16 Introduction to Chain Molecules Table 1.3 Name,Abbreviations,and R Group for Common Amino Acids Letter HOOC、 NH2 Name Abbreviation code R Group Alanine Ala A e H NH Arginine Arg R NH2 Asparagine Asn N Aspartic acid Asp D SH Cysteine Cys Glutamic acid Glu E Glutamine Gln Glycine Gly NH Histidine His H Me- -Me Isoleucine Ile I Me Leucine Leu L Me Lysine Lys K NH2
Table 1.3 Name, Abbreviations, and R Group for Common Amino Acids Alanine Ala A Me Arginine Arg R H N NH NH2 Asparagine Asn N NH2 O Aspartic acid Asp D OH O Cysteine Cys C SH Glutamic acid Glu E OH O Glutamine Gln O O NH2 Glycine Gly G H Histidine His H N NH Isoleucine Ile I Me Me Leucine Leu L Me Me Name Abbreviation Letter code R Group HOOC NH2 R Lysine Lys K NH2 Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 16 5.11.2007 8:21pm Compositor Name: JGanesan 16 Introduction to Chain Molecules
Addition,Condensation,and Natural Polymers 17 Table 1.3(continued) Letter HOOC、NH2 Name Abbreviation Code R Group R Me Methionine Met M Phenylalanine Phe F Proline Pro OH Serine Ser M Threonine Thr T Tryptophan Trp W Tyrosine Tyr Y Me Valine Val some of the more common amino acids are listed in Table 1.3.In proline (Pro)the nitrogen and the a-carbon are part of a five-atom pyrrolidine ring.Since some of the amino acids carry substituent carboxyl or amino groups,protein molecules are charged in aqueous solutions,and hence can migrate in electric fields.This is the basis of electrophoresis as a means of separating and identifying proteins. It is conventional to speak of three levels of structure in protein molecules: 1.Primary structure refers to the sequence of amino acids in the polyamide chain. 2.Secondary structure refers to the regions of the molecule that have particular spatial arrange- ments.Examples in proteins include the o-helix and the B-sheet
some of the more common amino acids are listed in Table 1.3. In proline (Pro) the nitrogen and the a-carbon are part of a five-atom pyrrolidine ring. Since some of the amino acids carry substituent carboxyl or amino groups, protein molecules are charged in aqueous solutions, and hence can migrate in electric fields. This is the basis of electrophoresis as a means of separating and identifying proteins. It is conventional to speak of three levels of structure in protein molecules: 1. Primary structure refers to the sequence of amino acids in the polyamide chain. 2. Secondary structure refers to the regions of the molecule that have particular spatial arrangements. Examples in proteins include the a-helix and the b-sheet. Table 1.3 (continued) Methionine Met M S Me Phenylalanine Phe F Proline Pro P H N O OH Serine Ser S OH Threonine Thr T HO Me Tryptophan Trp W N Tyrosine Tyr Y OH Valine Val V Me Me Name Abbreviation Letter Code R Group HOOC NH2 R Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 17 5.11.2007 8:21pm Compositor Name: JGanesan Addition, Condensation, and Natural Polymers 17
18 Introduction to Chain Molecules 3 Tertiary structure refers to the overall shape of the molecule,for example,a globule perhaps stabilized by disulfide bridges formed by the oxidation of cysteine mercapto groups.By extension the full tertiary structure implies knowledge of the relative spatial positions of all the residues. Hydrogen bonding stabilizes some protein molecules in helical forms,and disulfide cross-links stabilize some protein molecules in globular forms.Both secondary and tertiary levels of structure are also influenced by the distribution of polar and nonpolar amino acid molecules relative to the aqueous environment of the protein molecules.In some cases,individual proteins associate in particular aggregates,which are referred to as quaternary structures. Examples of the effects and modifications of the higher-order levels of structures in proteins are found in the following systems: 1. Collagen is the protein of connective tissues and skin.In living organisms,the molecules are wound around one another to form a three-stranded helix stabilized by hydrogen bonding. When boiled in water,the collagen dissolves and forms gelatin,thereby establishing a new hydrogen bond equilibrium with the solvent.This last solution sets up to form the familiar gel when cooled,a result of shifting the hydrogen bond equilibrium. 2. Keratin is the protein of hair and wool.These proteins are insoluble because of the disulfide cross-linking between cysteine units.Permanent waving of hair involves the rupture of these bonds,reshaping of the hair fibers,and the reformation of cross-links,which hold the chains in the new positions relative to each other.We shall see in Chapter 10 how such cross-linked networks are restored to their original shape when subjected to distorting forces. 