Chapter 15
Silicones and Silicone-Modified Materials Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/22/18. For personal use only.
Halogenated Siloxane-Containing Polymers via Hydrosilylation Polymerization J . H u and D. Y . Son
1
Department of Chemistry, P.O. Box 750314, Southern Methodist University, Dallas, T X 75275-0314
The synthesis and characterization o f new halogenated poly(carbosiloxane)s are described. The polymers are prepared v i a hydrosilylation polymerization using platinum-divinyldisiloxane complex as catalyst (Karstedt's catalyst). The monomers are either commercially available or can be prepared v i a a simple literature preparation. The polymers can be modified by either performing reactions on the pre-made polymer, or by modifying the monomers prior to polymerization. Insertion of cyclic tetrasiloxanes into the polymers is possible, and in certain cases the resulting polymers undergo spontaneous crosslinking reactions to form insoluble gels. A mechanism for this crosslinking process is proposed.
Organosilicon polymers have attracted much research interest due to their potentially useful properties. For example, the preparation, characterization, and applications o f poly(siloxanes) (1), poly(carbosilanes) (2), poly(carbosiloxanes) (3-8), and poly(silanes) (9) have been extensively documented in the literature. Recent research has focused on developing methods for modifying these polymers i n order to prepare a variety o f substituted derivatives with altered properties. Different approaches used to modify organosilicon polymers have included free-radical hydrosilylation of polysilanes (10), nucleophilic substitution i n poly(chlorosilylenemethylenes) (11), hydrosilylation of poly(silylenemethylenes) (12), and deprotonation of poly(carbosilanes) (13). The development of such types o f tunable polymers is a major goal in current research. Hydrosilylation polymerization has proven to be an extremely effective method for the synthesis o f many types of organosilicon polymers. F o r example, hydrosilylation polymerization has been utilized extensively i n the synthesis o f both poly(carbosilanes) (14-23) and poly(carbosiloxanes) (3-8). These polymerizations
Corresponding author.
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© 2000 American Chemical Society
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usually proceed cleanly with a minimum of side-reactions. Since a typical polymerization involves reaction between A and B monomers, near exact stoichiometry is required to obtain polymers of reasonably high molecular weight. In this report we outline the synthesis and characterization of new halogenated poly(carbosiloxanes), as well as results indicating that these unique polymers can be successfully and easily modified. A n additional benefit of this system is that the chemical substitutions can be brought about by either modifying the preformed polymer, or by polymerizing the substituted monomers. Certain cyclic siloxanes can also be inserted into the polymers, whereupon a rapid crosslinking reaction takes place after isolation of the products. 2
2
Results and Discussion Polymer Synthesis and Characterization. A s part of our investigation into the synthesis of modifiable organosilicon polymers, we recently reported a general synthetic route to a variety of &/s(trialkylsilyl)dihalomethane compounds (24), of which compounds 1 and 2 seemed to be ideal monomers for hydrosilylation polymerization (Scheme 1). Unfortunately, hydrosilylation polymerizations utilizing various combinations of 1 and 2 were not successful in the presence of platinum catalyst. For example, reaction between 1-C1 and 2-Cl did not occur. Use of elevated temperatures induced reactions to occur i n some instances, but the products were impure and highly colored. It was at this point that we decided to include commercially available reactive monomers i n our experiments. A s a result, we discovered that hydrosilylation polymerization occurred between 1-C1 and 1,1,3,3tetramethyldisiloxane or between 1-C1 and 1,1,3,3,5,5-hexamethyltrisiloxane to yield polymers 3-Cl and 4-Cl , respectively, in quantitative yield (Scheme 2). The reaction period was rather lengthy but necessary for complete reaction to occur. The reaction progress was monitored by noting the disappearance of the S i - H absorbance peak in the IR spectrum. Immediately after polymerization, the polymers were viscous, transparent oils. Polymer 3-Cl , however, slowly crystallized on standing. 2
2
2
2
2
2
2
2
1 3
Polymers 3-Cl and 4-Cl were characterized using H and C NMR spectroscopy, IR spectroscopy, elemental analysis, and gel permeation chromatography. The H N M R spectra show a lack of endgroup resonances and also indicate that the polymerizations proceeded cleanly with a minimum of side reactions. A n expanded view of the *H N M R spectrum for polymer 3-Cl is shown i n Figure 1(a). G P C molecular weight data are listed in Table I (entries 1 and 2). O n repeating these polymerization reactions, we found the G P C data to be consistent from batch to batch as long as we utilized a starting monomer molar ratio as close to 1:1 as possible. These molecular weight data in addition to the lack of visible endgroups in the *H N M R spectra suggest that cyclization during polymerization cannot be ruled out. Indeed, during the preparation of polymer 4-Cl , small amounts of a crystalline material were isolated and determined by X-ray diffraction studies to be an intramolecular cyclization product (Figure 2) (25). 2
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2
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. M e Me . ^Si-CXs-Si-^ Me Me 1-Br ;X = Br 1-CI ; X = CI
Me Me H-Si-CX -Si-H Me Me 2-Br ; X = Br 2-CI ; X = CI 2
2
2
2
2
Scheme 1. Potential monomers for hydrosilylation polymerization.
