In Situ Generated ABA Block Copolymers from CO₂, Cyclohexene Oxide, and Poly(dimethylsiloxane)s

Chain-transfer polymerization reactions with siloxanes, CO2, and cyclohexene oxide have been conducted, utilizing two β-diiminate (BDI) zinc-based catalysts, BDICF3(1)-ZnEt and BDICF3(2)-ZnEt ((BDICF3(1))H = [CH­(CCF3NC6H4-2,6-C2H5)2] and (BDICF3(2))H = [CH­(CCF3NC6H4-2,6-CH­(CH3)2)2]). The correlation between equivalents of siloxane and the corresponding molecular masses and glass transition temperatures is exhibited. Furthermore, the in situ preparation of ABA block copolymers from carbon dioxide, cyclohexene oxide, and α,ω-bis­(hydroxymethyl)­poly­(dimethylsiloxane)­s is presented. This reaction was found to strongly relate to a robust Lewis acid catalyst like the outlined complexes. The polymer properties can be tuned by varying the amount of chain-transfer agent or changing the catalyst. The resulting polymer structures and incorporation of siloxanes were revealed by 29Si NMR spectroscopy, 1H NMR spectroscopy, ESI-MS, GPC, and DSC

Electron-Deficient β‑Diiminato-Zinc-Ethyl Complexes: Synthesis, Structure, and Reactivity in Ring-Opening Polymerization of Lactones

A series of β-diiminato zinc­(II) complexes bearing two electron-withdrawing trifluoromethyl groups in the pentane ligand backbone were successfully isolated ((BDI^CF₃-I)H = [CH­(CCF₃N­C₆H₄-2,6-CH­(CH₃)₂)₂]; (BDI^CF₃-II)H = [CH­(CCF₃N­C₆H₄-2,6-C₂H₅)₂], and (BDI^CF₃-III)H = [CH­(CCF₃N­C₆H₄-2,6-CH₃)₂]). The solid-state structures illustrate differences in the ligation of the Zn atom compared to a literature known BDI-ZnEt complex. All catalysts show good activities in the ring-opening polymerization of the cyclic ester (<i>rac</i>)-β-butyrolactone (BL), whereas only BDI^CF₃-II-ZnEt and BDI^CF₃-III-ZnEt are active initiators for the polymerization of (<i>rac</i>)-lactide (LA)

2‑Methoxyethylamino-bis(phenolate)yttrium Catalysts for the Synthesis of Highly Isotactic Poly(2-vinylpyridine) by Rare-Earth Metal-Mediated Group Transfer Polymerization

Highly isotactic poly­(2-vinylpyridine) (P2VP) was synthesized by the group transfer polymerization of the prochiral 2-vinylpyridine (2VP) with 2-methoxyethylamino­bis­(phenolate)yttrium complexes. Isotacticities of up to <i>P</i>_<i>m</i> = 0.92, narrow molecular weight distributions, and high molecular weights were achieved by steric modifications of the variable bisphenolate ligand structure. The resulting polymer samples were characterized by thermoanalysis (DSC, TGA), GPC, and ¹³C NMR. The origin of the isotactic microstructure was attributed to an enantiomorphic site control mechanism based on ¹³C NMR mechanistic studies and allowed new insights into ¹³C pentad assignments

Synthesis of Lewis Acidic, Aromatic Aminotroponiminate Zinc Complexes for the Ring-Opening Polymerization of Cyclic Esters

Three novel aminotroponiminate (ATI) zinc complexes <b>I</b>–<b>III</b> (<b>I</b> = [(Ph₂)­ATI]­Zn–N­(SiMe₃)₂, <b>II</b> = [(C₆H₃-2,6-C₂H₅/Ph)­ATI]­Zn–N­(SiMe₃)₂, and <b>III</b> = [(C₆H₃-2,6-CH­(CH₃)₂/Ph)­ATI]­Zn–N­(SiMe₃)₂) were synthesized and tested in the ring-opening polymerization of the lactones β-<i>rac</i>-butyrolactone (BBL) and <i>rac</i>-lactide (LA). The ligands, with two of them literature unknown, were readily obtained via a three-step synthesis from tropolone. Forming a five-membered metallacycle with zinc, the complexes were further structurally examined via single-crystal X-ray analysis and compared with that of the established, 6-ringed β-diiminate (BDI) complex <b>IV</b> ([CH­(CMeNPh)₂]­Zn–N­(SiMe₃)₂). The influence of the varying metallacycle ring size on the polymerization was evaluated. <i>In situ</i> IR measurements indicate a higher catalytic activity of the novel ATI complexes <b>I</b>–<b>III</b> for BBL compared with the BDI system <b>IV</b>. The activity and degree of control were further improved by an <i>in situ</i> generated alkoxy initiating group generated after the addition of 2-propanol. An enhanced initiator efficiency allowed the synthesis of polymers with controlled molecular weights and narrow polydispersities. Furthermore, <b>II</b> and <b>III</b> exhibited a high activity in the ring-opening polymerization of <i>rac</i>-LA. Hereby, reaction time and initiator efficiency could also be optimized at a higher temperature or by the addition of 2-propanol

Versatile 2‑Methoxyethylaminobis(phenolate)yttrium Catalysts: Catalytic Precision Polymerization of Polar Monomers via Rare Earth Metal-Mediated Group Transfer Polymerization

The present study is one of the first examples for rare earth metal-mediated group transfer polymerization (REM-GTP) with non-metallocene catalyst systems. 2-Methoxyethylaminobis­(phenolate)yttrium trimethylsilylmethyl complexes were synthesized and showed moderate to high activities in the rare earth metal-mediated group transfer polymerizations of 2-vinylpyridine, 2-isopropenyl-2-oxazoline, diethyl vinylphosphonate, diisopropyl vinylphosphonate, and <i><i>N,N</i></i>-dimethylacrylamide as well as in the ring-opening polymerization of β-butyrolactone. Reaction orders in catalyst and monomer were determined for the REM-GTP of 2-vinylpyridine. The mechanistic studies revealed that the catalyst systems follow a living monometallic group transfer polymerization mechanism allowing a precise molecular-weight control of the homopolymers and the block copolymers with very narrow molecular weight distributions. Temperature-dependent reaction kinetics were conducted and allowed conclusions about the influence of the bulky substituents around the metal center on the polymerization activity. Additional polymerization experiments concerning the combination of REM-GTP and ROP to obtain block copolymers were performed