Mechanism‐Based Design of a ROMP Catalyst for Sequence‐Selective Copolymerization

Abstract

We report here the mechanism-based design of a ring-opening metathesis polymerization (ROMP) [1] catalyst which preferentially assembles a mixture of cyclooctene and norbornene into an alternating copolymer. While there have been reports of regular copolymerizations, highly selective examples are uncommon outside of free-radical polymerization, with the 1:1 alternating copolymers of ethylene and carbon monoxide, [2] or of epoxides and carbon dioxide, [3] being perhaps the most prominent examples. Given that the exemplary functional characteristics of biopolymers, for example, structural or catalytic functions in polypeptides, information storage and transmission in polynucleotides, and molecular recognition in polysaccharides, derive ultimately from sequence-selective copolymerization of simple monomer units, one can presume that synthetic polyolefins, as high-performance materials, would be substantially enhanced if even simple sequenceselective copolymerizations could be achieved routinely. Treating regular copolymerization more generally as a problem in metal-catalyzed organic synthesis, one sees that the issue is not stereoselectivity, or even regioselectivity. We seek to introduce into polymerization catalysts the element of chemoselectivity, which is the most basic kind of selectivity in organic synthesis. We encode the sequence information in the catalyst itself, and in this present study succeed in the most primitive, two-component, alternating sequence of ring-opening metathesis copolymerization.Phosphine ligand 1 was prepared by sequential alkylation and arylation of phenyldichlorophosphine with tBuMgCl and ortho-methoxyphenyllithium, isolation by careful distillation, cleavage of the methyl ether by BBr 3 , and then deprotonation with NaH. Reaction of one equivalent of 1 and [(Cy 3 P) 2 RuCl 2 ( = CHPh)] (2, Scheme 1) resulted in phosphine exchange, followed by elimination of NaCl. A similar approach has been reported by Hoveyda and co-workers for the preparation of a tethered, second-generation metathesis catalysts. [4] The resulting carbene complex 3 was purified by column chromatography under rigorous exclusion of oxygen, and checked by 1 H and 31 P NMR spectroscopy, as well as ESI-MS, to confirm that a single compound was prepared. Importantly, no other carbene species was present (analytical details and further information on the synthesis of 1 and 3 can be found in the Supporting Information). Polymerization of norbornene, cyclooctene, and mixtures thereof, together with 0.05 mol % catalyst, were conducted under dry N 2 at room temperature in either dichloromethane or cyclooctene as solvent. In experiments with cyclooctene as solvent, the norbornene is consumed quantitatively after 17 h. If a less-coordinating solvent, such as CH 2 Cl 2 , is used, then norbornene is consumed much more rapidly, with phenomenological rates comparable to those observed with catalyst 2 under the same conditions. The resulting polymer was isolated by removal of solvent at 0.01 mbar pressure until no further weight los...

Recent advances in ring-opening metathesis polymerization, and application to synthesis of functional materials

“…Appropriately designed dualsite Ru-carbene complexes catalyze the alternating copolymerization of norbornene and a large excess of cyclooctene, wherein one site of the complex shows chemoselectivity while the other site does not. [81][82][83] The combination of polar 2,3-difunctionalized 7-oxanorbornene derivatives and nonpolar cyclic olefins, including cyclooctene, also works satisfactorily. 84,85 The alternating copolymers form well-controlled micrometer-scale aggregates by complementary noncovalent interactions when diaminopyridine and thymine side chains are introduced.…”

Catalytic Enantioselective Olefin Metathesis in Natural Product Synthesis. Chiral Metal‐Based Complexes that Deliver High Enantioselectivity and More

“…Design of a chiral Ru complex, utilized in the racemic form, to promote sequence-selective copolymerization processes, [xxx] constitutes a notable demonstration of how chiral catalysts can deliver unusual selectivity profiles. [xxxi] As illustrated in Scheme 12, the stereogenic-at-Ru complex 41 has a relatively more stable diastereomeric form ( 41 and 42 ), where the carbene resides beneath the sterically less demanding phenyl substituent of the phosphine ligand.…”

Synthesis of Copolymers by Alternating ROMP (AROMP)

“…With the exception of Grubbs’s ring opening insertion metathesis (ROIMP) approach, in which alternation is controlled by equilibration,6 these rely on the pairing of a bulky but strained monomer with an unhindered and only slightly strained monomer. In all cases except one,7 a significant excess of one of the monomers is required for high levels of alternation 8…”