Organometallic calcium and strontium borohydrides as initiators for the polymerization of ε-caprolactone and l-lactide: combined experimental and computational investigations

Abstract

[Ca(BH4)2(THF)2] (1a), a known compound, was easily prepared following a convenient new procedure from [Ca(OMe)2] and BH3·THF in THF. Reaction of 1a with KCp* (Cp* = (η(5)-C5Me5)) and K{(Me3SiNPPh2)2CH} in a 1 : 1 ratio in THF resulted in the corresponding dimeric heteroleptic mono-borohydride derivatives [Cp*Ca(BH4)(THF)n]2 (2a) and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (3a), respectively. Both compounds were fully characterized and the solid-state structure of 3a was established by single crystal X-ray diffraction. Compounds 1a, 2a, and 3a, together with the earlier reported compounds [Sr(BH4)2(THF)2] (1b), [Cp*Sr(BH4)(THF)2]2 (2b), and [{(Me3SiNPPh2)2CH}Sr(BH4)(THF)2] (3b), were used as initiators for the ROP of polar monomers. The general performances of the complexes in the ROP of ε-caprolactone and l-lactide demonstrate a relatively good control of the polymerization under the operating conditions established. α,ω-Dihydroxytelechelic poly(ε-caprolactone)s (PCLs) and poly(lactide)s (PLAs) were thus synthesized. DFT calculations on the initiation step of the ROP of ε-CL were carried out. Gibbs free energy profiles were determined for the three calcium complexes highlighting slightly more active calcium complexes as compared to strontium analogues, in agreement with experimental findings.

Group 1 and group 2 metal complexes supported by a bidentate bulky iminopyrrolyl ligand: synthesis, structural diversity, and ε-caprolactone polymerization study

“…The calcium complex 7 also showed comparable activity towards the ROP of ε-CL with magnesium analogues (5 and 6) with narrow PDI values and controlled molecular weight distributions (Table 2, entries 7-11). Indeed, the sluggish reactivity of the calcium complexes is very similar to that observed in some previously reported studies using other calcium complexes for ROP of ε-CL, 45,46 we have noted living polymerization characteristics at room temperature without using any initiating agent like alcohol (entry 9, PDI = 1.5 and M w = 50 483) indicating that the triphenylmethyl group on the ligand backbone strongly influences the activity of calcium complexes towards the ROP of ε-CL. We anticipated that strontium (8) and barium (9) complexes could be more active than those of magnesium and calcium complexes having bulky iminopyrrolyls due to the larger ionic radii of Sr 2+ and Ba 2+ ions.…”

(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis

“…Under these conditions, all complexes successfully ring-open polymerized CL at room temperature in toluene ( Table 2). (14), B2-Yb-O1 87.59 (14), B2-Yb-S 164.57 (14), C1-Yb-N 65.84(9), C1-Yb-O1 84.99(9), C1-Yb-P1 32.11 (7), C1-Yb-P2 35.31 (7), C1-Yb-S 67.90 (7), N-Yb-O1 150.84 (10), N-Yb-S 87.21 (7), O1-Yb-S 81.77 (7), P1-C1-P2 121.8 (2). Running the polymerization in a more diluted reaction medium remained successful (Table 2, entries 9, 10).…”

“…Calculation studies revealed that the negative charge is mainly localized on the carbene carbon atom of the ligand backbone and that the charge delocalization from carbon to nitrogen is less important. 6,14,16,20,[36][37][38][39][40][41][42][43][44][45][46][47][48][49] In this context, some very efficient rare-earth metal initiators ligated by {CH(PPh 2 NSiMe 3 ) 2 } − have been unveiled for polymerization reactions. Mostly six membered metallacycles (N-P-C-P-N-M) are formed by chelation of the two imine groups to the metal center, adopting a pseudo-boat conformation.…”

Chiral alkaline earth metal complexes with M–Se direct bond (M = Mg, Ca, Sr, Ba): syntheses, structures and ε-caprolactone polymerisation

“…Indeed, the moderate reactivity of the strontium complexes is very similar to that observed in previously reported studies using other strontium complexes for ROP of 3-caprolactone. 29 Since the larger ion radius barium complexes have been reported to be more active than the calcium and strontium congeners in ROP, 30,31 we tested compound 6a or 6b as a catalyst and observed an enhanced rate of polymerisation (Table 1, entries 6-10). In the case of strontium, higher reactivity was observed for conversion of 3-caprolactone to poly-caprolactone and up to 500 3-CL units were successfully converted in high yields (75-90 per cent), within 15 and 10 minutes respectively, at 25 C. The control over the ROP process was rather good, affording PCLs, featuring a considerable match between the observed (as determined by GPC) and calculated molar mass values, as well as moderate dispersity data (PDI ¼ M w /M n < 1.94).…”