Probable Structures of Polymers of Lower Olefins A. WACHTER
Shell Development Company, Emeryville, Calif.
M
ANY attempts (8, 10, 11, 19) have been made to describe the mechanism of the catalytic polymerization of olefins in general terms which would not only account for the polymer compounds known to be formed but would also enable predictions to be made of the polymer structures from any pure olefin or mixture of olefins of known structure. The need for some theory, or set of working rules, which will enable a fairly reliable prediction to be made of the structural composition of a polymer is evident when, to take an example at random, the polymerization of any pentene to decenes is considered. There are 377 isomers of CloHzo (Y),all of which might be considered possible dimer products, if only the elementary rules of valence are applied and if every conceivable rearrangement is permitted. Early theories were seriously hampered by the scarcity of reliable data available on the products of polymerization. Almost all have postulated some mechanism for the polymerization process involving a particular catalyst or type of catalyst. I n some cases they fail to predict polymers which are found experimentally, in others they predict many more structures than are found. Coincident with recent developments in polymerization processes for the production of gasoline, a number of studies have been made of the reactions between the lower olefins. These enable a wider, more searching test to be made of the applicability and usefulness of any schemes for predicting polymer structures. The isomerization possibilities of olefins have begun to be fully appreciated only recently, chiefly as a result of the researches of Whitmore and co-workers (21). Not only changes in the position of the double bond but also transformations in the carbon skeleton may occur even a t moderate temperatures in the presence of polymerization catalysts. Thus, under conditions usually applying in polymerization experiments, the following reactions may occur to confuse the attempt to describe simply the mechanism of polymer formation:
Working rules are proposed for predicting the catalytic polymerization products of acyclic olefins. They are based on the assumption of a mechanism in which a hydrogen atom and an unsaturated alkyl radical from one olefin molecule saturate the double bond of another olefin molecule. Consideration is given to the possibilities of rearrangement of the original olefin before, and of the primary polymer after, polymerization. Comparisons are made showing satisfactory agreement between the structures predicted and those found experimentally.
and considerable care must be exercised in deciding whether a polymer structure is the direct product of polymerization of the olefin with which the experiment was started. These possibilities must also be kept in mind when applying the scheme of working rules presented here to the prediction of polymer structures. Two principal types of catalyzed olefin polymerization processes may be differentiated. I n one, the reaction as ordinarily conducted proceeds a t a comparatively low rate and rarely goes beyond a Cl8 to Cm product. In the other, the polymerization goes a t a very high rate, forming polymers of an oily or even resinous nature of extremely high molecular weight. The latter type is probably a chain reaction with mechanism and kinetics very different from the former (3). Only the first type of polymerization will be considered here. When the conditions of polymerization are very severe, cracking of the reacting materials or their products may occur. with rupture of many bonds giving rise to a large variety of incongruous polymers which do not originate from simple multiplication of the original olefin molecules. Polymers resulting from such severe processes and from methods of thermal polymerization will not be considered in this paper.
Polymerization Rules The generalizations and working rules presented here are supported by evidence on the constitution of the polymers of acyclic olefins; they are put forward as working hypotheses of polymerization, not as a description of the actual mechanism. Their chief use is to predict the products of a polymerization. They are considered to apply only to catalyzed polymerizations which give polymers whose molecular weights are the result of simple addition of the original olefins. It is considered that polymerization of PRIMARY PROCESS, two olefins proceeds primarily by the addition of a hydrogen atom and an unsaturated alkyl radical to a double bond. The process may be represented by the diagrammatic equation :
1. The original olefin may rearrange wholly or partly to isomers in which the double bond is in a different position or in which the carbon skeleton is changed, These isomers may combine with themselves or with the unchanged olefin. 2. Each of the primary polymers may rearrange to one or more isomeric structures. 3. The primary polymers or their isomers may combine with the original olefin or its isomers in further polymerization. 4. The rearranged polymer products may depolymerize to give different lower olefins. For example, a trimer of a lower olefin may rearrange and then depolymerize to give a monomer and a dimer, the latter being different in structure from any of the dimers originally made. 5. The depolymerization products from rearranged polymers may enter into further polymerization reactions.
