August, 1958
DILUTE GELLINGOF POLYALKOXYSILANES
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their thanks t o Mrs. D. K. Schults and Mr. J. P. work described herein. Mr. Bianchi also measBianchi for performing most of the experimental ured the molecular weights.
DILUTE GELLING SYSTEMS. 111. POLYALKOXYSILANES BY F. P. PRICE General Electric Research Laboratory, Schenectady, N . Y . Received April 28, 1868
Mixtures of dimethyldichlorosilane and methyltrichlorosilane have been allowed to react in chloroform with either ethylene glycol or decamethylene glycol. The extents of reaction a t gelation were measured as functions of both dilution and functionality. It was found, as expected, that both increasing dilution and decreasing average functionality increased the extent of reaction at the gel point. The variables of the system were correlated using the theory proposed in Paper 1 of this series. The theory indicates that the ethylene glycol polymers and the decamethylene glycol polymers both have about the same chain stiffness. However these polymers are significantslyless stiff than the polyesters described in Paper I1 of the series.
Introduction Previous work on soluble, highly polyfunctional polymethylsiloxanes has indicated that the molecules of these polymers may be regarded as spherical particles of micro-microgel. These polymers have very low viscosities in solution and show very little influence of molecular weight upon viscosity. Furthermore, when placed in a range of solvents, their viscosities change only very slightly as compared to linear polymers.2 All of these observations trend to support the view that these polyniethylsiloxanes are highly cross-linked, rather rigid molecules. I n general these siloxanes are prepared a t rather high dilut,ion where ring formation would be favored. It would be worthwhile knowing whether the behavior exhibited by the siloxanes was peculiar to these polymers or whether other highly functional polymers would behave in a like manner when prepared a t high dilution. The second paper in this series3 reports the results of some experiments on the effect of dilution on the extent of reaction a t gelation of polyester systems. It was shown that as expected both increasing dilution and decreasing functionality of the system increased the extent of reaction a t the gel point. It was further shown that these effects could be satisfactorily correlated by the theory proposed in the first paper of this series.4 It also was found that reactive groups were buried in the highly internally cross-linked molecule and that because of the inability of the structure to rearrange these groups became inert. At sufficiently high dilutions this effect made the reaction stop short of gelation even though 10-15% of the reactive groups remained. It is of interest to determine whether these phenomena occur in other potentially gelling systems. This paper describes some work of a similar nature on the gelation of mixtures of dihydric alcohols, dimethyldichlorosilane and methyltrichlorosilane.
Experimental Materials.-Chloroform was of C.P. grade obtained from the Mallinclcrodt Chemical Works. Dimethyldichlorosilane and methyltrichlorosilane were obtained from the Silicone Products Department, Chemical and Metallurgical Division, General Electric Company. Ethylene glycol was obtained from Union Carbide and Carbon Corp. Decamethylene glycol was obtained from Columbia Organic Chemical Company, Columbia, S.C. All these reagents were redistilled before use. Where necessary the distillation was carried out in vacuo. Method.-The appropriate chlorosilane mixture was weighed into the desired volume of chloroform in a threenecked flask and placed in a thermostat. The thermostat was maintained at 30 f 1". An equivalent amount of glycol was contained in a small bulb inserted through one neck of the flask. The other two necks were equipped uith a nitrogen inlet tube and an outlet tube, respectively. The latter led into a small trap (ea. 10-ml. capacity) and thence into a stirred flask of water. The main reaction flask could be stirred violently by a magnetic stirrer. The reaction was started by twisting the bulb containing the glycol so that its contents ran down into the main reaction flask. The HCl evolved was trapped in water after being swept out by a slow stream of dry nitrogen (ca. 100 cc./ min.). The HC1 was titrated a t intervals with standard NaOH. The time of gelation was determined by noting when bubbles in the polymer solution failed to rise. Where necessary the extent of reaction at gelation was determined by extrapolation. This extrapolation usually was short as the system generally was evolving HCl only very slowly when it gelled. In order to calculate the dilution (vol. of solution/vol. of polymer) it is necessary to know the density of the polymer. This was found t80be 1.475 for decamethylene glycol polymers and 1.501 for ethylene glycol polymers.
