Introduction to Polymer Chemistry Frank W. Harris Wright State University, Dayton. OH 45435 Polymers are extremely large molecules that are essential to our verv existence. Thev are a main constituent of our food (starch, protein, etc.), our clothes (polyester, nylons, etc.), our houses (wood cellulose, alkvd , and our bodies . ~. a i n t setc.), ~pdytnucleicacids), pruteins, etc.). Hence, it is rwamal~leto assume the educntion of every chemist shcdd, at least, include an introduction to their chemistry and properties. The obiectives of this paper are (1) to introduce the reader to the types of chemical reactions that are used to prepare polymers and (2) to acquaint him or her with the structural parameters that result in the unusual physical properties displayed by these molecules. Polymer Synthesis There are two maior w e s of ~olvmerization methods used . t ". . to convert small molecules (monomers) into polymers. These methods were originally referred to as addition and condensation polymerization. Depending on the author, addition ~olvmerizationis now called chain, chain-prowth, or chainieaction polymerization. Condensation is now referred to as step-prowth or step-reaction polymerization. T h e major distinEtionj hctnern these two methods. which, hupefully, will become apparent i r m the tollowing d:icussion. result irum the differenrrs in the kinelics of the p d y m e r i ~ s tion reactions. Chain-Reaction A Polymerization (Addition) The monomers normally employed in this type of polymerization contain a carhon-carbon double bond that can oatticinate in a chain reaction. As in the chain reactions studied in organic chemistry, e.g., the free-radical halogenation of alkanes. the mechanism of the ~olvmerizationconsists of three distinct steps. In the hitiition Step an initiator molecule(s) is thermally decomposed or allowed to undergo a chemical reaction to generate an "active species." This "active species," which can he a free radical, a cation, an anion, or a coor~clinationcomplex, then'initiatri the polymerization by adding to the monumrr's carhn-carbon duuble bond. The reaction occurs in such a manner that a new free radical, cation, anion, or complex is generated. The initial monomer hecomes the first repeat unit in the incipient polymer chain. In the Propagation Step, the newly generated "active species" adds to another monomer in the same manner as in the Initiation Step. 'rhis procedure is repentrd ovw and over again until thr find step of the vrw:rss, Termination, occurs. In this step, the growing chain'terminates through reaction with another arowina chain, bv reaction with another species in the po~ymerEationkixture,br by the spontaneous decomposition of the active site. Under certain conditions, anionic and coordination polymerizations can be carried out without the Termination Step to generate so-called "living" polymers. The following are several general characteristics of this type of polymerization: 1) Once initiation occurs, the polymer chain forms very quickly, i.e., I." n-1 to .. t..n-6 w ...
The~mteutrotionuf"acti\r sl,wirr" is wry low. Pcr rrnml~lr,in free radic.rl polymrri~arwnsthe a m e n t r s t w n ut trrr rad~calsis appnrxim;mly Ib-'.II. Ilenre,thep~~lvtneri,ario~~ mixlurr n.nclit.< primarily of newly-formed polymer and unreacted monomer. 3) Since the carbon-carbon double bonds in the monomers are, in effect,converted to two single carbon-carbon bonds in the polymer, 21
Table 1. Typical Polymers Produced by Chain-Reaction Polymerizations Chemical Name
Repeat Unit
Applications
Polyethylene
+CH*-CH.t
film, housewares
Polypropyiene
-tCH,-CHt
rope, automotive and appliance
I CH ,
Polystyrene
*HA-CHt
Poly(viny1 chloride)
I I C,,H; +CH~-CH+
I CI
parts
packaging, insulation floorcovering, wire and cable insulation
CH.,
I
-tCH.-CHt
Poly(methy1 methacrylate)
I
CO CH,
Poly(viny1 alcohol)
-HI--CHt
I
automotive parts (tail and signalhght lenses, etc.),display signs water-soluble thickening agent
Table 2. Typlcal Copolymers Produced by Chaln Reaction Copolymerizations Comonomers
Common Name SBR rubber
ABS resins mod acrylic^
A~olications
CH2=CH-CH%H,. C6HsCH=CH2 tires, shoe soles CH2=CH-CN, CH,%Happliance housings CH=CH2. CeHsCH=CHt CH2%H-CN, CH2=CH-CI clothing CHA
ionomew
I ,CH,--C-C0,H (acid group converted to metal salt in
CH.=CH,.