All of these reactions may occur in a simple experiment in which a single pure olefin is polymerized. Consequently, additional well-planned experiments are usually required, 822
JULY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
823
The term “acceptor” refers to the reacting olefin molecule secondary butenes < 2-methyl-1-propene; 2-methyl-1- and which becomes saturated by addition of a hydrogen atom and 2-butene > secondary pentenes > higher olefins. an olefin radical, both supplied by the “donor” olefin. Rule 3. The di- and trimeric polymerization products This implies, therefore, that there is no difference in kind of the lower olefins are far less easily polymerized than the between the primary step in polymerization and the addition monomer. For example, 2-methyl-1-propene is much more of hydrogen or hydrogen bromide to the carbon-to-carbon easily polymerized to octenes than the latter to hexadecenes; double bond of an olefin. No consideration need be given and the dimer is more readily polymerized than the trimer. to the role of the catalyst except t o state that it is the ATom ACTIVEIN POLYMERIZATION. The rules concerning means whereby the bonds of the reacting molecules are the selection of the atoms which actively participate in the activated. primary process of polymerization in both the donor and Kline and Drake (10) proposed a mechanism in which the acceptor molecules can be defined more explicitly. principal reaction involves rupture of a carbon-carbon bond Rule 1. According to the mechanism assumed for the to give a methyl and an olefin radical which then add to the primary process, the double bond in the acceptor molecule is double bond of another olefin molecule. Although prefersaturated by a hydrogen atom and an olefin radical supplied ence for a carbon-carbon rupture is reasonable in view of the by the donor molecule. The active hydrogen atom in the lower energy of dissociation of the carbon-carbon bond donor molecule is taken as the one attached to a terminal (82.5 kcal.) than the carbon-hydrogen bond (100 kcal.) carbon having a double bond or to that double-bonded carbon (18),this mechanism fails to explain many facts of polymeriatom to which the smaller alkyl group is attached. A few zation ahd predicts structures which are not found (6). examples illustrating applications of this rule are The recent experimental studies of Morikawa, Trenner, and Taylor (16)show that the apparent activation energy reCH3 quired to break the carbon-carbon bond in ethane and proHsC-CH=CH-H *, H3C-&=CE *--CHs, pane on the catalyst surface is higher than that required to H~C-CHZ-CH=CH *-CH,j break the carbon-hydrogen bond, and they also conclude that the presence of the double bond in the olefin markedly where the active hydrogens are starred. weakens the carbon-hydrogen bonds. I n the light of Kharasch and Fienner’s conclusions (9) ’In order to employ the primary process (I) for any two on the relative electronegativity of organic radicals, this is reacting olefins, it is necessary to decide which one acts as equivalent to stating that the active hydrogen comes from acceptor and which as donor, and to decide also which hythe more negative of the two carbon atoms linked by a drogen and carbon atoms in both are active in the reaction. double bond. The following paragraphs summarize those distinguishing The carbon atom in the donor molecule actively participatcharacteristics of olefins that usually enable a decision to be ing in the addition, by means of which the olefin radical bemade. comes attached to the acceptor, is the one to which the active ACCEPTOR AND DONORMOLECULES.When a pure olefin hydrogen atom is attached. is polymerized and it is definitely known that only one Rule 2. In the accepto-de one carbon with a molecular form is reacting under the polymerizing conditions, double bond becomes a f 6 h e d to the active hydrogen from no decision need be made concerning which are the acceptor the donor, the other to the olefin radical. The hydrogen atom and donor molecules. goes to the terminal double-bonded carbon atom or to the When two different olefins are cross-polymerized, the acone with the smallest alkyl group, and the olefin radical to ceptor olefin (the one in which the double bond becomes the adjoining double-bonded carbon atom. When the saturated) will probably be the one with the more easily acceptor olefin contains a tertiary carbon atom with a double activated double bond. The possibility exists of special bond, the olefin radical from the d o w r always becomes cases in which the distinction is not pronounced and therefore attached to it. of two different cross-polymerizing olefins acting both as Evidence which supports this selection of points of attachacceptor and donor to approximately the same extent. ment and which is unobscured by possibilities of rearrangeThe criteria to be applied are not a t present capable of ments is found in the fact that isopropyl benzene is the explicit statement. For each member of a pair of olefins chief primary product from reaction of propene with benzene under consideration, it must be decided which one contains the more easily activated double bond on the basis of their (14). Some examples embodying the application of these rules general chemical behavior. In weighing chemical evidence governing the primary process are as follows: it is considered that behavior with remect to addition reactions (e. g., addition of hydrogen haGdes to the double bond) is more important than other types of Acceptor Donor Primary Polymer chemical reactions-for example, substitution. A few commonly made generalizations which may be H CH,H H used as a rough guide follow. H H $ H3C-&-&=CH2 f Rule 1. I n the homologous series of olefin hyI 1 I drocarbons the ease of polymerization increases H H I!€ .f; from ethylene upward and is a t the maximum with the pentenes. Rule 8. Structural differences in isomers exert a m a r k e d i d u e n c e upon the activity of the double bond. I n general, the tertiary olefins such as 2 methyl-1-propene and 2-methyl-2-butene are more readily polymerized t h a n t h e c o r r e s p o n d i n g isomeric secondary olefins 2-butene and 2-pentene. The lower olefins may be arranged as follows with respect to relative t e n d e n c y for polymerization according to rules 1 and 2: ethylene < propene