Me Me ^-Si-CCI -Si-^ Me Me 1-CI
Me Me H-Si-^O-Si-^-H Me Me
2
v
2
Karstedt's catalyst
MeCl Me Me Me SI-C-Si-CH CH -Si4o-Si)-CH2CH2 I I I I * I ' X Me Me MeCl Me 2
2
3- CI ; x = 1 4- CI ; x = 2 2
2
Scheme 2. Hydrosilylation polymerization.
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Table I. G P C molecular weight data (referenced to polystyrene standards). entry
Polymer
1
3-Cl
2
2
4-Cl
2
3
3-Cl(Me)
a
3-Cl(Me)
b
4 5
3-Cl(SiMe H)
6
3-Cl(Me)
M
2
c
n
M
w
8600
14500
16500
26700
4800
7700
3200
5600
4400
6700
7800
13100
a
from reaction of 3-Cl with one equivalent of methyllithium from reaction of 3-Cl with 1.5 equivalents of methyllithium "prepared from monomer modification route 2
b
2
Polymer Modification Reactions. A s part of our investigation we wished to demonstrate that the polymers we obtained could indeed be modified i n a relatively facile manner. Previous work has shown the - S i C C l S i - moiety in small molecules to be very reactive towards a variety of reagents, particularly organolithium compounds (26,27). T o demonstrate that this chemistry is applicable to the same moiety i n polymers, we performed two basic reactions on polymer 3-Cl outlined i n Scheme 3. In both cases, reaction did proceed as expected. The H N M R spectrum of polymer 3Cl(Me) obtained from the reaction of 3-Cl with one equivalent of methyllithium exhibited a new peak at 8 1.53 due to the added methyl group (Figure 1(c)). However, the integration indicated that approximately 25% of the C C 1 groups remained unreacted. Utilizing an excess of methyllithium decreased the amount of unreacted C C 1 groups i n 3-Cl(Me). The second reaction depicted i n Scheme 3 is simply a lithium-halogen exchange followed by nucleophilic attachment of a dimethylsilyl group. Unlike the first reaction, the *H N M R spectrum of 3-Cl(SiMe H) indicated that nearly complete substitution occurred. However, unidentified side reactions may also have occurred during this reaction, evidenced by the presence of several minor new peaks in the spectrum (around 8 0.0). It is also apparent that some cleavage of the polymer backbones took place in both of these reactions, as evidenced by a decrease in the G P C molecular weight values (Table I, entries 3-5). 2
2
l
2
2
2
2
When modifying polymers, it is often possible to modify the monomer prior to polymerization. The advantage of this method is that the resulting polymer would possess essentially 100% substitution and would be free of contaminants from polymer modification reactions. T o illustrate the feasibility of this approach, we investigated the possibility of modifying the monomer, 1-C1 , prior to polymerization. W e first prepared ( C H = C H S i M e ) C C l M e i n 54% isolated yield from the reaction of 1-C1 with methyllithium. Subsequent hydrosilylation polymerization utilizing ( C H = C H S i M e ) C C l M e and 1,1,3,3-tetramethyldisiloxane as monomers proceeded smoothly i n the manner used previously, yielding polymer 3-Cl(Me) in quantitative yield as a viscous o i l . The H N M R spectrum of 3-Cl(Me) (Figure 1(b)) prepared 2
2
2
2
2
2
2
J
2
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1
ppm
I 2
1
'
• '
'
'
' ' '
I' 1
' ' *
1
' '
1
• I
0
]
Figure 1. H N M R spectra of (a) polymer 3-Cl , (b) polymer 3-Ci(Me) prepared via modification of monomer, and (c) polymer 3-Cl(Me) prepared via modification of polymer (1.0 equiv M e L i ) . 2
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Figure 2.