3.The globular proteins albumin in eggs and fibrinogen in blood are converted to insoluble forms by modification of their higher-order structure.The process is called denaturation and occurs, in the systems mentioned,with the cooking of eggs and the clotting of blood. 4. Actin is a fascinating protein that exists in two forms:G-actin(globular)and F-actin (fibrillar). The globular form can polymerize(reversibly)into very long filaments under the influence of various triggers.These filaments play a crucial role in the cytoskeleton,i.e.,in allowing cells to maintain their shape.In addition,the uniaxial sliding of actin filaments relative to filaments of a related protein,myosin,is responsible for the working of muscles. Ribonucleic acid (RNA)and deoxyribonucleic acid (DNA)are polymers in which the repeat units are substituted esters.The esters are formed between the hydrogens of phosphoric acid and the hydroxyl groups of a sugar,D-ribose in the case of RNA and D-2-deoxyribose in the case of DNA.The sugar rings in DNA carry four different kinds of substituents:adenine(A)and guanine (G),which are purines,and thymine (T)and cytosine(C),which are pyramidines.The familiar double-helix structure of the DNA molecule is stabilized by hydrogen bonding between pairs of substituent base groups:G-C and A-T.In RNA,thymine is usually replaced by uracil(U).The replication of these molecules,the template model of their functioning,and their role in protein synthesis and the genetic code make the study of these polymers among the most exciting and actively researched areas in science.As with the biological function of proteins,we will not discuss these phenomena in this book.However,as indicated previously,DNA plays a very important role as a prototypical semiflexible polymer,as it is now readily obtainable in pure molecular fractions of varying lengths,and because it is readily dissolved in aqueous solution.It is also a charged polymer,or polyelectrolyte,and thus serves as a model system in this arena as well. 1.5 Polymer Nomenclature Considering that a simple compound like C2HsOH is variously known as ethanol,ethyl alcohol, grain alcohol,or simply alcohol,it is not too surprising that the vastly more complicated polymer molecules are also often known by a variety of different names.The International Union of Pure and Applied Chemistry (IUPAC)has recommended a system of nomenclature based on the
3. Tertiary structure refers to the overall shape of the molecule, for example, a globule perhaps stabilized by disulfide bridges formed by the oxidation of cysteine mercapto groups. By extension the full tertiary structure implies knowledge of the relative spatial positions of all the residues. Hydrogen bonding stabilizes some protein molecules in helical forms, and disulfide cross-links stabilize some protein molecules in globular forms. Both secondary and tertiary levels of structure are also influenced by the distribution of polar and nonpolar amino acid molecules relative to the aqueous environment of the protein molecules. In some cases, individual proteins associate in particular aggregates, which are referred to as quaternary structures. Examples of the effects and modifications of the higher-order levels of structures in proteins are found in the following systems: 1. Collagen is the protein of connective tissues and skin. In living organisms, the molecules are wound around one another to form a three-stranded helix stabilized by hydrogen bonding. When boiled in water, the collagen dissolves and forms gelatin, thereby establishing a new hydrogen bond equilibrium with the solvent. This last solution sets up to form the familiar gel when cooled, a result of shifting the hydrogen bond equilibrium. 2. Keratin is the protein of hair and wool. These proteins are insoluble because of the disulfide cross-linking between cysteine units. Permanent waving of hair involves the rupture of these bonds, reshaping of the hair fibers, and the reformation of cross-links, which hold the chains in the new positions relative to each other. We shall see in Chapter 10 how such cross-linked networks are restored to their original shape when subjected to distorting forces. 3. The globular proteins albumin in eggs and fibrinogen in blood are converted to insoluble forms by modification of their higher-order structure. The process is called denaturation and occurs, in the systems mentioned, with the cooking of eggs and the clotting of blood. 4. Actin is a fascinating protein that exists in two forms: G-actin (globular) and F-actin (fibrillar). The globular form can polymerize (reversibly) into very long filaments under the influence of various triggers. These filaments play a crucial role in the cytoskeleton, i.e., in allowing cells to maintain their shape. In addition, the uniaxial sliding of actin filaments relative to filaments of a related protein, myosin, is responsible for the working of muscles. Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are polymers in which the repeat units are substituted esters. The esters are formed between the hydrogens of phosphoric acid and the hydroxyl groups of a sugar, D-ribose in the case of RNA and D-2-deoxyribose in the case of DNA. The sugar rings in DNA carry four different kinds of substituents: adenine (A) and guanine (G), which are purines, and thymine (T) and cytosine (C), which are pyramidines. The familiar double-helix structure of the DNA molecule is stabilized by hydrogen bonding between pairs of substituent base groups: G–C and A–T. In RNA, thymine is usually replaced by uracil (U). The replication of these molecules, the template model of their functioning, and their role in protein synthesis and the genetic code make the study of these polymers among the most exciting and actively researched areas in science. As with the biological function of proteins, we will not discuss these phenomena in this book. However, as indicated previously, DNA plays a very important role as a prototypical semiflexible polymer, as it is now readily obtainable in pure molecular fractions of varying lengths, and because it is readily dissolved in aqueous solution. It is also a charged polymer, or polyelectrolyte, and thus serves as a model system in this arena as well. 1.5 Polymer Nomenclature Considering that a simple compound like C2H5OH is variously known as ethanol, ethyl alcohol, grain alcohol, or simply alcohol, it is not too surprising that the vastly more complicated polymer molecules are also often known by a variety of different names. The International Union of Pure and Applied Chemistry (IUPAC) has recommended a system of nomenclature based on the Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 18 5.11.2007 8:21pm Compositor Name: JGanesan 18 Introduction to Chain Molecules
Polymer Nomenclature 19 structure of the monomer or repeat unit [7].A semisystematic set of trivial names is also in widespread usage;these latter names seem even more resistant to replacement than is the case with low molecular weight compounds.Synthetic polymers of commercial importance are often widely known by trade names that have more to do with marketing considerations than with scientific communication.Polymers of biological origin are often described in terms of some aspect of their function,preparation,or characterization. If a polymer is formed from a single monomer,as in addition and ring-opening polymerizations, it is named by attaching the prefix poly to the name of the monomer.In the IUPAC system,the monomer is named according to the IUPAC recommendations for organic chemistry,and the name of the monomer is set off from the prefix by enclosing the former in parentheses.Variations of this basic system often substitute a common name for the IUPAC name in designating the monomer. Whether or not parentheses are used in the latter case is influenced by the complexity of the monomer name;they become more important as the number of words in the monomer name increases.Thus the polymer(CH2-CHCI)n is called poly(1-chloroethylene)according to the IUPAC system;it is more commonly called poly(vinyl chloride)or polyvinyl chloride. Acronyms are not particularly helpful but are an almost irresistible aspect of polymer terminology. as evidenced by the initials PVC,which are widely used to describe the polymer just named.The trio of names poly(1-hydroxyethylene),poly(vinyl alcohol),and polyvinyl alcohol emphasizes that the polymer need not actually be formed from the reaction of the monomer named;this polymer is actually prepared by the hydrolysis of poly(1-acetoxyethylene),otherwise known as poly(vinyl acetate).These same alternatives are used in naming polymers formed by ring-opening reactions; for example,poly(6-aminohexanoic acid),poly(6-aminocaproic acid),and poly(g-caprolactam)are all more or less acceptable names for the same polymer. Those polymers which are the condensation products of two different monomers are named by applying the preceding rules to the repeat unit.For example,the polyester formed by the condensation of ethylene glycol and terephthalic acid is called poly(oxyethylene oxyterphthaloyl)according to the IUPAC system,but is more commonly referred to as poly(ethylene terephthalate)or polyethylene terephthalate.The polyamides poly(hexamethylene sebacamide)and poly(hexamethylene adipamide) are also widely known as nylon-6,10 and nylon-6,6,respectively.The numbers following the word nylon indicate the number of carbon atoms in the diamine and dicarboxylic acids,in that order.On the basis of this system,poly(E-caprolactam)is also known as nylon-6. Many of the polymers in Table 1.1 and Table 1.2 are listed with more than one name.Also listed are some of the registered trade names by which these substances-or materials which are mostly of the indicated structure-are sold commercially.Some commercially important cross-linked polymers go virtually without names.These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts.These materials are called resins and,at best, are named by specifying the monomers that go into their production.Often even this information is sketchy.Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins,for which typical structures are given by Structure (1.IV)and Structure (1.V),respectively: (1.V) (1.V)