packaging
polymer)
energy is released making the polymerization exothermic with cooling often required. 41 Chain-reactions normallv afford . oolvmers . with hieh " molecular weights, i.e., 1O4-lo7. 5 ) Polymers can be obtained that contain secondary chains (branches)attached to the main chain (backbone).Far example, free radicals sometimes abstract hydrogens from a formed polymer chain, thereby generating new free radicals along the backbone which initiate secondary polymerizations. 6 ) Crosslinked systems can form where all the primary chains are interconnected with secondarv chains. For examole. erowine
Two or more different monomers are often employed in a chain-reaction polymerization to yield a polymer containing the corresponding repeat units. Such a process is referred to as copolymerization, and the resulting product is called a copolymer. By varying the copolymerization technique and the amounts of each monomer, one can use as few as two monomers to prepare a series of copolymers with considerably different properties. The amount of different materials that can be prepared increases dramatically as the number of monomers employed increases. Thus, it is not too surprising that the majority of synthetic polymers used today are copolymers. The following are four different types of copolymer structures that have been prepared: Volume 58
Number 11 November 1981
837
--
-
CHAIN REACTION A POLYMERIZATION
Propagation
Initiation Free Radical
Free radical
0
II
+
C.RC-0-CH-CH,
CH,=CHI
-
0
-
II
C,,H,COCHCH,CH,CHI
etc.
Cationic CHI CH-ii
Cationic Il 0
+ RF,
HifBF~OHl-
I CH ,
CH,
1BF.OHI
I + CH2=C I
-
CH,
CH,
CHs
I
I CH,-C-CH?-C' I CH
(BF,OHIF
ete
I CH,
Anionic
Anionic N~NH,
UH,
N~+NH,
Coordination Complex CH, CH,
\
Coordination Complex
I
CHGH,
/
FH
I
CH,
I
CH ,
Termination
Complex Precipitate
CH ,
CH,
1
CH
"?Hi
\ "' /
.I/ + CH,=CH ~.~ \ I'H
Ti
; '
-CH,
CH,
A-& '$6 y; - il/ /
..~ \ CHI , :
-
CH,CH,
\Ci
I
2 R-CH,-CH;
\T{'
\y'
/ ,.\ ,,, \
I
CH,,
CH: I I CH.,
R-CH.CH,-CHCH?-R
Cationic
CHI
I
I
CH,
-
Free Radical
CH:,
-
I R-CH9-CtlBF,OH)F I
CHA R-CH,-C=CH?
I
+ H+(B&oH)
CH,
Anionic R-CH,-CH-NB*
I
+
CJL
NH,
-
R-CH,-CH,
I C..H,
+ NH?.
