I
LESLIE C. BATEMAN British Rubber Producers’ Research Association, Welwyn Garden City, Herts, England
New Polymers from Natural Rubber A new method for making graft polymers by milling offers possibilities in both rubber and plastics industries
CHEMICAL
modification of natural rubber by synthetic polymers has been the subject of considerable empirical study. The special feature of the work reviewed here is emphasis on determining the structure of the products and understanding the reactions involved. I t is hoped to establish correlations between polymer constitution and physical and technological properties that will permit a more rational approach to the modification of rubber to obtain desired properties. Present knowledge is rudimentary; until recently, for example, no evidence was available to show how, or indeed whether, certain rubber-synthetic polymer compounds behave differently from intimate mixtures of the components. Attention has been given to two main aspects at the British Rubber Producers’ Research Association. T h e first is the preparation of graft polymers from natural rubber and its homologs in solutions and emulsions. Scientific and technological reports of this work have been published, so that the aim here is to present a unified summary in the light of recent findings. The second is the preparation of block interpolymers from solid rubber by mechanical action-work which has a wider significance in demonstrating that mastication and milling provide rational manifestations of what may be termed the mechanicochemistry of elastic substances. Polymerizations in Liquid Systems
Polyisoprenes as Chain-Transfer Agents. I n view of the activity of alkylbenzenes and similar substances as chain-transfer agents toward polymer radicals (75), polyisoprenes, which contain comparably labile a-methylenic C-H bonds, would be expected to interact with growing polymer chains by a process expressed schematically as :
Polymer rodical
Pdy-
isoprene
Graft polymer
704
Model experiments with the diisogutta (molecular weight ca. 15,000) have been extensively investigated for prene, dihydromyrcene, indicated that isoprenoid structures can react in this this purpose (7). T h e results have some way (3). With styrene and methyl puzzling features, but leave no doubt that the transfer mechanism depicted methacrylate, the well-known kinetic above does not describe the behavior of consequences of such interaction were high-molecular polyisoprenes toward observed-an unchanged rate of polypolymerizing monomers. merization in the presence of the transfer agent, and a reduced degree of polyTwo peculiar and apparently contramerization in accordance with Mayo’s dictory features stand out. The first equation PVL = P,-l C [ R H ] / [ M ] , is that rubber, in contrast to dihydrowhere P and Po are molecular weights in myrcene (DHM), strongly retards the polymerization of methyl methacrylate, the presence and absence of the transfer with the other polyisoprenes showing agent, RH, and C is a transfer constant. intermediate behavior in a regular With vinyl acetate, more complicated sequence. behavior was encountered ; the addition
+
Polymerization rate
None
DHM
1.0
1.0
of dihydromyrcene reduced the rate of polymerization in a manner which can be rationally and quantitatively explained as “degradative chain transfer” -that is, the polymer radical abstracts a hydrogen atom to produce a polyisoprenyl radical, but this is insufficiently reactive to initiate polymerization of the relatively inactive monomer.