structure of the monomer or repeat unit [7]. A semisystematic set of trivial names is also in widespread usage; these latter names seem even more resistant to replacement than is the case with low molecular weight compounds. Synthetic polymers of commercial importance are often widely known by trade names that have more to do with marketing considerations than with scientific communication. Polymers of biological origin are often described in terms of some aspect of their function, preparation, or characterization. If a polymer is formed from a single monomer, as in addition and ring-opening polymerizations, it is named by attaching the prefix poly to the name of the monomer. In the IUPAC system, the monomer is named according to the IUPAC recommendations for organic chemistry, and the name of the monomer is set off from the prefix by enclosing the former in parentheses. Variations of this basic system often substitute a common name for the IUPAC name in designating the monomer. Whether or not parentheses are used in the latter case is influenced by the complexity of the monomer name; they become more important as the number of words in the monomer name increases. Thus the polymer (CH2–CHCl)n is called poly(1-chloroethylene) according to the IUPAC system; it is more commonly called poly(vinyl chloride) or polyvinyl chloride. Acronyms are not particularly helpful but are an almost irresistible aspect of polymer terminology, as evidenced by the initials PVC, which are widely used to describe the polymer just named. The trio of names poly(1-hydroxyethylene), poly(vinyl alcohol), and polyvinyl alcohol emphasizes that the polymer need not actually be formed from the reaction of the monomer named; this polymer is actually prepared by the hydrolysis of poly(1-acetoxyethylene), otherwise known as poly(vinyl acetate). These same alternatives are used in naming polymers formed by ring-opening reactions; for example, poly(6-aminohexanoic acid), poly(6-aminocaproic acid), and poly(e-caprolactam) are all more or less acceptable names for the same polymer. Those polymers which are the condensation products of two different monomers are named by applying the preceding rules to the repeat unit. For example, the polyester formed by the condensation of ethylene glycol and terephthalic acid is called poly(oxyethylene oxyterphthaloyl) according to the IUPAC system, but is more commonly referred to as poly(ethylene terephthalate) or polyethylene terephthalate. The polyamides poly(hexamethylene sebacamide) and poly(hexamethylene adipamide) are also widely known as nylon-6,10 and nylon-6,6, respectively. The numbers following the word nylon indicate the number of carbon atoms in the diamine and dicarboxylic acids, in that order. On the basis of this system, poly(e-caprolactam) is also known as nylon-6. Many of the polymers in Table 1.1 and Table 1.2 are listed with more than one name. Also listed are some of the registered trade names by which these substances—or materials which are mostly of the indicated structure—are sold commercially. Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers that go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol–formaldehyde and urea–formaldehyde resins, for which typical structures are given by Structure (1.IV) and Structure (1.V), respectively: OH n (1:IV) N N O N O n (1:V) Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 19 5.11.2007 8:21pm Compositor Name: JGanesan Polymer Nomenclature 19
