganic Poly(dimethylsiloxane) + Me Me O n Si + 2n HCl C Me C Si Cl n Me Cl n H2O Me + Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 14 5.11.2007 8:21pm Compositor Name: JGanesan 14 Introduction to Chain Molecules
Addition,Condensation,and Natural Polymers 15 are macromolecules which make up a large part of the bulk of the vegetable kingdom.Cellulose and starch are,respectively,the first and second most abundant organic compounds in plants.The former is present in leaves and grasses;the latter in fruits,stems,and roots.Because of their abundance in nature and because of contemporary interest in renewable resources,there is a great deal of interest in these compounds.Both cellulose and starch are hydrolyzed by acids to D-glucose, the repeat unit in both polymer chains.The configuration of the glucoside linkage is different in the two,however.Structure(1.D)and Structure(1.ID),respectively,illustrate that the linkage is a B-acetal- hydrolyzable to an equatorial hydroxide-in cellulose and an o-acetal-hydrolyzable to an axial hydroxide-in amylose,a starch: OH H OH H H 0 (1.D HO HO H OH OH H H H (1.⑩ ● HO Amylopectin and glycogen are saccharides similar to amylose,except with branched chains. The cellulose repeat unit contains three hydroxyl groups,which can react and leave the chain backbone intact.These alcohol groups can be esterified with acetic anhydride to form cellulose acetate;this polymer is spun into the fiber acetate rayon.Similarly,the alcohol groups in cellulose react with CS2 in the presence of strong base to produce cellulose xanthates.When extruded into fibers,this material is called viscose rayon,and when extruded into sheets,cellophane.In both the acetate and xanthate formation,some chain degradation also occurs,so the resulting polymer chains are shorter than those in the starting cellulose.The hydroxyl groups are also commonly methylated,ethylated,and hydroxypropylated for a variety of aqueous applications,including food products.A closely related polysaccharide is chitin,the second most abundant polysaccharide in nature,which is found for example in the shells of crabs and beetles.Here one of the hydroxyls on each repeat unit of cellulose is replaced with an-NHCO-CH3 amide group.This is converted to a primary amine-NH2 in chitosan,a derivative of chitin finding increasing applications in a variety of fields. As noted above,proteins are polyamides in which o-amino acids make up the repeat units,as shown by Structure (1.IID): (1.⑩ These molecules are also called polypeptides,especially when M<10,000.The various amino acids differ in their R groups.The nature of R,the name,and the abbreviation used to represent
are macromolecules which make up a large part of the bulk of the vegetable kingdom. Cellulose and starch are, respectively, the first and second most abundant organic compounds in plants. The former is present in leaves and grasses; the latter in fruits, stems, and roots. Because of their abundance in nature and because of contemporary interest in renewable resources, there is a great deal of interest in these compounds. Both cellulose and starch are hydrolyzed by acids to D-glucose, the repeat unit in both polymer chains. The configuration of the glucoside linkage is different in the two, however. Structure (1.I) and Structure (1.II), respectively, illustrate that the linkage is a b-acetal— hydrolyzable to an equatorial hydroxide—in cellulose and an a-acetal—hydrolyzable to an axial hydroxide—in amylose, a starch: O H O H HO H H OH H OH O H O H HO H H OH H O OH (1:I) O H O H HO H OH H H OH O H O H HO H H OH H O OH (1:II) Amylopectin and glycogen are saccharides similar to amylose, except with branched chains. The cellulose repeat unit contains three hydroxyl groups, which can react and leave the chain backbone intact. These alcohol groups can be esterified with acetic anhydride to form cellulose acetate; this polymer is spun into the fiber acetate rayon. Similarly, the alcohol groups in cellulose react with CS2 in the presence of strong base to produce cellulose xanthates. When extruded into fibers, this material is called viscose rayon, and when extruded into sheets, cellophane. In both the acetate and xanthate formation, some chain degradation also occurs, so the resulting polymer chains are shorter than those in the starting cellulose. The hydroxyl groups are also commonly methylated, ethylated, and hydroxypropylated for a variety of aqueous applications, including food products. A closely related polysaccharide is chitin, the second most abundant polysaccharide in nature, which is found for example in the shells of crabs and beetles. Here one of the hydroxyls on each repeat unit of cellulose is replaced with an –NHCO–CH3 amide group. This is converted to a primary amine –NH2 in chitosan, a derivative of chitin finding increasing applications in a variety of fields. As noted above, proteins are polyamides in which a-amino acids make up the repeat units, as shown by Structure (1.III): H N R O n (1:III) These molecules are also called polypeptides, especially when M 10,000. The various amino acids differ in their R groups. The nature of R, the name, and the abbreviation used to represent Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 15 5.11.2007 8:21pm Compositor Name: JGanesan Addition, Condensation, and Natural Polymers 15