Experiments with High-Molecular Polyisoprenes. A further critical consequence of the transfer mechanismthat the transfer agent should be attached to some of the polymer molecules-cannot be tested in experiments with lowmolecular olefins. This difficulty can be overcome, however, if the transfer agent is itself a high polymer, provided that the base polymer and the synthetic polymer differ sufficiently in, say, solution properties for them and the graft polymer to be identified in the product mixture. Polymerizations of methyl methacrylate in rubber (molecular weight ca. 500,000), gutta-percha (molecular weight ca. 70,000), and chicle-
Polymer mdecule
/
Polyisoprene radical
F-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Pol yisoprene Chicle 0.88
Gutta 0.60
Rubber 0.5
This retardation, which is independent of whether benzoyl peroxide or azoisobutyronitrile is used as initiating catalyst, suggests that rubber is involved in the polymerization. Admittedly, the three high-molecular polyisoprenes may contain traces of impurities which act as retarders, but this seems unlikely, since special attention was given to purification with this in mind and the specimens used simulated dihydromyrcene in oxidation characteristics. T h e second striking feature is that combination of rubber with synthetic polymer occurs when benzoyl peroxide is used as catalyst, but not when azoisobutyronitrile is used (a simple mixture then resulting). This is inconsistent with any transfer process involving attack of the growing polymer chain on the base polymer. The rubber-poly(methy1 methacrylate) compound produced in the presence of benzoyl peroxide is undoubtedly a graft interpolymer and the question arises, how is it formed if not by a transfer process? The most likely explanation is that some of the radicals produced on dissociation of the benzoyl peroxide directly abstract hydrogen atoms from the rubber to leave polyisoprenyl radicals as initiating sites for polymer growth. The inability of azoisobutyronitrile to react similarly is in keeping with its inability to introduce cross links into rubber, whereas benzoyl peroxide does so sufficiently readily to be classed as a vulcanizing agent. Experimental
examination of this possibility is in progress, using isotopically labeled catalysts. S o definite reason can be advanced as to why transfer does not occur with the high-molecular polyisoprenes and yet almost certainly does so with dihydromyrcene. Some subtle physical factor must be responsible, and this is possibly to be found in the phenomenon of microphase separation (74). Constitution of Rubber-Methyl Methacrylate Graft Polymer. \$'hen the total reaction product is dissolved in benzene and titrated with methanol, a separation into three fractions occurs, the first and third being rubber and poly(methyl methacrylate), respectivelv, while the second was originally identified as the graft polymer (76). From elemental analysis and molecular weight measurements, the over-all polymeric composition of the graft polymer can be calculated. From further experiments to determine the degree of conversion a t which free rubber failed to separate, and making the not unreasonable assumption that each rubber molecule then has one attached polymer chain, it is possible to deduce the size of the poly(methy1 methacrylate) side chains. The surprising result was that, while the rubber portion had the expected molecular weight (near to that of the free rubber), the methacrylate side chains had a molecular weight smaller by some IO2 than that of the free polymer formed concurrently. This disparity was long inexplicable, but it has recently been found that the unique colloidal properties of the graft polymer do not allow the polymeric mixtures to be clearly separated by the procedure adopted. In benzene-methanol, the graft polymer strongly solubilizes the rubber, so that what had been considered simply a graft polymer was in fact a colloidal graft polymer-rubber complex. Similar behavior in the analogous gutta-percha system is expressed quantitatively as:
anoi1ialous in respect to the size of the attached polymer chains, which are similar to the free poly(methy1 methacrylate) molecules (2). This result has been confirmed by complete chemical degradation of the rubber portion of the graft polymer under conditions where the methacrylate side chains are unaffected and can be subsequently isolated and examined directly.
Properties of Rubber-Methyl Methacrylate Graft Polymers. The differing polarities of rubber and poly(methyl methacrylate) are responsible for some novel properties of the graft polymer both in solution and in solid form (7). The addition of methanol to a benzene solution of the graft polymer at first produces a marked decrease in viscosity, accompanied by development of turbidity. After a certain addition little further change is apparent, and coagulation does not occur until a large excess of methanol has been added. T h e initial changes reflect the decreasing solubility and self-association of the rubber component in the presence of the nonsolvent methanol, which result in formation of a colloidal sol rather than a coagulum because of the stabilizing action of the still soluble methacrylate side chains. The noncombined rubber is also solubilized by the methacrylate component of the graft polymer, as is evident from the fallacious separative procedure referred to above. Essentially the reverse process of insolubilization of the methacrylate component to form a sol stabilized by the still soluble rubber can be realized in suitable solvents. These configurational changes may be represented as:
.
z
+---
21 41
31 39
16 50
, 4
\
I
I I
X.
I
Sol stabilized by rubber
Y. Sol stabilized by methacrylate 0
5
11 23
28 35
54
80
SYhen a graft polymer containing, say, 30y0 of methacrylate is treated as above and the solvents are evaporated, X and Y differ considerably in physical properties. In appearance X resembles the original rubber, while Y is harder, with a glossy plastic surface. More signifi-
Table I. Tensile strength, kg./sq. cm. Elongation at break, % MIOO, kg./sq. cm. Msoa, kg./sq. cm. Hardness, deg. B.S.l.