20 Introduction to Chain Molecules 1.6 Structural lsomerism In this section,we shall consider three types of isomerism that are encountered in polymers.These are positional isomerism,stereo isomerism,and geometrical isomerism.We shall focus attention on synthetic polymers and shall,for the most part,be concerned with these types of isomerism occurring singly,rather than in combinations.Some synthetic and analytical aspects of stereo isomerism will be considered in Chapter 5.Our present concem is merely to introduce the possibilities of these isomers and some of the associated vocabulary. 1.6.1 Positional Isomerism Positional isomerism is conveniently illustrated by considering the polymerization of a vinyl monomer.In such a reaction,the adding monomer may become attached to the growing chain end(designated by *)in either of two orientations: (1.VI) (1.B) (1.V四 Structure (1.VI)and Structure (1.VID),respectively,are said to arise from head-to-tail or head-to- head orientations.In this terminology,the substituted carbon is defined to be the head and the methylene is the tail.Tail-to-tail linking is also possible. For most vinyl polymers,head-to-tail addition is the dominant mode of addition.Variations from this generalization become more common for polymerizations which are carried out at higher temperatures.Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride.The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects.In many cases,the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group.Head-to-tail attachment is also sterically favored,since the substituent groups on successive repeat units are separated by a methylene carbon.At higher polymerization temperatures,larger amounts of available thermal energy make the less-favored states more accessible.In vinyl fluoride,no resonance stabilization is possible and steric effects are minimal.This monomer adds primarily in the head-to-tail orientation at low temperatures and tends toward a random combination of both at higher temperatures.The styrene radical,by contrast,enjoys a large amount of resonance stabilization in the bulky phenyl group and polymerizes almost exclusively in the head-to-tail mode.The following example illustrates how chemical methods can be used to measure the relative amounts of the two positional isomers in a polymer sample. Example 1.3 1,2-Glycol bonds are cleaved by reaction with periodate;hence poly(vinyl alcohol)chains are broken at the site of head-to-head links in the polymer.The fraction of head-to-head linkages in poly(vinyl alcohol)may be determined by measuring the molecular weight before (subscript b) and after (subscript a)cleavage with periodate according to the following formula: Fraction=44(1/M-1/Mp).Derive this expression and calculate the value for the fraction in the case of Mp=10 and Ma=10
1.6 Structural Isomerism In this section, we shall consider three types of isomerism that are encountered in polymers. These are positional isomerism, stereo isomerism, and geometrical isomerism. We shall focus attention on synthetic polymers and shall, for the most part, be concerned with these types of isomerism occurring singly, rather than in combinations. Some synthetic and analytical aspects of stereo isomerism will be considered in Chapter 5. Our present concern is merely to introduce the possibilities of these isomers and some of the associated vocabulary. 1.6.1 Positional Isomerism Positional isomerism is conveniently illustrated by considering the polymerization of a vinyl monomer. In such a reaction, the adding monomer may become attached to the growing chain end (designated by *) in either of two orientations: (1.VI) ∗ X H + H H X H X ∗ H X X ∗ X H H (1:B) (1:VII) Structure (1.VI) and Structure (1.VII), respectively, are said to arise from head-to-tail or head-tohead orientations. In this terminology, the substituted carbon is defined to be the head and the methylene is the tail. Tail-to-tail linking is also possible. For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases, the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene carbon. At higher polymerization temperatures, larger amounts of available thermal energy make the less-favored states more accessible. In vinyl fluoride, no resonance stabilization is possible and steric effects are minimal. This monomer adds primarily in the head-to-tail orientation at low temperatures and tends toward a random combination of both at higher temperatures. The styrene radical, by contrast, enjoys a large amount of resonance stabilization in the bulky phenyl group and polymerizes almost exclusively in the head-to-tail mode. The following example illustrates how chemical methods can be used to measure the relative amounts of the two positional isomers in a polymer sample. Example 1.3 1,2-Glycol bonds are cleaved by reaction with periodate; hence poly(vinyl alcohol) chains are broken at the site of head-to-head links in the polymer. The fraction of head-to-head linkages in poly(vinyl alcohol) may be determined by measuring the molecular weight before (subscript b) and after (subscript a) cleavage with periodate according to the following formula: Fraction ¼ 44(1/Ma–1/Mb). Derive this expression and calculate the value for the fraction in the case of Mb ¼ 105 and Ma ¼ 103 . Hiemenz/ Polymer Chemistry, 2nd Edition DK4670_C001 Final Proof page 20 5.11.2007 8:21pm Compositor Name: JGanesan 20 Introduction to Chain Molecules