POLYMERIZATION CHARLES C. WINDING CORNELL UNIVERSITY, I T H A C A , N. Y .
T
HE use of the polymerization reaction for the production of commercial products has expanded so rapidly during the last 10 years that polymerization has become one of the most important unit processes. The remarkable expansion of the vigorous plastics industry; the tremendous impetus of the wartime synthetic rubber program; and the steadily increasing pressure on the petroleum industry to produce more gasoline, have focused attention on polyrncrization in a manner that is secondary only to that given atomic fission and radioactive reactions. This increasing importance ha9 resulted in a correspondingly greater amount of research and number of publications dealing with polymerization, until the literature on this process has become so voluminous that a short review cannot hope to cover it completely. The situation as it existed prior to 1942 is reviewed in many recent books dealing with polymerization, high polymers, or plastics and resins (22, 26, 62, 65, 66, 85, 89, 92, 97, 106, 117, i20, 1255). These books provide good coverage of the types of polymers and the chemistry of the reaction, but only infrequently was a n attempt made to describe processes and equipment (66, 106, 126). Although the work of various investigators prior to this period laid the foundation for t h e present conccpts regarding the mechanism and the kinetics of the polymerization reaction (58, 47, 61, 90,92, 94, 100, 131),important advances and more general agreement have occurred during the past 6 years. This review on the unit process of polymerization will endeavor to cover present concepts, methods, equipment, and important developments that have occurred since the start of the war. It is accepted generally that the process of addition polymerization differs fundamentally from condensation polymerization. Addition polymerization is accomplished by a series of reactions that may be interrelated in an extremely complicated manner so that the over-all result differs markedly from simple, nonrepetitive reactions involving the same functional groups. The polymer produced has a recurring structural unit that is identical with the monomers from which i t is formed. Polycondensations, on the other hand, involve a repetitive sequence of simple condensation reactions which usually proceed i n a n orderly stopwise fashion (72), particularly if the polymerants are limited to bifunctional molecules. Polymers formed by condensation reactions have recurring structural units which lack certain atoms present in the monomers from which they are formed. If the reactants are limited to only two reactive groups per molecule, polycondensation reactions form chain polymers that in many respects resemble linear addition polymers, but the mechanism and kinetics of the reaction by which they are formed are entirely different. Because of these differences, addition and condensation polymerization will be considered separately although a large percentage of the. references include information on both types of polymeric reactions.
cessation. To this should be added transfer reactions (38), which, although not required to explain the formation of addition polymers, do play an important role in the kinetics of the reaction. These reactions may be represented as follows where M represents an ethenic monomer, X a solvent molecule, and the asterisk indicates a n activated monomer or growing chain:
1. Initiation or activation
M-M* 2. Propagation
M*
AM:
-Mn-l]
M : +Mm-Mvz M,*+M -M,+M* M:+X -M,,+X*
+ MZ
4. Cessation
M,*+M;f,-Mn+m M i M2Mm
+
+
M m
Transfer reactions also can be considered t o be cessation reactions in the sense that they terminate the growth of any particular chain, but they have no effect on the over-all rate of polymerization because a new active nucleus is produced as each growing chain is terminated. However, they may play a n important role in the formation of branched chains (94). This type of addition polymerization usually is called vinyl polymerization as vinyl compounds of the type
X
/ cHz=\
ordinarily form the monomers, but it has been pointed out (109, 149)that other structures involving the double bond also undergo this type of polymerization. The mechanism of the initiating reaction has been a subject of much discussion and investigation (1,16, 19,56, 43,48, 60,67, 89, 109, 116, 129, 182, 138, 140). As most commercial polymerization reactions employ peroxide catalysts, and i t has been shown that peroxides decompose to produce free radicals (20, 61, 96, 111, ldl),the free radical process of activation is one of the most important methods of initiating polymerization reactions, although at least three other methods of activation are possible (109). The decomposition of peroxides into free radicals can be represented as follows: (RC02)Z
ADDITION POLYMERIZATION Poly-addition reactants are practically always ethenic compounds and the polymerization reaction involves carbon-carbon double bonds. This reaction, in contrast to the polycohdensation reaction, differs markedly from nonrepetitive reactions involving carbon-carbon double bonds. It has been shown (48, 89,107,109,133,140) that this reaction is actually composed of three entirely different reactions: initiation, propagation, and
+ [M
3. Transfer
RCOz.
+ R . + GO,
The free radical then is capable of adding on to a monomer to provide the nucleus for the growing chain:
Re + M - R M * The active center is transferred continually to the end of the chain; this permits monomers to add on in rapid succession until the cessation reaction stops the growth.
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RMn.
+ RMm.
