Chapter 17
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Organic-Inorganic Hybrid Polymers from Atom Transfer Radical Polymerization and Poly(dimethylsiloxane) Krzysztof Matyjaszewski, Peter J. Miller, Guido Kickelbick, Yoshiki Nakagawa, Steven Diamanti, and Cristina Pacis Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213
A summary of the synthesis of hybrid materials composed o f inorganic siloxanes and polymers prepared by atom transfer r a d i c a l polymerization (ATRP) is given. Hydrosilation of vinyl or hydrosilyl terminal and pendant poly(dimethylsiloxane) ( P D M S ) w i t h an attachable initiator yielded macroinitiators for A T R P . Polymerization of styrene and (meth)acrylates resulted i n triblock copolymers with increased molecular weights and reduced polydispersities. L o w polydispersity living anionic P D M S was terminated with an attachable initiator containing a benzyl chloride moiety. A T R P o f styrene from such a monofunctional macroinitiator resulted in formation of a block copolymer with well-defined segments. A tetrafunctional initiator was synthesized from a cyclotetrasiloxane core toward formation o f star polymers by A T R P . Finally, a silsesquioxane initiator was used i n styrene ATRP yielding a polymer with a bulky inorganic tail group.
A s technology i n a variety o f fields improves, the need for specialized high performance materials becomes a necessity. One major goal is the combination o f properties not accessible by one kind o f material. For example, i n microlithography and electronic applications, a combination of low dielectric properties and mechanical strength is required while in medicine, oxygen permeability and biocompatibility is paramount. The list o f examples is ever increasing. T o fulfill these requirements scientists and engineers are using copolymers o f varying compositions and/or architectures to obtain the desired properties. One such set o f copolymers that is gaining interest are the inorganic / organic hybrid materials. Inorganic polymers generally have specific properties w h i c h organic analogues do not possess, making them desirable i n hybrid materials. Polyphosphazenes w h i c h have high thermal stability and biocompatibility - and polysilylenes possessing photoconductive, photorefractive and nonlinear optical properties - are two common examples ( i ) . However, the most widely studied inorganic polymers are the polysiloxanes. Poly(dimethylsiloxane) ( P D M S ) has a high oxygen permeability, chain flexibility and thermal conductance, making it attractive for a variety o f applications ranging from biomedicine to thermal transfer f l u i d technologies ( i ) . Unfortunately, the homopolymer o f P D M S is unattractive from a
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2000 American Chemical Society
In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
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mechanical standpoint due to its low dimensional stability. Therefore, the copolymerization of P D M S with tougher materials can lead to improved properties. In many cases, organic polymers have been found good candidates to fill this role. Typically, polysiloxanes are synthesized by ionic mechanisms leading to polymers with a high degree of terminal functionality ( i ) . Materials related to polysiloxanes are silsesquioxanes (Scheme 1) (2). These cubic siloxane molecules have been produced with a number of functional species such as styrene and methacrylate moieties protruding from one corner of the structure (3, 4). Classic free radical polymerization of these molecules has been shown to produce an inorganic / organic hybrid homopolymer. Furthermore, cubes have been synthesized with pendant silyl hydrido and silanolate moieties which can be used for further functionalization (5). R a
R
l f
R , R3, R , R , R , R7, Rg = OH 2
4
5
6
Scheme 1 Organic polymers have been more widely studied than the inorganic analogues. In terms of block copolymers functionality is again paramount. T y p i c a l l y , this functionality is achieved through l i v i n g ionic polymerizations. However, the underlying mechanisms rely on stringent reagent purity and are limited to a select number of monomers. Recently, controlled free radical polymerization has provided an alternative to the ionic techniques. In particular, atom transfer radical polymerization ( A T R P ) is most effective (6, 7). The method utilizes a dynamic equilibrium between active (P -) and dormant ( P - X ) radical species facilitated by halogen atom (X) transfer mediated by a transition metal (Mt) species (Scheme 2) (8). For polymerizations that exhibit first order kinetic consumption of monomer and molecular weights predetermined by the ratio of consumed monomer to initially infused initiator, the value of the equilibrium constant must be sufficiently low such n
n
M k. -X
+
m
Mt L
P • n
X M t JTl+1L 1
1
+
2 XMt
m + 1
L
