INTRODUCTION TO POLYMER LABORATORY EXPERIMENTS Granular Polymerization of Methyl Methacrylate RICHARD H. WILEP University of North Carolina, Chapel Hill,North Carolina
THISmaterial is part of that furnished the author's students in a laboratory course in polymer chemistry. About twenty experiments have been designed to provide experience with: (1) manipulation and testing of polymers, (2) principal types of polymers, and polymerization reactions, and (3) chemical reactions of polymers. A short discussion provides a theoretical background, somewhat like that found in the GatterrnanWieland text, for each division of the work and for each experiment. The material which follows is selected from sections introducing the student to condensation and addition polymerization and t o an experiment on granular polymerization as one of the addition polymerization methods. The experiment is that used by the author's students t o illustrate granular polymerization.
In this equation R is a divalent radical such as -CHeCH2CHpCH2CH2-. The terms condensation polymer and addition polymer are often used as names of the products formed in the respective processes. This terminology is confusing because in some cakes polymers with the same structural units can be formed by either a condensation polymerization or an addition polymerization. Thus eaminocaproic acid and carpolactam form polylners with the same unit.
DEFINITIONS
It would seem that the same polymer is to be named either a condensation or an addition polymer depending on the method by which it is prepared. To avoid this apparent inconsistency condensation polymers have been defined as in the preceding paragraph with the further qualification that they can be degraded to monomers differing in composition from the structural unit (5). The linear polyamide cin be degraded by hydrolysis to eaminocaproic acid. A molecule of water is added to each unit to form the monomer in a reversal of the condensation. Therefore, the polyamide is properly classified as a condensation polymer. This expanded definition, it may be noted, includes proteins and cellulose as condensation polymers. The division of polymer chemistry into condensation and addition types is useful because the reactions by which the two types are formed are fundamentally different. Addition polymerization is a chain reaction which proceeds with extreme rapidity. Each chain is formed rapidly and, as a result, low molecular weight products are not usually isolatable. If the reaction is interrupted before completion, unreacted monomer and high molecular weight polymer are obtained. On the other hand, condensation polymerization is a reaction which usually proceeds slowly. The poly-esterification reaction has been shown t o be kinetically similar t o monomolecular esterification (8)and there is reason t o believe that other poly-condensations proceed as do related monomolecular condensations. The reaction can be considered simply as a reaction of functional groups.
Polymers are substances, usually of high molecular weight, which contain recurring structural units. For instance, the unit in polyethylene is -CHzCH,while that in a polyamide is -CONH(CH2),-. These units are repeated hundreds or thousands of times to give typical polymers. Literally, however,.the word polymer means many parts and the trimer of formaldehyde is as much within this literal meaning as is a polyethylene with 1000 units in its chain. The phrase "high polymer" has been adopted to.provide a name more clearly descriptive of the high molecular weight polymers. The use of this term is helpful in emph* sizing that there is a basis for such a distinction even though the question as to what is meant by "high" remains to be answered. Polymerization is an intermolecular combination functionally capable of proceeding indefinitely (1, 2). Addition polymerization is a combmation of unsaturated molecules. For example, carbon atoms linked by a double bond are converted to carbon atoms in a long chain. Thus, ethylene, CH2=CHz, is converted to polyethylene,-CH~CH~(CH2CH2)~-CH~CH2-. Condensation polymerization is a combination of reactants with the loss of some simple molecule such as water. Thus, the amino and carboxylic groups in an amino acid condense with elimination of water to form a linear polymer: zNHlRCOzH
-
-NHRCO(NHRCO),-INHRCO-
+ XHSO
zN&C&C&C&CH2CH2COzH
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If the polymerization is intemupted before completion, a low molecular weight polymer is obtained; not a mixture of monomer and polymer. Furthermore, heating a low viscosity polymer, such as a polyamide in which amide formation proceeds by heating, will continue the polymerization until a very high viscosity results. To prepare "viscosity?stable" condensation polymers it is necessary to add a trace of a nionofunctional reactant to furnish end groups for the short chain. ADDITION POLYMERIZATION
In addition to polymerization the intermolecular comhination capable of proceeding indefinitely is the comhination of one unsaturated molecule with another. The reaction is characteristic of a great variety of unsaturated compounds and is one of the most important general .reactions of organic chemistry. In Table 4, there are listed a variety of polymerizahle and nonpolymerizable monomers. Many others might he included since attempts have been made to polymerize most known types of unsaturated compounds. Even so, no general statement relating structure to polymerizability has heen formulated. Attempts at such generali-
zations areunsuccessful in explaining such facts as the failure to obtain high molecular weight products from propylene when both ethylene and isohutylene are readily converted to such. Price (4) has presented a tentative explanation for some similar problems in relating structure to copolymerization tendency and Thompson (5) has collected,many of the facts. Addition polymerization has the characteristics of a chain reaction. The reaction is extremely rapid. If interrupted before completion only monomer and polymer--no dimers, trimers, etc.-are found. The reaction is subject to initiation, surface effects, and inhibition as are other chain reactions. The two commonly used types of initiators are free radical generators, such as hydrogen peroxide, henzoyl peroxide, or sodium persulfate, and ionic reagents of an electrophilic type, such as aluminum chloride or boron trifluoride. The use of the minimum amount of initiator gives the highest molecular weight product since fewer chains are competing for available monomer. Similarly a t the lowest effective temperature, where rate of decomposiition of initiat.or is lowest and concentration of initiating radicals is therefore l o ~ ~ e sthe t , highest molecular
--
TABLE 1 Polymerizable Unsaturated Compounds Ezamplest Polymerizable
Types 1. Unsubstituted types
.