RMn
+ RMm
Ample evidence has been accumulated to show that catalyst residues appear in the finished polymer (16, 30, 71, 102, 112, 115, 149). T o this extent, peroxides arc not true catalysts because they do not remain unchanged throughout the reaction, but the amounts u.sed are so small that these initiators are usually called catalysts. The olefinic double bond also will undergo simple addition reactions through polar or ionic mechanisms. This method of activation is important in low temperature polymerizations xvhere boron fluoride, aluminum chloride, titanium tetrachloride, or st,annic chloride catalysts are used (I, 26, 100, 103, 109, 139, 147). The monomers activated by this type of catalyst contain groups promoting release of electrons at the double bond and the catalysts are substances having particularly strong affinit'y for coordinating with a pair of electrons. If such a complex is formed the polar effect on the double bond is strong enough to produce a cationic condition on one carbon atom; this decreases the activation energy to the point where the chain initiation reaction occurs readily. The mechanism of catalytic polymerization by phosphoric acid also has been considered to be ionic (32). A wide variety of catalysts have been used t o promote polymerization reactions. One entire class consists of oxidizing agents of various types (127, 130, l S 4 , 141). Organic peroxides such as benzoyl, dibutyl, dicaproyl, dicaprylyl, as well as cuinene and terl-butyl hydroperoxides are frequently used. Hydrogen peroxide either by itself or mixed with ferric compounds or sodium hydroxide is satisfactory for some applications. Ammonium perchlorate, sodium thiosulfate, sodium borate, alkaline metal persulfates and perborates, and alkaline earth persulfates all have been used. I n addition, the metallic halides mentioned form anot'her class of catalysts; the petroleum industry employs phosphoric and sulfuric acid and various metal pyrophosphates
(46). The propagation reaction represents the addition of the nionomer t o the activat,od end of the growing chain by a bimolecular reaction. Under normal conditions its rate is usually large in comparison with the rate of the initiation reaction (68, 67, 109), as well as that of the cessation reaction. The over-all rat>eis a composite of the rates of all four types of reactions, but it is dependent also on the method of carrying out the polymerization. Since this is true, some mention of methods should be made before considering the kinetics of the reaction. METHODS
OF A D D I T I O N P O L Y M E R I Z A T I O N S
Polymerization may take place in a liquid monomer, in solution, in an emulsion or suspension, or in the vapor phase (56, 149). If polymerization occurs in the liquid mononier itself, the entire contents of the vesscl gradually polymerize and form a solid mass a t room temperature, provided the polymer is soluble in the monomer or monomers. The mass becomes more and more viscous until it solidifies. I n other cases the polymer is not soluble in the excess monomer and will precipitate out after a certain degree of polymerization is reached. I n eit'her case, t h e process is known as bull; or mass polymerization and is of particular importance in the formation of plastic shapes by the casting process. Usually it is difficult to remove t,he heat.of polymerization in a bulk reaction, particularly if agitation must. be avoided, because of the high viscosity t h a t exists relatively early in the progress of the reaction. This method is usually carried out under conditions that require a relatively long reaction time so t h a t the heat may be removed slowly over a long period. In addition to the difficulty of temperature control, the product usually has a relatively low average degree of polymerization i n d a wide range of molecular weight; both of these mag be undesirable.
Vol. 40, No. 9
Polymerization in solution simplifies the removal of the hcat of polymerization, but the presence of a solvent slov-s down the reaction rate and tends to produce polymers of low or modcrate molecular weight. I n addition, it is difficult t o strip off the last traces of solvent from the extremely viscous polymer, and even traceh of solvent may have a considerable effect on the properties of the finished material. This method, hon-ever, is relatively easy t o carry out and the heat of polymerization may be removed by the use of a reflux condenser if the desired tenipcrature is near the boiling point of the solvent. Emulsion and suspension polymerization actually differ only in the size of the suspended droplets but this difference produces important variat'ions in the process. I n emulsion polymerization a dispersion of finely divided drops of the monomer in water is obtained by the use of soap or other emulsifying agents. Sta'nilizers, cat'alysts, and modifiers are also added. At the end of the reaction, a latex of colloidal polymer suspended in water is obtained; this must be coagulated before the polymer can bc recovered by filtration, washing, and drying. To a certain degree, this disadvantage is offset by many important uses for the latexes themselves: The temperature of the r e a d o n is relatively easily controlled, since the high heat capacit,y of the aqueous medium prevents local overheating. The rate of polymerization is usually higher than that for other methods and it is possible to obtain polymers of higher avcrage molecular weight (66). In some cases, such as the synthetic rubbers, the problem of removing. the emulsifying, stabilizing, and modifying agents aft'er coagulation is not important, as actually they niay have beneficial results. In other cases, the impurities occasioned by the presence of addit,ion agents may be a distinct disadvantage. Emulsion polymerization received a tremendous amount of attent'ion during the war years, particularly in the field of synthet,ic rubbers (21, 31, 46, 0'0, 56, 57, 58, 64, 66, 88, 91, 101, 116, 187, 130, 149). I n dispersion, bead or pearl, polymerization no attempt is made to obtain a true emulsion; in fact, every effort is made to avoid extremely small droplets. Liquid globules of monomer are kept in suspension by just t,he