Polymers
1. Introduction: Prior to the early 1920's, chemists doubted the existence of molecules having molecular weights greater than a few thousand. This limiting view was challenged by ermann !taudinger , a "erman chemist with ex#erience in studying natural com#ounds such as rubber and cellulose. $n contrast to the #revailing rationali%ation of these substances as aggregates of small molecules, !taudinger #ro#osed they were made u# of macromolecules com#osed macromolecules com#osed of 10,000 or more atoms. e formulated a polymeric structure polymeric structure for rubber , based on a re#eating iso#rene unit &referred to as a monomer. (or his contributions to chemistry, !taudinger received the 19)* +obel Pri%e. The terms polymer and and monomer were were derived from the "ree roots #oly &many, mono &one and meros &#art. -ecognition that #olymeric macromolecules mae u# many im#ortant natural materials was followed by the creation of synthetic analogs having a variety of #ro#erties. $ndeed, a##lications of these materials as fibers, flexible films, adhesives, resistant #aints and tough but light solids have transformed modern society. !ome im#ortant exam#les of these substances are discussed in the following sections.
2. Writing Formulas for Polymeric Macromolecules : The re#eating structural unit of most sim#le #olymers not only reflects the monomer&s from which the #olymers are constructed, but also #rovides a concise means for drawing structures to re#resent these macromolecules. (or #olyethylene, arguably the sim#lest #olymer, this is demonstrated by the following euation. ere ethylene ðene is the monomer, and the corres#onding linear #olymer is called high/density #olyethylene &P. P is com#osed of macromolecules in which n ranges from 10,000 to 100,000 &molecular weight 210 ) to * 10 3 .
$f 4 and 5 re#resent moles of monomer and #olymer res#ectively, 5 is a##roximately 10 /) 4. This #olymer is called #olyethylene rather than #olymethylene, &/6 2/n, because ethylene is a stable com#ound &methylene is not, and it also serves as the synthetic #recursor of the #olymer. The
two o#en bonds remaining at the ends of the long chain of carbons &colored magenta are normally not s#ecified, because the atoms or grou#s found there de#end on the chemical #rocess used for #olymeri%ation. The synthetic methods used to #re#are this and other #olymers will be described later in this cha#ter. 7nlie sim#ler #ure com#ounds, most #olymers are not com#osed of identical molecules. The P molecules, for exam#le, are all long carbon chains, but the lengths may vary by thousands of monomer units. 8ecause of this, #olymer molecular weights are usually given as averages. Two ex#erimentally determined values are common Mn , the number average molecular weight, is calculated from the mole fraction distribution of different si%ed molecules in a sam#le, and Mw , the weight average molecular weight, is calculated from the weight fraction distribution of different si%ed molecules. These are defined below. !ince larger molecules in a sam#le weigh more than smaller molecules, the weight average : w is necessarily sewed to higher values, and is always greater than : n. ;s the weight dis#ersion of molecules in a sam#le narrows, :w a##roaches : n, and in the unliely case that all the #olymer molecules have identical weights &a #ure mono/dis#erse sam#le, the ratio : w < :n becomes unity.
Many polymeric materials having chain-like structures similar to polyethylene are known. Polymers formed by a straightforward linking together of monomer units, with no loss or gain of material, are called addition polymers or chain-growth polymers. A listing of some important addition polymers and their monomer precursors is presented in the following table.
Some Common Addition Polymers
ame!s" Polyet$ylene low density &=P
Formula
Monomer
#ses
soft, waxy solid
film wra#, #lastic bags
Polyet$ylene ethylene >&62/62 n> high density &P 62?62
rigid, translucent solid
electrical insulation bottles, toys
Polypropylene >@62/ &PP different grades 6&6*A n>
#ro#ylene 62?66*
atactic soft, elastic solid isotactic hard, strong solid
similar to =P car#et, u#holstery
Poly!%inyl c$loride" >&62/ &PB6 66l n>
vinyl chloride 62?66l
strong rigid solid
#i#es, siding, flooring
vinylidene >&62/66l2 n> chloride 62?66l2
dense, high/melting solid
seat covers, films
>@62/ 6&63)A n>
styrene 62?663)
hard, rigid, clear solid soluble in organic solvents
toys, cabinets #acaging &foamed
Polyacrylonitrile >&62/ &P;+, Crlon, ;crilan 66+ n>
acrylonitrile 62?66+
high/melting solid soluble in organic solvents
rugs, blanets clothing
Polytetrafluoroet$y lene >&6(2/6(2 n> &PT(, Teflon
tetrafluoroethyl ene 6(2?6(2
resistant, smooth solid
non/stic surfaces electrical insulation
Poly!met$yl met$acrylate" &P::;, =ucite, Plexiglas
>@62/ 6&6*6C26 *A n>
methyl methacrylate hard, trans#arent 62?6&6*6C solid 26*
Poly!%inyl acetate" &PB;c
>&62/ vinyl acetate 6C6C6*n 62?6C6C6 soft, sticy solid > *
latex #aints, adhesives
cis&Polyisoprene natural rubber
>@62/ 6?6&6*/ 62A n>
Poly!%inylidene c$loride" &!aran ; Polystyrene &P!
>&62/62 n>
Polyc$loroprene &ci >@62/ s D trans 6?66l/ &+eo#rene 62A n>
ethylene 62?62
Properties
lighting covers, signs sylights
iso#rene 62?6/ 6&6*?62
soft, sticy solid
reuires vulcani%ation for #ractical use
chloro#rene 62?6/ 66l?62
tough, rubbery solid
synthetic rubber oil resistant
