Synthesis and Properties of Silicones and Silicone-Modified Materials

Chapter 5

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Ru-Catalyzed Hydrosilylation Polymerization An Overview of RuH (CO)(PPh ) -Catalyzed Hydrosilylation Copolymerizations of α,ω-Diketones with α,ω-Dihydrido-oligodimethylsiloxanes and Polymerizations of ω-Dimethylsilyloxy Ketones 2

3 3

Joseph M. Mabry, Matthew K. Runyon, Jyri K. Paulasaari, and William P. Weber* Κ. B. and D. P. Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, C A 90089 *Corresponding author: email: [email protected]

Activated dihydridocarbonyltris(triphenylphosphine)ruthenium (Ru) catalyzes the hydrosilylation copolymerization of both aromatic and aliphatic α,ω-diketones with α,ω-dihydrido-oligodimethylsiloxanes to yield poly(silyl ether)s. The effect of siloxane chain length on copolymer T has been evaluated. Ru also catalyzes the polymerization of both aromatic and aliphatic ω-dimethylsilyloxy ketones to yield poly(silyl ether)s. Chiral centers affect the NMR spectra of these poly(silyl ether)s. The susceptibility of these polymers to hydrolytic degradation, which depends on structure, will be discussed. g

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© 2003 American Chemical Society

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Properties of Poly(silyl ether)s Poly(dimethylsiloxane) (PDMS) is a crystalline polymer, with a melting point ( T J near -40 °C and a glass transition temperature (T ) near -125 °C (7). Many copolymers, which contain oligodimethylsiloxane (ODMS) units, no longer exhibit detectable T s, but still have T s close to that of PDMS (7). While PDMS is biocompatible (2), the strength of the Si-O-Si bond linkage makes it resistant to hydrolysis and thus biodégradation. Poly(silyl ether)s, on the other hand, contain Si-O-C bond linkages, which are susceptible to acid or base catalyzed hydrolysis (3). This hydrolytic instability may make them attractive for various applications, such as the controlled release of drugs, or as materials whose degradation will limit their long-term environmental impact (4). There is also interest in the possible utility of poly(silyl ether)s as membranes (5), sensor materials (6), and elastomers (7). Poly(silyl ether)s may also be applicable to various space applications because of their stability to high temperatures and ultraviolet radiation (8). Some poly(silyl ether)s exhibit excellent flame retardance (9). g


m

g

Previous Synthetic Methods to Prepare Poly(silyl ether)s A general method to prepare poly(silyl ether)s is the equilibration polymerization of either dialkoxysilanes (70), dihalosilanes (77), or diamino silanes (8,12) with diols. Acid catalyzed ring-opening polymerization of 2-sila1-oxacyclopentanes has also been reported (75). Due to the instability of poly(silyl ether)s to acid or base, methods, which use neutral conditions, are favored. Among these are transition metal catalysis. High molecular weight poly(silyl ether)s have been obtained by rhodium or palladium catalyzed crossdehydrocoupling of bis(Si-H) compounds with α,ω-diols (14-17). Poly(silyl ether)s containing pendant chloromethyi groups have been synthesized by quaternary ammonium chloride catalyzed reaction of bis(oxetane)s or bis(epoxide)s with dichlorosilanes (18-21). Palladium catalyzed condensation copolymerization of bis(Si-H) compounds with para-quinones has also been successfully applied (22). We have reported the ruthenium catalyzed dehydrogenative silylation condensation copolymerization of ortho-quinones with α,ω-dihydrido-oligodimethylsiloxanes shown in Figure 1 to yield polycyclic aromatic poly(silyi ether)s (23,24). We have also reported the ruthenium catalyzed competitive condensation/addition of α-diketones with α,ω-dihydrido-oligodimethy lsiloxanes (25,26).

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ρ +


-fO

H-Si-O-Si-O-Si-O-Si-O-Si-H

Ο-Si-O-Si-O-Si-O-Si-O-Si-^·

Figure 1. Ru catalyzed condensation copolymerization.

Ru Catalyzed Hydrosilylation Copolymerization of Aromatic α,ω-Diketones The catalyst, RuH (CO)(PPh ) (Ru), preparedfromruthenium trichloride hydrate (27), was activated with styrene in toluene at 125 °C for 3 min. The color of the activated catalyst solution is red (28). The styrene activates the catalyst by removal of hydrogen and production of ethylbenzene. One triphenylphosphine ligand is also lost. This produces the highly coordinately unsaturated catalyst "Ru(CO)(PPh ) " shown in Figure 2 (29). Approximately one mole percent of the catalyst was routinely used. Among the advantages of the Ru catalyst is that it does not equilibrate the ODMS units. Siloxane equilibration is observed in many cation and anion catalyzed reactions (30). This Ru catalyzed hydrosilylation copolymerization is unusual in that, while the reacting solution is brown, completion of the reaction is often indicated by the return of the red color associated with the initial activated Ru catalyst. In this way, the reaction is comparable to a titration. 2