No?lpol~lrnerirable
2. Monosubstituted ethylene. RCH=CH2
3. 1,2-Disubstituted ethylene.
4.
1,l-~isubstituted ethylene.
RCH=CHR
R2C=CH9
CHFCCL (or F.) (CHs)aC=CH? CHx=C(CHJCO?R CH1=CCIC02R CH2=C(CHa)CH=CHs CH-CCICH=CH3 CH2=C(CHa)C(CHa)=CH, CeHs(CHJC=CH2 (CeH,),C=CI12 (ROIC),C=CHX
5. Trisubst,ituted ethylene
6. Tetrasubstituted ethylene8
t If s. compound f o r m high molecular weight polymers, it ia listed as palymeriaable; conversion to low molecular weight products is not considered polymerization. * Copolymerisability has been reported.
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weight polymer is formed. Effective inhibitors are the polyhydroxy and polynitro aromatic compounds. The mechanism of the free radical type polymerization has been the subject of much study. Four steps have been postulated as parts of the over-all reaction. These are initiation, propagation, cessation, and chain transfer. In the initiation step the free radical generator dissociates into free radicals. In propagation the free radical unites with monomer to form a new radical and the process is repeated until a long carbon chain is produced. In cessation, the radical at the end of the polymer chain is destroyed, for example, by disproportionation or combination. Chain transfer takes place when the activity of the free radical is transferred to another molecule. . These may be summarized:
-
1. 1nitiat.ion:
(PhC0)10* 2. Propagation: Ph'
+M
3. Cessation:
PhCO2.
-,, -
PhM.
+ Ph' + COX
(x-l)M
PhM.
by combination:
RCH d H , + RCH2CHa where M is CH.CH%
2PhMSm
FaBYH Me$+
BPS + YH + CH2=CMe2 + CH?=CMet
---
FsBYH FaBYMe&+ Med-CH?-CMeaC,
+
etc.
Several methods are available for radical type addition polymerization. Four widely used types, based on the physical systems used, are: (1) bulk, in which a liquid monomer and catalyst are converted to a polymer; ( 2 ) solut,ion, in which a solvent is added to the bulk system; (3) emulsion, in which the liquid monomer and catalyst are dispersed in another phase, usually water; and (4) granular, in'which droplets of liquid monomer are polymerized in another phase, usually water. All of these have been adapted to the polymerization of normally gaseous monomers under pressure. The methods are all of general utility with the qualification that aqueous emulsion techniques c a n n ~ be t used with water-sensitive ingredients such as aluminum chloride initiator or vinyl ether monomers. GRANULAR POLYMERIZATION
by disproportionation:
2RCHzCHx. R is PhM,. -
trifluoride acts as a transfer agent which transfers a proton to the unsaturate to form a carbonium ion which is is the chain initiator. These reactions are summarized:
Ph.M.Ph
4. Chain transfer:
This mechanism is supported by various types of evidence (6a). For instance, fragments of initiators and inhibitors have been identified as terminal groups of the polymer chain. Carbon tetrachloride has been shown to react according to the chain transfer reaction with styrene (6b),and with vinyl acetate (7), and with ethylene (8) while carbon tetrabromide and bromoform react similarly with olefins (9). The products in these transfer reactions may be of low molecular weight, where x is one to ten, or may be much higher in molecular weight, where x is about fifty (10). The mechanism also explains the kinetic data which show the reaction to be first order with respect to monomer and half order. with respect to initiator concentration; i.e,
Much less is known about the mechanisn~of ionic type addition polymerization. For many years it was assumed that the catalyst, boron trifluoride or aluminum chloride, for example, combined or reacted with the monoher to form a carbonium ion in the initiating step (11, 13). Recent studies ( I S , 14, 15) have shown that pure boron trifluoride does not initiate the polymerization of isobutylene in the absence of some third material such as water. This finding has been made the basis of a mechanism which proposes that the boron
