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1.2 Polymer Structure

The properties of polymers are strongly influenced by details of the chain structure. These details include the overall chemical composition and the sequence of monomer units in the case of copolymers, the stereochemistry or tacticity of the chain, and the geometric isomerization in the case of diene-type polymers for which several synthesis routes may be possible.

1.2.1 Copolymers

Often, it is possible to obtain polymers with new and desirable properties by linking two or three different monomers or repeating units during the polymerization. Poly-mers with two different repeating units in their chains are called copolymers. When there are three chemically different repeating units, the resulting polymer is termed a terpolymer. Commercially, the most important copolymers are derived from vinyl monomers such as styrene, ethylene, acrylonitrile, and vinyl chloride.

The exact sequence of monomer units along the chain can vary widely depending upon the relative reactivities of each monomer during the copolymerization process. At the extremes, monomer placement may be totally random or may be perfectly alternating, as illustrated in Figure 1-4. The actual sequence of monomer units is determined by the relative reactivities of the monomers as will be described for free-radical copolymerization in Section 2.2.1. Under special circumstances, it is possible to prepare copolymers that contain a long block of one monomer (A) followed by a block of the other monomer (B). These are called AB-block copolymers. ABA-triblock copolymers have a central B block joined by A blocks at both ends. A commercially important ABA-triblock copolymer is polystyrene-block-polybutadiene-block-polystyrene or SBS, a thermoplastic elastomer (see Section 9.2.3). In addition to these copolymer structures, graft copolymers can be prepared by polymerizing a monomer in the presence of a fully formed polymer of another monomer. Graft copolymers are important as elastomers (e.g., SBR) and high-impact polymers (e.g., high-impact polystyrene and acrylontrile−butadiene−styrene or ABS resin).

Figure 1-4

Figure 1-4 Possible structures of copolymers containing A and B repeating units.

1.2.2 Tacticity

In addition to the type, number, and sequential arrangement of monomers along the chain, the spatial arrangement of substituent groups is also important in determining properties. The possible steric configurations of an asymmetric vinyl-polymer chain can be best represented by drawing the chain in its extended-chain or planar zigzag conformation, as illustrated in Figure 1-5. A conformation describes the geometrical arrangement of atoms in the polymer chain while configuration denotes the stereochemical arrangement of atoms. Unlike the conformation, the configuration of a polymer chain cannot be altered without breaking chemical bonds. For long, flexible polymer chains, the total number of conformations is nearly infinite. The extended-chain conformation for vinyl polymers is often the lowest-energy conformation.

Figure 1-5

Figure 1-5 Two forms of stereochemical configuration of an extended-chain vinyl polymer having a substituent group R other than hydrogen.

As illustrated in Figure 1-5, several different placements of the asymmetric substituent group, R, are possible. As examples, a substituent group may be a methyl group as in polypropylene, a chlorine atom as in poly(vinyl chloride), or a phenyl ring as in polystyrene. In one configuration, all the R groups may lie on the same side of the plane formed by the extended-chain backbone. Such polymers are termed isotactic. If the substituent groups regularly alternate from one side of the plane to the other, the polymer is termed syndiotactic. Polymers with no preferred placement are atactic. More complicated arrangements of substituent groups are possible in the case of 1,2-disubstituted polymers; however, these are commercially less important and will not be discussed here.

In general, tactic polymers (i.e., isotactic or syndiotactic) are partially crystalline, while atactic polymers are amorphous indicating the absence of all crystalline order. In addition to crystallinity, other polymer properties, such as thermal and mechanical behavior, can be significantly affected by the tacticity of the polymer as later examples will show. Whether a specific polymer will be atactic, isotactic, or syndiotactic depends upon the specific conditions of the polymerization, such as the temperature and choice of solvent, as will be discussed in Chapter 2. Commercial polypropylene is an important example of an isotactic polymer. Atactic and syndiotactic forms of this polymer can also be prepared by controlling the polymerization conditions. Atactic polypropylene is an amorphous, tacky polymer with no commercial importance. Commercial poly(vinyl chloride) (PVC) is an example of a polymer with imperfect tactic structure. Although the overall structure of commercial-grade PVC can be reasonably characterized as atactic, there are populations of repeating units whose sequences are highly syndiotactic and that impart a small degree of crystallinity to the commercial resin. Space-filling (CPK) models of a short PVC chain having eight repeating units with all isotactic and all syndiotactic placements of the chlorine atoms are shown in Figure 1-6. Using special polymerization methods, PVC with very high syndiotactic or isotactic content can be made (see Section 9.1.2); however, these crystalline stereoisomers of PVC offer no important advantage compared to the commercial plastic. In the case of polystyrene, syndiotactic polystyrene has been obtained by metallocene polymerization (see Section 2.2.3) and is being studied as an alternative to the atactic “crystal grade” plastic for some applications (see Section 9.1.2).

Figure 1-6

Figure 1-6 Views of computer-generated chains of eight repeating units (octamer) of vinyl chloride with isotactic (top) and syndiotactic (bottom) structures are shown. These views are looking down on the chain with the chlorine atoms (large spheres) sitting at the base of each chain. Small light gray spheres represent hydrogen atoms while larger dark gray spheres identify the carbon atoms.