3

3

3 2

RuH (CO)(PPh ) 2

3

3

125-130 °C toluene

Figure 2. Activation of catalyst. Aromatic α,ω-diketones, e.g. 1,4-diacetylbenzene and 4,4'-diacetyldiphenylether, were independently reacted with 1,3-dihydridotetramethyidisiloxane (TMDS), 1,5-dihydridohexamethyltrisiloxane (HMTS), 1,7-dihydridooctamethyltetrasiloxane (OMTS), and 1,9-dihydridodecamethylpentasiloxane (DMPS). These reactions were carried out in the presence of activated Ru to

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yield poly(silyl ether)s. Copolymerization of 1,4-diacetylbenzene with DMPS is shown in Figure 3. DMPS was prepared by a triflic acid catalyzed reaction of TMDS and hexamethylcyclotrisiloxane (D ) (30). 3

I

I

I

I

i

H-Si-O-Si-O-Si-O-Si-O-Si-H


/ 4θ

I

JT\ f

ι

ι

ι

i

x

O-Si-O-SHO-Si-O-ShO-Sit-

X

)

Figure 3. Ru catalyzed reaction of 1,4 diacetylbenzene with DMPS The T s of the copolymers decrease as the length of the ODMS unit between the aromatic units increases. This is expected because of the flexibility of the ODMS segment. The nature of the aromatic unit in the copolymer also affects T s. The plot of the T s for the copolymers derived from 4,4'diacetyldiphenylether, while similar to that of those derived from 1,4diacetylbenzene, has a steeper slope. Both plots are shown in Figure 4. g

g

g

Number of Siloxane Units (η) 1

2

3

4

! 5

6

j

j -•-1,4-Diacetylbenzene HK-4,4'-diacetyldiphenyleéier !

Figure 4. Plots ofTgS vs. number of siloxane units. The NMR spectra for these α/ί-copolymers are complicated by the presence of chiral centers (*) as seen in Figure 5. The spectra are similar for both the 1,4diacetylbenzene and 4,4'-diacetyldiphenylether copolymers. The Ru catalyzed addition of the Si-Η bond across the C-0 double bond of the ketone results in formation of a single chiral center, so each polymer unit contains two chiral centers. This results in two different stereochemical environments for the Simethyl groups.

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H

J T \

H

0-Si-O-ShO-Si


Figure 5. Two chiral centers created in each monomer unit. The Si-methyl protons in the TMDS copolymers are all chemically equivalent. However, each is split by the nearest chiral center and then split again by the more remote chiral center. This produces four diastereotopic environments: RR', SS\ R S \ and R'S, which are distinct. This results in four singlets of equal intensity as seen in Figure 6A. In the case of the HMTS copolymers, each polymer unit contains three silicon atoms. Two of these are adjacent to a chiral center. Their diastereopic environments are only affected by this adjacent chiral center which can be either R or S. This results in two signals of equal intensity. The central silicon of the trisiloxane copolymer is affected by both chiral centers. This results in four diastereotopic environments: RR, SS, RS and SR. However, RS and SR are meso. This leads to three resonances in a 1:2:1 ratio as seen in Figure 6B. (A) (B)

Figure 6. ^HNMR spectra of aromatic copolymers. The OMTS copolymers contain four silicons, each of which is affected only by the nearest chiral center, which can be either R or S. The difference between these environments is larger for the Si-methyl groups which are adjacent to chiral center than those which are further removed. This leads to four resonances of equal intensity as seen in Figure 7A. The DMPS copolymers have five Si-methyl groups. Those, which are closest to the chiral center, experience the largest difference in diastereotopic environments. The inner pair of Si-methyl groups experience a smaller difference. Finally, the central Si-methyl groups are not affected by either chiral center because they are too remote. This results in five signals of equal intensity as seen in Figure 7B.

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(A)

0.02

0.00

-0.02

-0.04

-0.06

PPM II

0 . 1 2 0 . 1 0

0.08

0.06

PPM)

Figure 7. ^HNMR spectra of aromatic copolymers.

Ru Catalyzed Hydrosilylation Copolymerization of Aliphatic Diketones 2,5-Hexanedione and 2,7-octanedione (31,32) were independently reacted with the same four α,ω-dihydrido-oligodimethylsiloxanes above (33). The reaction is shown in Figure 8. A difference between the two systems is that the color change of the Ru catalyst from brown to red at the conclusion of the reaction previously reported with aromatic α,ω-diketones is not observed. I

l

0

l

H-SHO-fSHOtSi-H '

'

'

+

J

Synthesis and Properties of Silicones and Silicone-Modified Materials

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