Polydiacetylenes An Ideal Color System for Teaching Polymer Science Gordhan N. Patel Allied Corporation, Syracuse Research Laboratory, P.O. Box 6, Solvay, NY 13209 Nan-Loh Yang College of Staten Island, City University of New York, Staten Island, N Y 10301
curriculum have been extensively documented (1-3).%e wish to present here a series of experiments which illustrate, via color changes, a broad scope of fundamental phenomena in polymer science. The entire set can he carried out in a few laboratory hours. We have developed a system which, we helieve, is ideal to teach an extensive range of representative physical and chemical reactions in polymer science. This system,1 'mvolving diacetylenes, R-C=C-C=C-R, and their nolvmers. +CR-C--C-CR+-. exhibits readilv ohservad~eEolor changes during polym&ation (41, degradation (. 5.,) . . phase transitions 16). dissolution (7. . , 8). ., eelation (9). ion-exchange (10,11),and conformation changes (7-11). The visible nature of these transformations can be used to ereat
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thus he extremely u s e h pedagogically in t h e teaching of ~ o l v m escience. r The series of experiments presented in this paper requires no more than test tubes, filter paper, a heat source (a hot plate or a hair drier), and a UV source (direct sunlight or a- 20 watt short wavelength UV lamp). The monomers, required only in extremelv small auantities (about The experiments are suitable for students frbm high school level up to graduate level. Recently we had groups of high school students perform this series (12). They responded to the laboratory work with great enthusiasm and derived enormous satisfaction from the experiments. In the present paper, we describe some typical experiments on diacetvlenes and their . ~olvmers. No a t t e m ~ist made to " review the work on diacetylenes. Thorough reviews have been periodically published (13-15). Synthesis (16) of the diacetylenes used here and mechanisms of several processes such as polymerization (17), dissolution (81, crystallization ( 7 , 8 , 18),conformation changes (7-181, and degradation (19)have been described in detail elsewhere. Synthesis of diacetylenes and energetics aspects of all processes described here will he published elsehwere in a format suitable for teaching purposes 1201. ~-,
We have synthesized more than two hundred diacetylenes and their polymers. There is no single diacetylene which undergoes all possible physical and chemical reactions. However, 3BCMU and 4BCMU with substituent group -(CHz)a,~OCONHCH~COO(CH~)~CHa cover the broadest scope and are ideal for demonstration. The visual processes --
' M:ireua s dcscrioed in lnls paper nave not
)?I w e n aucq~alei) eo r\ lh regnrn lo the r lox c 11. Avo,o inhaimon and o rccl conlocl with skin. We recommend that laboratory safety procedures such as use Of a fume hood described in the following reference to be implemented. (1) Handbook of Laboratory Safety,The Chemical Rubber Company, Cleveland. Ohio, 1978. (2) J. CHEM. EDUC., 55, A71, A337 (1978) EDUC., 57, 203 (1980). (3) J. CHEM.
51.0
of 3BCMU have been descrihed elsewhere (21). Here we will focus on 4BCMU. Althoueh not all ~olvmersbehave exactly like polydiacetylenes, the kderlyin~pr~nciples conveyed here in this series are, of course, au~licable in general terms to all -. other polymer systems. Polydiacetylenes and Colors Typically, disuhstitnted diacetylenes, R-C=C-C=C-R, are solid. The suhstituent groups can he -(CH,),OH, -(CH2),OCONHY, -(CH&,OSO2Y, -(CH2),COOH and -(CHe),COOY, where Y is a substituted or unsuhstituted alkyl or aryl group. Diacetylenes polymerize in the solid state (4,14,15) either upon thermal annealing or upon exposure to high energy radiation such as ultraviolet light, X-ray and Gamma ray (see Fig. 1A and 1B).During polymerization, the crystal structure of the monomer is usually retained. This leads to nearly defect free single crystals of polymers. Polydiacetylenes have a highly conjugated backbone. The color of the oolvmer chain is due to this unsaturated backbone:
T h e Color Plates S h o w n on t h e Cover The color plates accompanying this al?icieare printed on the cover of this issue. Below are descriptions of each. Color Plate 1: Colorless solid coating of 4BCMU [R = ( C H z ) n O C 0NHCH&OO(CH2)3CH3] obtained by spraying five percent solution in ethanol and letting the solvent evaporate.Small black dot is a reference spot. Color Plate 2: After partial polymerization by a short wavelength UV lamp. Color Plate 3: After exposing the left-handside of the coating for about ten seconds for more than 50% polymerization. Color Plate 4: After heating the coating io about 75% (M.P. of the monomer). Plate 5: After heating the coating to about 1 4 5 T (M.P. of the
CDlOr
polymer). Color Plate 6: Aftercooling the coating to room temperature. Color Plate 7: After reexposing the coating to UV light. Color Plate 8: After dissolving the polymerized coating of photograph #7 in methylene chloride (a good solvent). Color Plate 9: After adding henane (a nonsolvent) in the yeliow solution of photograph 8. Addition of excess nmsolvent Causes precipitation of polymer (not shown). Color Plate 10: About 0.1 g of Poiy4BCMU was dissolved in c-dichlorobenzene (a poor solvent)to form yellow solution at 120DC(not shown). me solution was allowed to cwl at room temperature.The yellow solution iurned Into red colored soiid gel. The gel is so solidmat the beaker can
be turned upside dawn. Color Plate 11: The side groups of paly4BCMU were saponified by adding KOH solution in the yellow solution of photograph 8. The polymer. Poiy4KAU[R = -(CH&OCONHCH&OO-Ki] is now polyelecirolyte and is Soluble in water to form yellow solution. Color Plate 12: After adding dilute HCI in the solution in Color Plate 11. if the pH is below 2, the polymer precipitates (not shown).