1.2.3 Geometric Isomerism

When there are unsaturated sites along a polymer chain, several different isomeric forms are possible. As illustrated by Figure 1-7, 1,3-butadiene (structure A) can be polymerized to give 1,2-poly(1,3-butadiene) (B) or either of two geometric isomers of 1,4-poly(1,3-butadiene) (C and D). The numbers preceding the poly prefix designate the first and last carbon atoms of the backbone repeating unit. 1,2-poly(1,3-butadiene) has a vinyl-type structure, where the substituent group (ethene) contains an unsaturated site; therefore, this geometric isomer can be atactic, syndiotactic, or isotactic. In the case of the commercially more important 1,4-poly(1,3-butadiene), all four carbons in the repeating unit lie along the chain. Carbons 1 and 4 can lie either on the same side of the central double bond (i.e., cis-configuration, C) or on the opposite side (i.e., trans-configuration, D). The structure of polybutadiene used in SBR rubber (i.e., a copolymer of styrene and butadiene) is principally the trans-1,4 isomer with some cis-1,4- and 1,2-poly(1,3-butadiene) content.

Figure 1-7

Figure 1-7 Alternative pathways for the polymerization of 1,3-butadiene (A) to give 1,2-poly(1,3-butadiene) (B), cis-1,4-poly(1,3-butadiene) (C), or trans-1,4-poly(1,3-butadiene) (D).

1.2.4 Nomenclature

As the preceding examples illustrate, a very large number of different polymer structures are possible. In order to identify these as unambiguously as possible, it is important to have a robust nomenclature system. As is already evident, simple vinyl polymers are designated by attaching the prefix poly to the monomer name (e.g., polystyrene, polyethylene, and polypropylene); however, when the monomer name consists of more than one word or is preceded by a letter or number, the monomer is enclosed by parentheses preceded by the prefix poly. For example, the polymer obtained from the polymerization of 4-chlorostyrene is poly(4-chlorostyrene) and that from vinyl acetate is poly(vinyl acetate). Tacticity may be noted by prefixing the letter i (isotactic) or s (syndiotactic) before poly as in i-polystyrene. Geometric and structural isomers may be indicated by using the appropriate prefixes, cis or trans and 1,2- or 1,4-, before poly, as in trans-1,4-poly(1,3-butadiene).

Nomenclature rules for non-vinyl polymers such as condensation polymers are generally more complicated than for vinyl monomers. These polymers are usually named according to the initial monomer or the functional group of the repeating unit. For example, the most important commercial nylon, commonly called nylon-6,6 (66 or 6/6), is more descriptively called poly(hexamethylene adipamide) denoting the polyamidation of hexamethylenediamine (alternatively called 1,6-hexane diamine) with adipic acid (see Figure 1-3A). Similarly, the aliphatic nylon obtained by the polyamidation of hexamethylenediamine with a 10-carbon dicarboxylic acid, sebacic acid, is nylon-6,10 or poly(hexamethylene sebacamide) (see structure shown in Table 1-3).

In some cases, “common” names are used almost exclusively in place of the more chemically correct nomenclature. For example, the polycondensation of phosgene and bisphenol-A—the common name for 2,2-bis(4-hydroxyphenyl)propane—produces the engineering thermoplastic, polycarbonate (Figure 1-3B). Often, the prefix bisphenol-A is placed before polycarbonate to distinguish it from other polycarbonates that can be polymerized by using bisphenol monomers other than bis-phenol-A, such as tetramethylbisphenol-A.

For many years, the International Union of Pure and Applied Chemistry (IUPAC) and the American Chemical Society (ACS) have developed a detailed, structure-based nomenclature for polymers. In addition, an industrial standard (ASTM D-4000) for specifying specific commercial grades of reinforced and non-reinforced plastics has been offered by the American Society for Testing and Materials (ASTM).

The IUPAC structure-based rules for naming organic, inorganic, and coordination polymers have been compiled in several publications 38. Although such nomenclature provides an unambiguous method for identifying the large number of known polymers (more than 60,000 polymers are listed in the Chemical Abstracts Service (CAS) Chemical Registry System, semi-systematic or trivial names and sometimes even principal trade names (much to the displeasure of the manufacturer) continue to be used in place of the sometimes unwieldy IUPAC names. As examples, the IUPAC name for polystyrene is poly(1-phenylethylene) and that for polytetrafluoroethylene

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is poly(difluoromethylene)—a polymer more typically recognized by its trademark, Teflon. The IUPAC name for the polycarbonate of bisphenol-A mentioned earlier is poly(oxycarbonyloxy-1,4-phenyleneisopropylidene-1,4-phenylene).

For convenience, several societies have developed a very useful set of two-, three-, and four-letter abbreviations for the names of many common thermoplastics, thermosets, fibers, elastomers, and additives. Sometimes, abbreviations adopted by different societies for the same polymer may vary, but there is widespread agreement on the abbreviations for a large number of important polymers. These abbreviations are convenient and widely used. As examples, PS is generally recognized as the abbreviation for polystyrene, PVC for poly(vinyl chloride), PMMA for poly(methyl methacrylate), PTFE for polytetrafluoroethylene, and PC for bis-phenol-A polycarbonate. A listing of commonly accepted abbreviations is given in Appendix A at the end of this book.

Following IUPAC recommendations, copolymers are named by incorporating an italicized connective term between the names of monomers contained within parentheses or brackets or between two or more polymer names. The connective term designates the type of copolymer as indicated for six important classes of copolymers in Table 1-5.

Table 1-5 Scheme for Naming Copolymers

Type

Connective

Example

Unspecified

-co-

Poly[styrene-co-(methyl methacrylate)]

Statisticala

-stat-

Poly(styrene-stat-butadiene)

Random

-ran-

Poly[ethylene-ran-(vinyl acetate)]

Alternating

-alt-

Poly[styrene-alt-(maleic anhydride)]

Block

-block-

Polystyrene-block-polybutadiene

Graft

-graft-

Polybutadiene-graft-polystyrene

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