In the following experiment methyl methacrylate is polymerized by the granular technique. This technique is sometimes called. the suspension or pearl method (16). It has been used with vinyl acetate, methacrylates, acrylates, styrene, and ethyl acrylatestyrene copolymers (16, 27,18,19, $O,21, $9, $3,). The system used is somewhat like that used in emulsion polymerization. The monomer is suspended in droplets 1 to 5 mm. in diameter in an aqueous medium. Additional substances are added as dispersing agents to keep the droplets, which become sticky as the polymerization proceeds, from coalescing. Such substances are: bentonite, particles of which adhere to the droplets; glycerol, which increases the viscosity of the aqueous layer; or a colloidal substance such as starch ($0) or sodium polyrnethacrylate (18). These substances, sometimes most effectively used in combination, all prevent coalescence of the droplets. The polymerization then proceeds as a multitude of bulk polymerizations rather than an emulsion polymerization. This procedure is very useful since the polymer is prepared in a pure form, is easy to handle, and is ready for use as a molding powder without further operations. Preparation of Dispersing Agent. ' Sodium polymethacrylate is prepared as follows. Stir 25 g. of methacrylate with 500 ml. of cone. sulfuric acid for four hours on the steam bath. After standing overnight, pour the black solution onto ice to precipitate the polymethacrylic acid. Wash the precipitate three times by thorough mixing with water. Dissolve in a solution of 10 g. of NaOH in 200 ml. of water and dilute to 600 ml. Preparation of Monomer. Distill the methyl methacrylate monomer uuder aslight vacuum just prior touse to free it of stabilizer. Collect under oxygen-free nitrogen and keep cold until used. Polymerization. Dilute 25 ml. of the sodium poly-
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methacrylate solution t o 150 ml.. and place the diluted solution in a 500 ml. three-necked flask equipped with sealed stirrer, condenser, and thermometer. Place a buffer solution, prepared from 0.85 g. of disodium phosphate and 0.05 g. of monosodium phosphate in 5 g. of water, in the flask. Dissolve 0.5 g of benzoyl peroxide in 50 g. of the distilled methyl methacrylate and place this solution in the flask. Operate the stirrer a t aaspeed just sufficientto keep the ester in the form of drops 2 or 3 mm. in diameter. Heat in a water bath to 80-82°C. for 45 minutes to complete the polymerization. Collect the granules of polymer on a filter, wash with water, and dry. The size of the granules is determined by the speed a t which the stirrer is operated. Very rapid stirring will form an emulsion, not granules.
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Chemistry, Vol. I, High Polymers," Reinhold Publishing Corporation, New York, 1945: (a)pp. 37-45; (b)pp.43,116 (7) HARMON, J. H., U. S. Patent 2,396,261 (1946). (8) Imperial Chemical Industries, Brit. Patent 581,899 (1946). Chem. Abstr., 41, 3477 (1947). (9) KHARASCH. M. S., E. V. JENSEN,AND W. H. URBY,J. Am. Chem. Soc.,68, 154 (1946). (10) PETERSON, M. D., AND A. G. WEBER,U. S. Patent 2,395,a"* '3' (Nro,. (11) C. c., N . Y.~ ~ s&.,~44, 368 d (1943). . (12) MARVEL,C. S., AND E. C. HORNING, in H. Gilman, Editor, "Organic Chemistry." .. 2nd ed.. John Wilev and Sons. New York, 1944, p. 776. AND M. POLANYI, J . Chem. Sac.,252, 1947. (13) EVANS,A. (14) PLESCH,P. H., M. POLANYI,AND H. A. SKINNER,ibid., 257. 1947. (15) EVAN;, A. G., AND M. A. WEINRERGER, Nature, 159, 437 (1947). 06) HOEENSTEIN, W. P., AND H. MARK,J . Polymer Sei.,1, 127 ,.A,,.>
IlOdfii
LITERATURE CITED (1) CAROTHERS, W. H.,Cheni. Rev.,8, 358 (1931). (2) CAROTHERS, W.H., Tram. Faraday Soc.,32, 41 (1936). (3) FLORY,P. J., Chem. Ra,., 39, 137 (1946). (4) PRICE,C. C., J . Polymer Sci.,1,83 (1946). (5) BUEK,R. E.,.H. E. THOMPSON, A. J. WEITB, AND I. WIP LUMS,"Polymerization," Ileinhold Publishing Corporation, New York, 1937, pp. 13-52. (6) P r u c ~C. , C., in S. B. Twrss, Editor, "Advancing Fronts in
U. S. Patent 2,108,C R A ~ O RJ. D W. . C., AND J. MCGRATE, 044 (1938). MARKS,B. M., U. S. Patents 2,122,886 (1938). CRAWFORD, J. W. C., AND J. MCGRATH, U. S. Patent 2,191,520 (19401. D I ~ M A EH; , R., U. S. Patent 2,163,305(1939). HILTNER,J. R.,AND W. F. BARTOE,U. S. Patent 2,264,376 11941 - -- \,. White, J. O., U. S. Patent 2,401,445(1946). Fletcher, D.A.,and F. L.JOHNSTON, U. S. Patent 2,418,828 \