Volume 60
Number 3
March 1983
181
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Figure 2. Hydrogen bonded planar confnmation of urethane substituted polydiacetylenes. backbone can acquire a butatriene structure
R
I
+CxC=C=Ctx,
I
(14,15)
R
carbon atoms overalp. As a result the %electrons are delocalized along the backbone. The effective conjugation length "l,," over which the n-electrons are delocalized determines the color. A relationship between the color of the polymer and effective conjugationlength is explained in Table 1(22).There is no sharp demarcation line between different colors or between the varying 1,'s. I t suffices to state here that polymer molecules with electrons delocalized over a longer coniuaated " " system, i.e., with a higher l,, absorb a t longer wavelengths and aonear blue. If the electrons are delocalized over a shorter conjugated system, i.e., for lower l,, the polymer molecules absorb at shorter wavelenaths and aDuear vellow. Polvmer
!
..
the functionalities of their side groups are capable of hydrogen bond formation. Fieure 2 illustrates the general hvdroaen" bonded structures of polydiacetylenes. There are two hydrogen bonded chains, one on each side of the backbone. These hydrogen bonded chains keep the polymer backbone in one plane. Some of the important conformations the polymer chain can assume are shown in Figure 3. The backbone, the hydrogen bonds, the side groups, and the substituent Y of the polymer molecule are depicted in Figure 3 as the central zigzag line, two vertical dotted lines, horizontal solid lines, and heavy dots, respectively. The spatial arrangement of hydrogen bonds dictates the conformation and hence the color of the backbone. In the case of 4BCMU, the monomer molecules in the crvstal are nacked in such a wav that thev are hvdroaen . bonded. During polymerization, these hydrogen bonds remain intact (Fig. " 2). In order to retain the substitution solid solution, the backbone is compressed and planar in the case of partially polymerized 4BCMU, (Fig. 3A). A planar and compressed molecule of polydiacetylene will appear blue (Table lA), because the 1, is far greater than thirty repeat units. If the solid solution is destroyed, the backbone can then acquire a stretched conformation (Fie. 3B). The solid solution can be destroyed either by melting or dissolving the unreacted monomer molecules. The stretched conformation with a decreased l,, will lead to red color (Table 1B).The planarity of the backbone can he destroyed, if the hydrogen bonds are broken. The backhone can then rotate around the single bonds 182
Journal of Chemical Education
(PLANAR-COMPRESSED)
(PLANAR-STRETCHED)
A
B
(HIGHLY INTERRUPTED)
C
Figure 3. Some imponant conformations of urethane substituted polydiacetylenes. The color of the backbone is indicated.
Table I. Colors of Polydiacetylene Molecules and Theil Conformations A. Blue Color (I,=
> 30 r.u!l
1) Planar and Unstralned 2) Planar and Compressed 3) Planarity Periodically Interrupted (Planarity 30 r.u.1 8. Red Color (20 r.u. I, 10 r.u.1 1) Planar and Stretched 2) Periodically Interrupted (20 >Planarity 15 r.u.1 3) Slightly Twisted 4) Planar Butatriene Structure 5) Low Concentration of Structural Defects C. Yellow Color (I, 6 ru.) 1) Highly Nonpianar 2) Highly Stretched 3) Highly Twisted 4) Planarity Periodically Interrupted (Planarity < 6 r.u.) 5) lsomerization With the Side Groups 61 Hioh Concentration of Structural Defects
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