Enantioselective Living Coordinative Chain Transfer Polymerization: Production of Optically Active End-Group-Functionalized (+)- or (−)-Poly(methylene-1,3-cyclopentane) via a Homochiral C₁-Symmetric Caproamidinate Hafnium Initiator

Abstract

The homochiral, configurationally stable, C 1-symmetric dimethyl, pentamethylcyclopentadienyl, N-(S)- or N-(R)-1-phenylethyl-caproamidinate hafnium preinitiators (S C ,S Hf )-1 and (R C ,R Hf )-1, respectively, can be used in combination with the borate coinitiator [PhNHMe2]­[B­(C6F5)4] (B) and excess equivalents of ZnEt2 for the enantioselective living coordinative chain transfer polymerization (LCCTP) of 1,5-hexadiene to provide (+)- and (−)-poly­(methylene-1,3-cyclopentane) (PMCP) of tunable molecular weight and very narrow polydispersity. Use of excess I2 to quench LCCTP provides the corresponding (+)- and (−)-iodo-terminated PMCP products. This enantioselective LCCTP process serves to establish access to readily available, practical, and scalable quantities of optically active liquid crystalline end-group-functionalized xPMCP building blocks for material science and technology discovery.

Zirconium Permethylpentalene Amidinate Complexes: Characterization, Bonding, and Olefin Polymerization Catalysis

“…While amidinate complexes have been widely studied for homogeneous catalysis, examples of solid-supported amidinate catalysts for olefin polymerizations are very scarce. ,, Sita and co-workers successfully developed a Cp*ZrMe­{(NEt) (N t Bu)­CMe} initiator immobilized on a polymer support to polymerize α-olefins . Furthermore, Sita’s group has hugely contributed to the study of both living coordination polymerization and living coordinative chain transfer polymerization of alkenes and on the acidic nature of amidinate fragments within group 4 metal complexes. − …”

Optical Purity as a Programmable Variable for Controlling Polyolefin Tacticity in Living Coordinative Chain Transfer Polymerization: Application to the Stereomodulated LCCTP of α,ω-Nonconjugated Dienes

“…Poly­(methylene-1,3-cycloalkane)­s are obtained from the highly regioselective coordinative cyclopolymerization of α,ω-nonconjugated dienes. − As Scheme reveals, the stereochemical microstructures of these materials are more complex by virtue of two stereodefining steps that are involved in propagation. The first step is enantioface complexation and migratory insertion ( i ) of the α-end of the diene that establishes the relative configuration of every other stereocenter in the microstructure or, formally, overall tacticity.…”

“…The remaining four possible tetrads that arise with cis / trans mixtures are then given as MmR, MrR, RmM, and RrM and are not shown. Most importantly, the stereochemical microstructures of PMCP and PMCH are both amenable to 13 C NMR analysis, and in the case of PMCP this has included a quantitative determination of values for both α and σ at the tetrad level for different systems., ,, As a final consideration, the enantioselective coordinative polymerization of 1,5-hexadiene to yield optically active PMCP is well precedented. , According to Scheme , only the 1,3- trans , isotactic microstructure lacks mirror plane symmetry and is, therefore, chiral and can give rise to optical activity. In practice, however, as long as the isotactic criterion is met, a range of cis / trans ratios for PMCP have been shown to support optical activity due to the presence of an overall global noncentrosymmetry of each polymer chain.…”

“…In a preliminary study, we demonstrated some time ago that the LCCTP of 1,5-hexadiene to provide PMCP was achievable using a C s -symmetric CPAM Hf initiator . We have also just very recently reported the first example of enantioselective LCCTP using the readily available, optically pure (homochiral), and configurationally stable C 1 -symmetric caproamidinate hafnium preinitiators, ( S C , S Hf )- and ( R C , R Hf )- 1 that are shown in Scheme . , Importantly, in the case of 1,5-hexadiene, optically active cis / trans , isotactic PMCP materials were obtained of either handedness when the LCCTP of this monomer was conducted using either ( S C , S Hf )- or ( R C , R Hf )- 1 after activation by addition of 1.1 equiv of the borate co-initiator B1 in the presence of a fixed stoichiometric excess of ZnEt 2 . Further, when this LCCTP process is quenched by the addition of molecular iodine (I 2 ), practical quantities of optically active, iodo-terminated cis / trans , isotactic PMCP are produced.…”

“…Homochiral and enantiopure ( S C , S Hf )- 1 and ( R C , R Hf )- 1 were prepared according to the previously published procedures, , and different levels of optical purity (ee) were established by mixing measured quantities of these two enantiomers together prior to performing the LCP and LCCTP of 1,5-hexadiene and 1,6-heptadiene according to the conditions presented in Table . Also provided in this compilation are the relevant analytical and spectroscopic characterization data for the series of PMCP and PMCH materials that were obtained.…”