Nsi
Coordination Complex 1) Random copolymer. The repeat units are randomly distributed
along the polymer backbone. -A-A-B-B-B-A-A-B-A-B-B-B-B-A-A-B-A-A-A-A-B-A2) Alternatina co~olvmer.The repeat units are located in alternating positions along the backbone; -A-R-A-B-A-B-A-tl-A-B-A-B3) Block copolymer. The repeat units are located in long alternating segments. -A-A-A-A-A-A-A-B-B-8-B-B-B-B-B-B-A-A-A-A-A-A.A4) Graft copolymer. Branches containing one repeat unit are attached
to the main chain, which contains the other unit. -A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-
I
B
I
B
I
B
I
B
I
B
I B I B I B I
I I B 1 B
B
B
838
Journal of Chemical Education
StepReaction Polymerization (Condensation) This type of polymerization normally employs two difunctional monomers t h a t are capable of undergoing typical organic reactions. For example, a diacid can be allowed to react with a diol in the presence of an acid catalyst to afford a polyester. In this case, chain growth is initiated by the redction of one of the diacid's carboxyl groups with one of the diol's hydroxyl groups. T h e free carboxyl or hydroxyl group of the
STEP-REACTION POLYMERIZATION
L
resultine dimer can then react with an appropriate functional is repeated group inanother monomer or dimer. ~ h i i throughout the polymerization mixture until all of the monomers are converted to low molecular weight species, such as dimers, trimers, tetramers, etc. These molecules, which are called oligomers, can then further react with each other through their free functional groups. Polymer chains that have rnode;ate molecular weiehtsian he built in this manner. The .. htgh molecular weights common to chain-reaction polymerizations are usuallv not reached. This is due to the fact that as the molecular weight increases the concentration of the free functional groups decreases dramatically. In addition, the groups are attached to the ends of chains and, hence, are no loneer capable of moving- freely through . the viscous reaction medium. The followine are several general characteristics of this type
Conversely, the reactivity of pendant functional groups can be substantially increased by the presence of suitable neighboring groups. For example, the rate of transesterification of pendant phenyl-ester groups surrounded by pendant carhoxyl groups is many orders of magnitude faster than that of analogous phenyl acetates (7). ~
~
4) Step-reaction polymerizations normally afford polymers with moderate molecular weights, i.e., eparate p a r a m e k s are thenbetermine2 in solvents in each class and the composite solubility parameter (60) calculated from the following expression: 60%= an2 + aP2 + 6h2
where V is the total volume of the mixture, AEvl and AEv2 are the vaoorization enereies of comoonents 1and 2. VTand V ztheir molar volumes, A d 81 and & their volume fiac&s. Hence, the probability that a polymer and a solvent will he miscible increases as the difference between their 6's decreases. Although solvents are generally miscible if this difference is f3.5 (cal/cm")'", the difference between the 6's of a polymer and solvent can usually he no larger than f 1.5 (cal/cm3)'" for solution to occur. The solubility parameter for a mixture of solvents (?I,,) is approximated by the following equation: X1V161+ X2V9.82 x 1 v 1 + X2V2 where X is the mole fraction and V the molar volume of each component of the mixture. Polymers are often dissolved in 6,,
=
Literature Cited (11 Williams. David J.. "Polymer Science and Engineering." Prentim-Hall. Englawood Cliffs,NJ, 197I.p 19.
(21 R o d r i ~ e s Ferdinand. , "Prineipln of Polymer Systems.l. MeCrsw-Hill, New York, 1970. p p 35.39.
131 Rawe, A,, "Oqsnie Chemistry of Manomolecu1es.l. Marml Oekker, New York. 1967. (4) Billmeyer. Fred W , "Textbmk of Polymer Science."2ndEd.. Wiley-lntuluenm,New York, 1971. (51 Kaufman, Herman S., and FslmLfs, Joseph J., IEdilorsl. '"lntrduetion to Polymer Science sndTechnology," Wiley, New York. 1977. (61 MeCsffery, Edward M.. "Laboratory Preparation far Macromolocuiar Chemistry," McCraw-Hill. NevYork. 1970.p. 14. (71 Morawotz, H.. and Zimmering, P. E., J. Phys Chsm., 58.753 (19511. (81 Zulty, N. L..Faueher, LA., and Bonotto, S., in " E n ~ y ~ l ~ p e d iP~ i Iiym f ymymk i i i m a n d Teehnolngy," Vol. 6. (Editor: Bikales, N. M1 . Wiley, New York. 1967, p. 420. (91 Uhlmann. D. R., and Kolbeck, A. C..Scienti/icArnerkon. 233.96 (19751. (101 Cell,P.,JEMMSE.3.1 119811. (Ill Hansen,C. M . J . Point Tech..39,104 (19671. (12) Brandrup. L a n d Immergut, E. H., (Editors1,"Polymer Handbmk."2nd FA., Wiley. New York. 1975.
Volume 58
Number I I
November 1981
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