Results and Discussion In all the experiments reported in this paper the initial equivalents of chloride were equal to the initial equivalents of hydroxyl. Figure 1 summarizes the results of the present investigation on the et,hylene glycol-chlorosilane polymers. This figure illustrates the effect of dilution, D,on the extent of reaction at gelation, P,, for several values of p where P =
(1) F. P. Price, S. G . Martin and J. P. Bianchi, J. Polymer Sci., 22, 4 1 (1956). ( 2 ) F. P. Price, S. G . hIartin hnd J. P. Bianchi, ibid., 22, 40 (1056). (3) J. H. Gibbs, F. P. Price and B. 11. Zilnm, Tme J O U R N A L , 62, 973 (1958). (4) R. W. Kilb, ibid., 62, 969 (1958).
initial equiv. of chloride on methyl trichlorosilane total initial equiv. of chloride
Figure 2 shows the relationships of D to P, for the decamethylene glycol-chlorosilsiie polymers.. The solid curves on both these figures were drawn in accord with the theory outlined in paper I of
F. P. PRICE
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The magnitude of K expresses the sensitivity of the gel point t o dilution. The greater the value of K the greater is the effect of dilution in delaying gelation. In Table I are presented the values of K for these polymers along with the same quantity for the polyesters discussed in Paper I1 of this series.8 TABLE I
Polymer
Ethylene glycolchlorosilane Decamethylene glycolchloyosilane Polyes ters
I[
Atoms in smallest X 108 ring
Relative effective link length, b
20.1
5
1.0
8.1
13
1.0
5.8
11
1.5
Also included in the table are the number of atoms in the smallest possible ring and the relative effective link length. This last quantity was calculated from eq. 15 of Paper I of this series, asPC . suming that the ratio .&’/A1’ was unity. The only Fig. 1.-Effect of dilution on gel point for ethylene glycol undetermined quantity is the ratio of effective polymers: 0,p = 1.00; A, p = 0.69; 0,p = 0.50. link lengths (b,/+). In Table I the larger the relative effective link length, the stiffer is the polyISC mer chain compared to the ethylene glycol polymer chain. The figures in the last column of Table I indicate that both of the silicon-containing polymers have about the same stiffness and that the polyester polymers are considerably stiffer. In view of the approximate nature of the theory it is interesting and probably fortuitous that relative effective link lengths for the ethylene glycol and the decamethylene glycol polymers turn out to be equal. For whatever the comparison may be worth, it indicates that in a homologous series of polymers the effect of dilution on gel point depends directly on the molecular weight of the repeating unit and inversely on the three halves power of the number of atoms in the smallest possible ring. It is surprising that this relationship apparently holds for rings containing as few as five atoms. The decamethylene glycol polymers and the PC polyesters have about the same number of atoms Fig. 2.-Effect of dilution on gel point for decamethylene glycol polymers: 0,p = 1.00; A , p = 0.69; 0 , p = 0.50. in the smallest ring. Nevertheless the effect of dilution on gelation of the latter polymers is senthis series.4 The one adjustable parameter, K, of sibly less. It is reasonable that this difference this theory was determined by fitting the theory to should arise as a result of the polyester being stiffer, the points a t p = 1 in Figs. 1 and 2 to the equation since the ester link is less flexible than the siloxy 2Pcz = 1/(1 - 2.612~D) (1) link. Throughout the course of this work there was no This is the equation to which equation 14 of Kilb’s paper reduces with the assumptions of equivalent indication of the “buried group effect” found in the concentrations of reactants, and S =3/2. The polyesters.3 The absence of this effect is probably use of this value of X rather than a value of 5/2 is the result of the inherently greater flexibility of the reasonable since the lysis rate of siloxy bonds is siloxy link as compared to the ester link. Acknowledgments.-The author wishes to exmuch slower than the formation rate under the particular experimental conditions used. It can press his gratitude to J. H. Gibbs, R. W. Kilb and be seen that the agreement with theory is generally B. H. Zimm for much helpful discussion during the satisfactory. Only in Fig. 2 for p = 0.69 is there course of this investigation. Thanks are also due any substantial disagreement. The reason for the to Mrs. D. K. Schultz for performing much of the experimental work described herein. discrepancy is not known.
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