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

Three novel aminotroponiminate (ATI) zinc complexes I-III (I = [(Ph2)ATI]Zn-N(SiMe3)2, II = [(C6H3-2,6-C2H5/Ph)ATI]Zn-N(SiMe3)2, and III = [(C6H3-2,6-CH(CH3)2/Ph)ATI]Zn-N(SiMe3)2) were synthesized and tested in the ring-opening polymerization of the lactones ß- rac-butyrolactone (BBL) and rac-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 ß-diiminate (BDI) complex IV ([CH(CMeNPh)2]Zn-N(SiMe3)2). The influence of the varying metallacycle ring size on the polymerization was evaluated. In situ IR measurements indicate a higher catalytic activity of the novel ATI complexes I-III for BBL compared with the BDI system IV. The activity and degree of control were further improved by an in situ 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, II and III exhibited a high activity in the ring-opening polymerization of rac-LA. Hereby, reaction time and initiator efficiency could also be optimized at a higher temperature or by the addition of 2-propanol.

CO2-Controlled One-Pot Synthesis of AB, ABA Block, and Statistical Terpolymers from ß-Butyrolactone, Epoxides, and CO2.

Terpolymerizations of (rac)-ß-butyrolactone (BBL), cyclohexene oxide (CHO), and carbon dioxide were realized in one-pot reactions utilizing a Lewis acid BDICF3-Zn-N(SiMe3)2 (1) catalyst. The type of polymerization can be regulated and switched between ring-opening polymerization (ROP) of BBL and CHO/CO2 copolymerization by the presence of CO2 in the reaction mixture. Applying 3 bar CO2 to the three-component system leads to similar reaction rates for copolymerization and ROP and therefore to a terpolymer with a statistical composition, whereas 40 bar CO2 affords exclusive copolymerization of CHO/CO2. Two-dimensional NMR spectroscopy and polarimetry provided a deeper understanding of the underlying mechanism. Furthermore, copolymerization of cyclopentene oxide (CPO) and CO2 was performed, resulting in the highest reported TOF of 3200 h-1 together with 99% polycarbonate selectivity. Terpolymerizations of CPO/CO2 and BBL were successfully conducted using the established reaction pathways.

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 ß-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.