Highly Stereoselective Living Polymerization of Vinyl Ethers at Ambient Temperature Mediated by Chiral Titanium Complexes

Abstract

The synthesis of polymers from polar monomers such as vinyl ethers (VE) assumed significance because of their easy polymerizability and the excellent properties of the resulting polymers. [1] The classical cationic polymerization systems did not exhibit control over VE polymerization. [2] Polymerization of VE can be activated by using Lewis acids [3] or non-nucleophilic counterions. [4] However, although Lewis acid-catalyzed polymerizations of VE are closely related to those that take place in nature, in most cases it is difficult to control the stereoregularity of the obtained polymers. [5] There have been several reports on the synthesis of stereoregular poly(VE). [6] Interestingly, the ligand environment has been shown to influence the stereoregularity of the polymers obtained in the Pd-catalyzed polymerization of VE. [7] Masuda et al. reported that selected metallocene and nonmetallocene catalysts in combination with methylaluminoxane (MAO) could polymerize vinyl ethers with moderate isotacticity, but the origin of the isotacticity was not explained. [8] Sawamoto and co-workers improved the stereoregulation in cationic polymerization of VE by utilizing designed Lewis acids. [3, 5] They achieved isotactic polymers with a meso dyad of 90-92 % by carrying out the reaction in n-hexane at À78 8C in the presence of a bulky pyridine. An increase in reaction temperature yielded lower molecular weight atactic poly(VE)s. [5] We aimed to develop a new class of transition metal catalysts to produce highly stereoregular poly(VE) with high molecular weights at ambient temperature. We have already established the ability of aminotriolate-based titanium chlorides with C 3 and pseudo-C s symmetry and modified aminodiolate titanium dichlorides with pseudo-C 2 and pseudo-C s symmetry as effective catalysts for ethylene, a-olefin, [9, 10] and n-butyl vinyl ether (BVE) polymerizations. [11] Literature precedence for the use of titanatranes as olefin polymerization catalysts is very limited. [12] We previously developed a series of titanium precatalysts (Figure 1) for the polymerization of linear a-olefins; [10] [1-TiCl] and [2-TiCl] showed moderate activity whereas [3-TiCl 2 ] and [4-TiCl 2 ] showed very good activity with living behavior. Herein, we therefore examined possible stereoregulation in BVE polymerization by employing different titanium catalysts, in combination with MAO or borate, for which the bulkiness and the electronic nature of metal center could be manipulated by modifying the structure of the ligand. We have also substantiated a carbocationic mechanism for the polymerization of vinyl ethers.The precatalysts [1-TiCl], [2-TiCl], [3-TiCl 2 ], and [4-TiCl 2 ] in combination with MAO were able to polymerize BVE in excellent yields. The key feature of this study is how the ligand nature influences the reactivity of the titanium center towards polymerization of vinyl ethers. The titanium catalysts containing modified triethanolamine ligands have been effectively studied for the homo-and copolymerizations of...

100th Anniversary of Macromolecular Science Viewpoint: The Past, Present, and Future of Stereocontrolled Vinyl Polymerization

“…Sudhakar and co-workers also reported bulky titanium-derived Lewis acids for cationic polymerization but were unable to achieve similar success regarding stereoselectivity. 162,163 Sawamoto and co-workers additionally investigated various combinations of protic and Lewis acids for the stereoselective polymerization of vinyl ethers. 164 In the polymerization of IBVE, modest levels of isotacticity (68−86% m) were achieved using the Lewis acid SnCl 4 combined with a bulky phosphoric acid (Figure 8C).…”

“…A statistical analysis of triad distributions suggested enantiofacial monomer addition was controlled by the catalyst; however, similarly high levels of stereocontrol were not observed when exploring vinyl ethers bearing alternative pendant functionality, affording isotactic-enriched poly­(VE)­s with n -butyl (76% m ), t -butyl (69% m ), iso -propyl (88% m ), n -propyl (78% m ), and ethyl (64% m ) pendant groups. Sudhakar and co-workers also reported bulky titanium-derived Lewis acids for cationic polymerization but were unable to achieve similar success regarding stereoselectivity. , …”

Ionic Liquids as Solvents and/or Catalysts in Polymerization

“…This is a very effective way of recovering the catalyst system using ionic liquid-based homogeneous polymerization. Our attempts to polymerize 1-hexene with non-metallocene catalysts [100,101] 4 ]. The recovered catalyst system in IL was effective for further use without any significant loss in activity [97].…”

Tetradentate schiff base ligand/MCln initiating systems for the controlled cationic polymerization of isobutyl vinyl ether: Effects of the ligand framework

“…In cationic polymerization, Lewis acid catalysts play a pivotal role in the precise synthesis of well‐defined polymers with controlled molecular weights, narrow molecular weight distributions (MWDs), end functionality, and controlled tacticity . Indeed, controlled/living cationic polymerization has been established with many initiating systems through the adjustment of the Lewis acidity of the catalysts using additives such as weak Lewis bases and tetraalkylammonium halides .…”

“…Furthermore, alcohol or acetylacetone efficiently react with appropriate metal halides to allow for in situ ligand exchange reactions, generating real catalytic species that induce living polymerization . More interestingly, the ligand design of soluble complex catalysts, such as TiCl 2 (OAr) 2 and transition metal complexes with tridentate ligands, was demonstrated to be effective for stereospecific cationic polymerization.…”