Structure of intramolecular cyclization product isolated during
polymerization.
3-CI
2
1) n-BuLi/-78°C 2) Me HSiCi 2
3-CI(SiMe H)
3-CI(Me)
2
MeCl Me
Me
Me
-Si-C-Si-CHaCHa-Si-O-Si-CHaCHg MeX
Me
Me
Me
3-CI(Me); X = Me 3-CI(SiMe H); X = SiMe H 2
2
Scheme 3. Modification o f polymer 3-Cl . 2
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using this approach indicated that this product was considerably cleaner than polymer 3-Cl(Me) prepared by the polymer modification route. The molecular weight data is also similar to that obtained for polymer 3-CI (Table I, entry 6). The limitation of this method is that the choice of substitution on the monomer is restricted to groups that would not be reactive under the hydrosilylation polymerization conditions. For example, polymer 3-Cl(SiMe H) would be difficult to obtain v i a this route since the - S i M e H group in ( C H = C H S i M e ) C C l ( S i M e H ) would be reactive during the polymerization. 2
2
2
2
2
2
2
Polymer Insertion Reactions. It was of interest to us to increase the siloxane content of the synthesized polymers and thus change their properties. Since polymer 3-Cl contains an - S i - O - S i - linkage, it should be possible to insert cyclic siloxanes into the polymer backbone via the common ring-opening processes used to make polymeric silicones (Scheme 4) (28,29). This method would be an alternative to using long-chain siloxanes i n the original polymer synthesis (Scheme 2, x » 2 ) . A n initial experiment utilizing octamethylcyclotetrasiloxane (D ) as the insertion monomer proved to be successful. In a typical experiment, a sample of 3-Cl and two equivalents of D were heated in the presence of trifluoroacetic acid for 7 days. The resulting thick polymer gave a single peak in the G P C chromatogram and indicated a higher molecular weight than that of 3-Cl (M=13507, M =18935). The H N M R spectrum of this product is shown i n Figure 3, which indicates a relatively clean product. Reaction of polymer 3-Cl with tetramethylcyclotetrasiloxane ( D ' ) was next attempted, as this would provide additional reactive sites in the polymer i n the form of S i - H groups. These reactions were carried out i n chloroform using trifluoromethanesulfonic acid as catalyst (trifluoroacetic acid can be used but the reaction is slower). A s expected, a flowable colorless viscous o i l was obtained after workup and removal of volatiles. However, the o i l gradually turned into an insoluble gel-like solid on standing. This process occurred slowly in nitrogen but rapidly i n air. In addition, we observed the formation of gaseous acid during the crosslinking by noting a color change i n a piece of wet litmus paper held over the mixture. It is also noteworthy that gelation did not occur i f the polymer was kept in chloroform solution. A similar crosslinking occurred with the product from the reaction of polymer 3-Cl with tetramethyl-tetravinylcyclotetrasiloxane ( D ) . After removal o f chloroform solvent, the viscous o i l slowly crosslinked and became insoluble. L i k e the D ' crosslinking, this crosslinking occurred slowly in nitrogen but rapidly i n air. Overall however, the D product crosslinking occurs at a much slower rate than the D ' product crosslinking. T o further probe the mechanism of these crosslinking reactions, a reaction between polymer 3-Cl and 1,1,3,3,5,5-hexamethyltrisiloxane was prepared (Scheme 5). After mixing the reagents thoroughly, the flask was exposed to regular room lighting conditions. After one month, the mixture had become an insoluble gum-like solid. In a separate experiment, the mixture was exposed to a weak ultraviolet light source (a thin-layer chromatography plate visualizer). In this case, the crosslinking was more rapid with complete gelation occurring after one week. During this experiment, the reaction progress was monitored by IR spectroscopy. The decreasing intensity of the S i - H absorbance was readily apparent during the course of the reaction, with the peak disappearing completely after one week (Figure 4). Based on the results and observations described above, we believe the crosslinking reactions proceed via a radical mechanism. One possible mechanism is 2
4
2
4
J
2
w
2
4
v,
2
4
4
V l
4
4
2
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2
Me
M
