Zinc(II) complexes of Triaza and Amidinate ligands: Efficient initiators for the ring‐opening polymerization of ε‐Caprolactone and rac‐Lactide

• Published in: Applied organometallic chemistry Vol. 31; no. 12
• Main Authors: Kirubakaran, Bakthavachalam, Beesam, Raghavendra, Nareddula, Dastagiri Reddy
• Format: Journal Article
• Language: English
• Published: 01.12.2017
• Subjects: caprolactone; poly(lactide); poly(ε‐caprolactone); rac‐Lactide; ring‐opening polymerization; zinc complex
• ISSN: 0268-2605; 1099-0739
• DOI: 10.1002/aoc.3833

Zinc complexes supported by tertiary 1,3,5‐triazapenta‐1,3‐dienate ligand (L1) and N‐benzoyl‐N′‐arylbenzamidinate [aryl =2,6‐diisopropylphenyl (L2), phenyl (L3)] ligands have been synthesized and characterized. The reaction of L1H with ZnEt2 affords a mononuclear zinc complex [L1ZnEt] (1) in good yield. Tetra nuclear zinc complex [(L1)2Zn4O(OAc)4] (2) is prepared by treating L1H with one equivalent of Zn(OAc)2 in toluene. Further, dinuclear zinc complexes [L2ZnEt]2 (3) and [L3ZnEt]2 (4) are obtained in good yields from L2H and L3H with ZnEt2 in toluene respectively. The complexes 1–4 have been characterized by 1H/13C NMR spectroscopy and single crystal X‐ray diffraction studies. All of the complexes have been explored for their catalytic activity toward the ring‐opening polymerization (ROP) of ε‐caprolactone. It has been found that complex 1 is an active catalyst for the polymerization of ε‐caprolactone in presence of a cocatalyst benzyl alcohol (BnOH). While complex 2 is as active as 1 there is no need for a cocatalyst for the polymerization to proceed. Dinuclear zinc complexes 3 and 4 show very high activity for the ROP of ε‐caprolactone (CL) and rac‐lactide (LA) without requiring a cocatalyst. The resultant polymers are found to have very high molecular weight (Mn = 296 X 103 g mol−1) and relatively narrow polydispersity index compared to 1 and 2. Mono‐, di‐ and tetra nuclear zinc complexes were synthesized and used as initiators for the ring opening polymerization of ε‐caprolactone and rac‐lactide.