Polydispersity control in ring opening metathesis polymerization of amphiphilic norbornene diblock copolymers
Introduction
The advent of ring-opening metathesis polymerization (ROMP) has allowed the synthesis of many polymers previously unavailable with standard synthesis techniques [1], [2], [3], [4]. The high tolerance of ROMP catalysts to various functional groups along with their high activity enables facile synthesis of ampiphilic block copolymers. Ruthenium catalysts, such as Grubbs's catalyst and Schrock's Mo and W ROMP catalysts have found wide use in the synthesis of block copolymers [5].
An amphiphilic block copolymer contains a hydrophobic block and a hydrophilic block. By exploiting the thermodynamic phase separation of amphiphilic diblock copolymers, nanodomains of hydrophilic blocks can be formed, which can then be used to incorporate metal salts [6], [7], [8], [9], [10], [11], [12]. Norbornene can be used as the hydrophobic block and is a commercially available monomer that is easily purified. Grubbs and coworkers reported that norbornene cannot be polymerized in a living fashion using Grubbs's catalyst [13]. They achieved a polydispersity index (PDI) of 2.0 for norbornene homopolymers initiated with Grubbs's catalyst.
In this paper, a method of synthesizing block copolymers of narrow polydispersity of norbornene (NOR) and norbornene dicarboxylic acid (NORCOOH) with Grubbs's [(PCy3)2Cl2RuCHPh, Cy=cyclohexyl, Ph=phenyl] catalyst is reported. All but one of the synthesis were performed by polymerizing a 5-norbornene-2,3,-dicarboxylic acid bis trimethylsilyl ester (NORCOOTMS) precursor first, followed by the polymerization of unfunctionalized norbornene. The resultant diblock copolymers [NORCOOTMS]m/[NOR]n were converted to [NORCOOH]m/[NOR]n by precipitating the polymer solution in a mixture of methanol, acetic acid, and water. The PDIs of the copolymers with NORCOOTMS as the starting monomer were less than 1.26 in all cases, in contrast to the block copolymer with norbornene as a starting monomer, which had a PDI of 1.61. The block copolymer having a block ratio of NORCOOH:NOR=50:400 had a PDI of 1.26. Increased block lengths of NORCOOTMS resulted in decreased PDIs, as a NORCOOH:NOR=150:400 copolymer gave a nearly monodisperse PDI of 1.05. These reactions were performed without modification of the catalyst or addition of excess phosphine to control the rate of propagation [14], [15]. The method presented in this paper shows that the sequence of monomer addition is a factor for controlling the PDI of ROMP synthesized diblock copolymers.
Section snippets
Materials
Norbornene (NOR), ethyl vinyl ether and dichloromethane (CH2Cl2) were purchased from Aldrich. Bis(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride (Grubbs's catalyst) was purchased from Strem Chemicals. CH2Cl2 was distilled over calcium hydride under argon. 5-Norbornene-2,3,-dicarboxylic acid (NORCOOH) and 5-norbornene-2,3,-dicarboxylic acid bis trimethylsilyl ester (NORCOOTMS) were prepared according to literature [6].
All the solvents, monomers and catalysts required for polymer
Results and discussion
The PDI of ROMP polymers is related to the propagation rate constant (kp) and initiation rate constant (ki). The value of kp/ki should be less than 10 to produce a living polymer with a low PDI [5]. A smaller kp/ki (<10) results in complete catalyst initiation of monomers. When propagation starts after full initiation of monomers, the number of polymer chains remains constant, equal to the number of the original initiating species [16]. The catalytic activity and rate of initiation of ROMP
Summary
We have synthesized block copolymers composed of norbornene with relatively low polydispersity using Grubbs's catalyst [bis-(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride] as the initiator for a living ROMP. In order to control the polydispersity of the block copolymer, the bulkier 2-norbornene-5,6,-dicarboxylic acid bis trimethylsilyl ester (NORCOOTMS) was the first monomer to be polymerized. Because of steric interference between the NORCOOTMS monomers and inhibition of Grubbs's
Acknowledgements
This material is based upon work supported by the National Science Foundation Grants No. CTS-9875001, DMR-008008, and the Office of Naval Research Grant No. N00140010039.
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