unexplained. It is clear, however, that as the functionality of the reactants increased, the temperature a t the gel point or degree of reaction decreased. The exact numerical relationship between the gel temperature and degree of reaction is not known a t this time. However, preliminary calculations indicate that, though the gel temperatures fall in the same order as calculated, these temperatures represent somewhat higher degrees of reaction than calculated from the Flory equation. The problem of premature gelation causing incomplete reaction, which was pointed out in the Vicat softening data of several polymers, was thus confirmed in this work. Some factors which might influence the exotherm temperature a t gelation were studied briefly. The most complex system, MDI-3 with heterocyclic 8(125) resin, was chosen for this portion of the work. The MDI-3 contained a high level of acidity (see Table 11) to help ensure a slow reaction rate with the resins. and thus to permit efficient mixing before gelation. Variations in the time and degree of mixing did not affect the exotherm temperature at gelation. Further, the addition of a tertiary amine catalyst (blobay catalyst C-16) in amounts u p to 1% of the polymer Iveight greatly reduced the time before gelation, but had no effect on the temperature a t gelation. All of these observations indicate that the exotherm a t gelation was dependent only on the degree of reaction, and, in this system. was independent of mixing and catalysis of the N C O / O H reaction over wide ranges. The exothermic temperature a t gelation (100’ C.) for the heterocyclic 8(150) resin with MDI-2 is below that expected by comparison with the rest of the data in Table VI1 ; the authors have no explanation. While the degrees of mixing studies had no significant effect on the exotherm temperature a t gelation, it did have a direct effect on the maximum exotherm after gelation. As mixing efficiency was improved, the maximum exotherm increased. Thus very good mixing before gelation increased the extent of reaction which could occur after gelation, as would be expected.
These observations emphasize that in foam preparation with highly functional systems the reaction rate should be kept low enough to permit very efficient mixing before gelation, if high quality foams are to be obtained. Thus the catalyst level should be low and the mixing very rapid. The functionality may even be chosen a t such a high level that it is difficult to prepare outstanding foams. Thus the foam data in Table I11 point out this problem. Thc sample produced from heterocyclic 8(125) resin and MDI-2.5 isocyanate showed poor properties and was difficult to foam. The system was very insensitive to catalyst concentration and it was not possible to raise the foam exotherm above 120’ C. to improve the blowing efficiency. These problems all indicate that the foaming reaction was being controlled by the very early gelation of the polymer. Methods of improving mixing techniques so that the polymerization reaction can continue more effectively after gelation are being investigated. Acknowledgment
The authors express appreciation to the Physical Testing Group under Samuel Steingiser and W.J. Bartels, and to C. D. Ferrell, who were so helpful in the preparation and testing of the many materials used in this work. literature Cited
(1) Am. Sac. Testing Materials, Philadelphia, Pa., “ASTM Standards,” 1961. (2) D’Eustachio, D., Schreiner, E. R., Am. Soc. Heating Ventdating Engrs. Trans. 58, 331 (1952). (3) Flory, P. J., “Principles of Polymer Chrmistry,” Chaps. IX-1 and IX-2, Cornel1 University Press, Ithaca, N. Y., 1953. (4) Remington, W.J., Parker, R., Rubber World 138, 261 (1958). for review April 1, 1963 RECEIVED ACCEPTED July 10, 1963 Division of Organic Coatings and Plastic Chemistry, 142nd Meeting. .4CS. Atlantic City, N. J., September 1962.
POLY(ALKYLENE OXIDE) RUBBERS J . G. H E N D R I C K S O N , A.
E. GURGIOLO, A N D W.
E. P R E S C O T T
Polyol and Latex Research, T h e Dow Chemical Company, Freeport. T f x a s
A high grade rubber was made b y vulcanizing copolymers of propylene oxide and allyl glycidyl ether. The crude polymer contained both crystalline and amorphous fractions. The total polymer could be sulfur cured without fractionation or removing the catalyst-a partly hydrolized ferric alkoxide, resulting from the reaction of FeCI3 with propylene oxide. Variables in the vulcanization recipe were studied, and a tensile strength of 1980 p.s.i. with 56070 elongation was obtained. Improved tear and oil resistance were observed as compared with SBR rubber. Other physical properties compared favorably with commercial rubbers, except for compression set. based On a COpOl~merOf propylene oxide and allyl glvcidvl ether (PO-AGE) has been under investigation This application is promising for commercial development because the rubber has very good physical properties. including high tear resistance and resistance to oil s\ielling. Reports of vork bv others in this new rubber field are starting to appear: these describe the preparation of alkylene oxide-
A
hE\V SY\THETIC RUBBER
allyl glycidyl ether copolymers and also describe the vulcanization of these copolymers to rubber (7, 3, 5. 6. 8, 72). However, neither a n extensive study nor results using a n iron alkoxidetype catalyst have been published to date. The development of high molecular weight, solid poly(alkylene oxides) has been slow. They were first made with a catalyst derived from ferric chloride ( 7 7 ) . The early catalyst was a reaction product of anhydrous ferric chloride with excess VOL. 2
NO. 3
SEPTEMBER
1963
199