correct amount of agitation, a low ratio of water to monomer, and special stabilizers (66, 6 4 ) . The coagulation step usually can be omitted in thc dispersion process and the mechanism of polymerization is similar in nature t o that occurring in bulk rather then emulsion polymerization. Vapor phase polymerization is at present confined to petroleum refining, alt.hough occasionally there is some mention of i t in connection witahthc proparat,ion of high polymers (3'4, 69, 143). The vapor phase process as operated in petroleum refineries primarily produces dimers and trimers although some higher polymers are formed. To this extent it is not truly a polymerization process in the same sense that the other methods are. Condit,ionsand catalysts are chosen to give maximum yields of dimers and trimers and, under these conditions, a considerable amount of cracking also occurs. However, the petroleum industry commonly applies the term polymerization to these processes. They are described in the handbook sections of some of the petroleum journals (10, 1 1 ) , and are discussed in a few review articles (18, 33,54). As they differ so widely from the other, more numerous, polymerization processes, t,hey will not be t>aken up in greater detail in this short review although it should be rcalized that a large tonnage of high grade gasoline is being produced by vapor phasc polvmcrieation. REACTION KINETICS
For addition polymerizations carried out in the liquid monoiiier or in a solvent, and einplo\;ing peroxide catalysis, it has been observed ( I , 5, 70, 107) t,hat,the reactions proceed a t a rate proportional t o the square root of the catalyst concentration. This can be account,ed for on the hwsis of an initiation process of thc first order with respect t o the catalyst concentration and a cessation reaction of the second order with respect t o the activated molecules ( 1 , 93, 109). In some cases, particularly in clilutc solutions,
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September 1948
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COURTESY PLASTICS DIVISION, MONSANTC CHEMICAL COMPANY
Blending Tanks for Melamine-Formaldehyde Resin Solutions
the propagation reaction also is of second order with respect t o the monomer and activsted molecule concentrations (49). These conditions can be summarized as follows: Rate of initiation = K,[R.] Rate of propagation = Kb[ll/il[M?] Rate of cessation = K0[M;I2 where K is the rate constant, R . the cataiyst, M the monomer, and the asterisk again indicates activated conditions. If K, is much greater than K,, a concentration of activated molpcules such that the rates of initiation and cessation becomc equal is reached
K J R . 1 = K,[MX]2
rato curve for solvent polymerization is of the autocatalytic type (23)where the velocity increases as the reaction proceeds. These simplified kinetics apparently should only be regarded as part of the more complicated over-all problem and are strictly applicable only to dilute solutions. NIorc dctailed treatments of reaction rates are available; these can be used for concentrated solutions (1,63, 70, 123) and for other assumptions regarding the order of the activation step. For polymerizations carried out without catalysts, it has been observed generally (67, 109) that the process is second order with respect to monomer. This should result if the ini'iial activation by the absoyption of energy resulted in a dimerization t o a molecule having a n active center at both ends
C&
2CsHsCH=CH2
and
CsH5
I I ---+ * CHCHaCI-I,CN.
The rate of initiation then is proportional t o the square of the monomer concentration and
If this value is inserted in the propagation expression, Rate of propagation
=
Kb[M]
dsg$
and the rate of propagation becomes first order with respect t o monomer and half order with respect to catalyst. The rates of the propagation and cessations reactions for the bulk polymerization of vinyl acetate have been found t o be independent of the chain length (17, 18). I n some cases the reaction
Some experimental evidence is available (137) in which separate determinations were made of the rate of initiation and the rate of polymerization. The polymerization of divinyl compounds of the diallyl ester type is complicated by the fact that three-dimensional networks are formed. The polymerization of these compounds has been found (126) to proceed a t a zero order rate a t low concentrations, but the gel point is reached at about 25y0 conversion of the monomer.
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The kinetics of the ionic type of polymerization catalyzed by such compounds as aluminum chloride and boron trifluoride are markedly different from either the noncatalytic reactions or the lree radical mechanism. Instanees have been reported where the initiation reaction is first order with respect t o catalyst concentration but the over-all rate is third order or higher with respect t o monomer (109,147). The kinetics of polymerization reactions carried out by the emulsion technique are complicated by the fact that the polymerization does not proceed in the monomer phase but occurs in the aqueous medium (29,46, 57, 68,110, 161). The over-all rate of polymerization is much faster t,han when occurring in a solvent or in bulk under comparable conditions; and higher molecular weight polymers usually are obtained. I n the emulsion process soap micelles must exist-at least in the early stages of polymer growth-and t,here is evidence (67) t h a t most of the activation or growth reactions occur in these micelles during t,he early stages of the reaction. As it is possible t o carry out emulsion polymerizations without the use of a soap, a minor amount of activation must occur in the water phase outside the soap micelles. I n many cases the soap is completely absorbed on, or in, the polymer particles as the reaction continues and the problem of soap removal or conversion is a n important one in this process. At some stage in its growth, whether or not the reaction initiated in the soap micelles, the soap is absorbed on each particle and acts as a n emulsion stabilizer tending to prethat vent coagulation. Indicat,ions are available also
