Electronic and steric effects of methyl substituent in Samarium (III) phenolates on their initiation activities in polymerization of ϵ‐caprolactone

Abstract

Seven phenols with methyls substituting ortho or para hydrogens are firstly reported as ligands for Samarium (III) complexes. The resultant Samarium (III) phenolates were used as single component initiators for ring-opening polymerization (ROP) of e-caprolactone (CL). The results were in good order and met well with structural changes in phenol ligands. To explain the ordered correlation between ROP results and ligand structures of these phenolates, quantum chemical (QC) method was applied to optimize the most stable conformations for substituted phenols. QC data indicated that changes in charge distribution and geometric parameters of phenol ligands also followed certain order, which could be attributed to electronic and steric effects caused by methyl substituents. It was found that: methyl in phenol, especially single ortho one, would induce more space around metal center and easier nucleophilic attack from phenol oxygen to CL monomer, which means positive electronic effect and could induce increased initiation activity of corresponding phenolate. Meanwhile, two ortho methyls will take up considerable space around the metal center and bring along un-ignorable negative steric effect, which surpasses positive electronic effect also introduced by these two ortho methyls and finally leads to decreased activity in phenolates. This study revealed that there is correlation between the initiation characteristics of phenolates and the structures of their phenol ligands; QC calculation is a convenient, cheap, and helpful method to aid study on it.

Synthesis and structure of para‐toluene sulfonamide lanthanide complexes and their application in the polymerization of ε‐caprolactone

“…In complex 3 , the La(1)‐O(3) and La(1)‐O(5) coordination bond lengths (2.475 Å and 2.468 Å) are similar to the values for [La(O 2 CC 6 H 2 Me 3 –2,4,6) 2 (μ‐O 2 ‐CC 6 H 2 Me 3 –2,4,6)(DMSO) 2 ] 2 (2.459 Å) . The La(1)‐N(2) and La(1)‐N(3) coordination bond lengths (2.523 Å and 2.547 Å) are longer than that of [Li(DME) 3 ][MeCpLa(NPh 2 ) 3 ] (2.443 Å), but shorter than that of La(Me 2 NC 2 H 4 CP) 3 (2.898 Å) …”

Lanthanide chloride complexes of amine-bis(phenolate) ligands and their reactivity in the ring-opening polymerization of ε-caprolactone

“…One possibility is via insertion of the monomer into the metal-phenoxide bond, as has been proposed for Sm tris(phenolate) species. 28 In previous studies of ROP using amine-bis(phenolate) Ln species, the Ln-phenolate bonds were considered inert and the monomer only inserted into the metal amide or alkyl bond in these related lanthanide-amide andalkyl lactone ROP initiators. 15, 18 However, in our study no ligand related end groups that would suggest this insertion mechanism were observed in the 1 H NMR spectra of the polymer products.…”

“…27 Samarium tris(phenolate) complexes have recently been reported as initiators for e-CL ROP and their mechanism studied in some detail. 28 It was proposed that e-CL insertion occurs at all three Sm-O bonds and although in Scheme 2 insertion is only shown into one Ln-O bond, it may well be occuring at both reactive bond sites. After repeated coordination and migratory-insertion steps, termination is likely occuring via nucleophilic attack of the 'phenol' oxygen at the Ln metal centre and subsequent 'back-biting' of the growing polymer chain to yield cyclic polyesters of narrow polydispersity.…”

Direct synthesis of thiol-terminated poly(ε-caprolactone): a study on polymerization kinetics, mechanism and rare earth phenolates' structure–activity relationship

“…2,6-Di-tertbutyl-4-methylphenol and the lanthanide showed relative higher chemoselectivity, which might have resulted from substituent's electronic/steric effect and metal characteristic. 29,30 To demonstrate the versatility of rare earth catalysts, polymerization reactions with different monomer feed ratios were carried out to obtain varied molecular weights of PCLSH using 8 as the catalyst. On increasing [CL]/[MH] from 40 to 200, a wide range of molecular weights from 6870 to 20 450 g mol À1 were achieved within 2 h, which were in good accordance with the M n,theo values.…”