propylene oxide in a solvent. The product was a viscous, black tar. Although it possessed the activity needed to make high molecular weight poly(alky1ene oxide), a n improved catalyst was later made by partly hydrolyzing this reaction product (4). This improved catalyst was a rusty brown powder which produced a higher molecular weight polymer and about a 10% higher yield of isotactic moldable polymer from propylene oxide. Additional work showed that iron alkoxides would yield products of this type (4),as did later work with aluminum isopropoxide and zinc chloride (70). Also, a review article on this subject is available (2). While many "tricks" have been tried and many catalyst modifications have been made and tested in attempts to increase the solid yields ( 7 ) , the highest solid yields of high molecular weight polypropylene oxide made to date were below 50%. Experimental
The catalyst was prepared in a 5-liter glass pot equipped with a stirrer, condenser, thermometer, separatory funnel, and ice bath. Air was excluded as much as possible and a plastic sleeve was used to load the FeC13. Into the pot was weighed 162 grams of anhydrous FeC13; 1 liter of CCll was added. From a separatory funnel, 235 grams of propylene oxide was added. The temperature was held to 25" to 35' C . for addition of the first half and 35' to 40' C. during the last half. The mixture was next digested 30 minutes a t 40' C. One liter of hexane was added, and the reaction product was hydrolyzed with 27 grams of HzO separately dissolved in 250 ml. of propylene oxide and fed in dropwise. The temperature was kept a t 30' to 45' C . The crude catalyst was digested for 30 minutes and filtered. Then the catalyst was digested 60 minutes each time for four times, with decantation and filtration of the total catalyst after each digestion. These digestion solvents were: 1 liter of hexane a t 63' C . ; 1 liter of propylene oxide and 1 liter of hexane at 42' C . ; 1 liter of propylene oxide and 1 liter of benzene at 45' C.; 1 liter of dry benzene a t 80' C. After the final filtration, the catalyst was dried overnight in a vacuum oven a t 60' C. These high molecular weight alkylene oxide polymers are prepared by heating an epoxide monomer with 2 to 4% of iron catalyst a t 80" C. for 24 hours or longer. Different oxides have different reactivities so conversions and also solid yields are functions of the reactivity of monomer used. Typical results are summarized in Table I. The crude P O polymer is usually a deep brown, tough, rubbery-to-hard solid mass. Although the whole crude PO-AGE copolymer, catalyst and all, was vulcanized when making rubber, it was often desirable to separate the stereoregular polymer from the amorphous fractions. A common analytical technique was used to separate the two fractions. The product was freed of traces of residual monomer, chopped up, and dissolved in warm acetone, The iron was solubilized with concentrated HCI (a few grams per 100 ml.), and the clear yellow solution was chilled to -20' C. A gel of high molecular weight polymer separated which was filtered off, washed, and dried, Iron was separated from the filtrate with phosphoric acid. Evaporation of the acetone gave the liquid amorphous fraction. For polymers other than those based on propylene oxide, different fractionation techniques will, of course, be necessary depending on the solubility characteristics of the polymer. Discussion The current work consists of three phases: characterizing the propylene oxide polymers and copolymers; developing a 200
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
vulcanization recipe for the PO-AGE copolymers; and studying experimental variables of the process. An understanding of the mechanism and chemistry involved in making these new alkylene oxide polymers has developed hand in hand with the development of the Ziegler aluminum alkyl catalyst system (74) for olefin polymerization and the Natta concepts of stereospecific polymers and catalysts ( 9 ) . Price (70) showed that these solid epoxide polymers are also stereoregular, being a mixture of solid and liquid polymer. The solid fraction is stereoregular and crystalline. and it comprises 20 to 657, of the total product, depending on the epoxide and catalyst used. The amount and degree of crystallinity in these polymers was influenced by many factors, and the amount of crystallinity can be reduced as desired. I n particular, copolymers have reduced crystallinity as would be expected. The presence of different or bulky substituents interferes with the ability of the polymer to form spherulites which are synonymous with crystallinity. Interference increases with increasing percentage of comonomer. However, homopolymer containing bulky side groups substituted in regular fashion again have the ability for the chain to pack in a n orderly manner and crystallinity once again increases. The mixture of stereoregular and amorphous polypropylene oxides has low crystallinity because the amorphous material interferes sufficiently with the packing of the stereoregular chains so as to prevent them from forming spherulites. This characteristic may be of considerable value in affecting the physical properties of the PO-AGE rubbers. Such a mixture when vulcanized and then stretched would be expected to orient the stereoregular polymer chains causing the development of high crystallinity with a resultant increase in tensile properties. Such concepts are already known to exist in stretched rubber. This phase of work is under investigation. As described under Experimental, conversions to polymer of 95% or higher are obtained with 6% AGE in PO after 48 hours a t 80' C. using 47, iron catalyst. The crude polymer
Table 1.
Epoxide Polymerization at 80' C. with an FeCI3-PO-H2O Type Catolyst Catalyst,
Reaction, Hours
Converted,
Solid Yielda
Epoxide
70
yo
70
Propylene oxide (PO) Allyl glycidyl ether (AGE) 6y0 .4GE in PO Epichlorohydrin
4
48
100
40-45
8
137
87