46 33
This phenomenon can be ascribed to molecular rupture during cold mastication, occurring preferentially on the rubber component at or near the site of grafting, so that the methacrylate side chains are detached, either as free polymer or essentially so--i.e., with small polyisoprene fragments attachedand then contribute a separate and hardening phase to the final product. To obtain hard vulcanizates vulcanization must be carried out at temperatures above the softening point of the synthetic polymer. I t is thus possible by suitable control of mastication and vulcanization conditions to obtain from the same raw
-1 methacrylate), however, is considered to result from degradation under shear of a prior-formed interpolymer. The formation of the sandwich structures is attributed to combination of two rubber-poly(methy1 methacrylate) radicals. While the relative importance of combination and disproportionation as termination processes in polymerizations of methyl methacrylate in liquid systems is still debated, it is agreed that the former will come into prominence at the low temperature of mastication (7, 9, 70). A particularly interesting feature of the product composition is that interpolymer I is formed only in the initial stages of the reaction, and rather abruptly gives way to the formation of compound I1 (Figure 18). This change occurs at about the point of rapid Increase in rate of monomer conversion (see Figure 9), and is associated with the Trommsdorff or “gel” effect (20, 22), in which a substantial increase in bulk viscosity, which is favorable to reaction under mastication conditions, impedes the termination reaction by hindering the mobility of the radicals and thus reducing radicalradical contact.
Acknowledgment The author is indebted to several colleagues for help in preparing this paper, especially D. J. Angier. The work described forms part of a program of research undertaken by the Board of the British Rubber Producers’ Research Association.
Literature Cited (1 ) Allen, P. ( 2 ) Allen, P.
W., unpublished results. W.,Merrett, F. M., unpub-
lished results. Allen, P. W., Merrett, F. M., Scanlan, J., Trans. Faraday Sac. 51, 95 (1955). Angier, D. J., Ceresa, R. J., Watson, W. F., unpublished results. Angier, D. J., Watson, W. F., J. Polymer Sci., 20, 235 (1956). Angier, D. J., Watson, W. F., unpublished results. .4rnett, L. M., Petersen, J. H., J . Am. Chem. SGC.74, 2031 (1 952). Ayrey, G., Moore, C. G., Watson, I V . F., J. Pollmer Sci. 1 9 , l (1956). Bevington, J. C., Melville, H. W., Taylor, R. P., Zbid., 14, 463 (1954). Bickel, A. F., Waters, W. A, Rec. trav. chim. 69, 312 (1950). Bloomfield, G. F., J. Appl. Chem. 5 , 609 (1955).
Bloomfield, G. F., Merrett, F. 82., Popham, F. J., Swift, P. M., PrGC. Rubber Tech. Conf. 1954, 185. Brit. Rubber Producers’ Research Assoc., “Heveaplus,” Tech. Bull. 1 (1954). Dobry, A , , Boyer-Kawenoki, F.. J. Polvmer Sci. 2. 90 (1947). (15) Gregg, R . A., Mayo, ‘F. R’., Dzscus.nons Faraday Sac. 2, 328 (1947). (16) hlerrett, F. M., Trans. Faraday Soc. 50, 759 (1954). (17) Merrett, F. M., unpublished results. (18) Merrett, F. M., Wood, R . I., R o c . Inst. Rubber Ind. 3, 27 (1956). (19 ) Natl. Rubber Development Board, Rubber Deoelopiiient.r, 9, 2 (1 956). (20) Norrish, R. G. W., Smith, R. R., Nature 150, 336 (1942). (21) Pike, M.,Watson, W. F., J . Polymer Sci. 9,229 (1952). (22) Trommsdorff, E., Kohle, H., Lagally, P., Makromol. Chem. 1 , 1 6 9 (1948). (23) Watson, W. F., Trans. Inst. Rubber Ind. 29, 32 (1953). RECEIVED for review May 31, 1956 ACCEPTED October 27, 1956 Division of Rubber Chemistry, ACS, Cleveland, Ohio, May 16 to 18,1956. VOL. 49, NO. 4
APRIL 1957
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