Scheme 2 that the steady-state radical concentration is on the order o f that observed i n conventional free radical systems. This low radical concentration limits termination to negligible levels. Narrow molecular weight distributions ( M / M < 1.3) are w
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In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
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obtained when the rates of initiation and deactivation are greater than or equal to that of propagation. A T R P has been shown to be effective for polymerization of styrenes (6, 8-10), acrylates (6), methacrylates (6, 11-16), and acrylonitrile (17) using metals such as copper (6), iron (12), ruthenium (11) and nickel (18). The combination of control over chain length and functionality has allowed for the synthesis of numerous (co)polymers exhibiting a variety of compositions and architectures. Figure 1 demonstrates the general form of said structures. Linear, diblock, triblock, graft, network, star and hyperbranched polymers can a l l be synthesized with relative ease (19, 20). Since all that is needed to initiate A T R P are activated alkyl halides (21-23), a number of research groups have synthesized segments by different mechanisms (ionic, conventional free radical, step growth, etc.) as macroinitiators for the A T R P of vinyl monomers in the synthesis of block (24-34) and graft (co)polymers (34-39). This combination of mechanisms with A T R P is not limited to strictly organic segments. T w o primary examples resulting i n inorganic / o r g a n i c h y b r i d m a t e r i a l s are the g r a f t i n g o f p o l y s t y r e n e f r o m a poly(methylphenylsilylene) backbone (36) and growth o f polyacrylamide from a modified silica surface (40). This paper w i l l summarize work performed in our laboratory on the synthesis of hybrid materials using cyclic oligo- and linear polysiloxanes in conjunction with polystyrenes, acrylates and methacrylates synthesized by A T R P . Architectural variation w i l l also be discussed including linear, graft and star polymers. Results and Discussion A B A Triblock Copolymers. There has been considerable interest in the use of poly(dimethylsiloxane) i n d i - and triblock copolymers for applications such as thermoplastic elastomers and pressure sensitive adhesives (41-44). In terms o f inorganic / organic hybrids, copolymers have most often consisted of polystyrene and P D M S . For example, a diblock copolymer of poly(styrene-Z?-dimethylsiloxane) was produced by polymerization o f hexamethylcyclotrisiloxane (D3) from l i v i n g polystyryl lithium (45). Here, A B A triblock copolymers could not be produced by subsequent addition of styrene monomer to the living lithium silanolate solution due to the inability of the active species to reinitiate styrene polymerization. However, in other contributions triblock copolymers were prepared by coupling reactions using hydrosilation techniques (46) or by condensation of silanolates (47, 48) with organic polymers containing reactive silyl chlorides (49). In these cases, block copolymers of high purity and yield were difficult to obtain due to the need for exact stoichiometry and reagent purity required in the coupling of two macromolecular species. In terms of free radical chemistry, the best examples of inorganic / organic linear hybrid materials is the use of P D M S containing in-chain silylpinacolate moieties to initiate styrene and methacrylate polymerizations (41, 42). Due to the predominance of termination by radical coupling over disproportionation in styrene polymerization, segmental copolymers were produced. However, the polymerizations were not " l i v i n g " and polydispersities were often greater than 3. A z o terminal P D M S macroinitiators have also been reported (44). Here, formation of a mixture of block and homopolymers resulted due to initiation from both sides of the homolytically cleaved initiating species (44). In our laboratory the motivation for the synthesis of d i - and triblock copolymers was three-fold: 1) synthesis of the organic blocks by a " l i v i n g " polymerization technique such that segment length and copolymer composition could be controlled, 2) prevention of coupling of macromolecular species and 3) use of a technique that would allow for the synthesis of a plethora of copolymers based on variation of the organic monomer. Considering these qualifications, A T R P of v i n y l monomers from mono- and difunctional P D M S macroinitiators was used. Scheme 3 illustrates the synthesis of A B A triblock copolymers from terminal functionalized P D M S . The method utilizes hydrosilation of attachable initiators to commercially available vinyl
In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000. x
x
Y = halogen or functional group X = halogen
Homopolymer Statistical Copolymers Gradient Copolymers
Figure 1. Summary of architectures available through A T R P .
Hyperbranched
Comb / Graft Copolymers
X
Diblock Copolymers
-^/\/\/\/\/\/\/\/\/\/\r Y
Crosslinked
Triblock Copolymers
-^y\y\/\yv\y\/Yr
