Living ring-opening polymerization of ɛ-caprolactone with combinations of tert-butyllithium and bilky aluminium phenoxides

Abstract

The polymerization of -caprolactone (-CL) with a combination of tert-butyllithium (t-BuLi) and bis(2,6-di-t-butylphenoxy)methylaluminum [MeAl(ODBP) 2 ] in toluene at 0°C proceeded in a living manner to give polymers with narrow molecular weight distributions (MWD) within a few minutes, while the polymerization with t-BuLi alone gave a polymer with much broader MWD. The yield of the polymer did not reach 100 % at the Al/Li ratio of 5, because the excess MeAl(ODBP) 2 coordinates with -CL to protect it from the attack by the propagating species. The polymerization with t-BuLi/EtAl(ODBP) 2 gave polymers in quantitative yields regardless of Al/Li ratio, and also narrower MWD even for higher molecular weight polymers. IntroductionAnionic ring-opening polymerizations of -caprolactone (-CL) by alkali-metal alkyls or alkali-metal alkoxides are generally very fast, but are problematic in the control of polymer structure due to intra-and / or intermolecular transesterification reactions, that lead to the formation of macrocycles and to the broadening of molecular weight distribution (MWD). Decrease in the nucleophilicity of the initiator, through the modification of the counterion for example, has been found favorable to the propagation of linear chains compared to the macrocyclization process [1]. Thus, the living polymerizations of -CL have been attained by use of aluminum and zinc alkoxides such as Al(O i Pr) 3 [2], Zn[OAl(OR) 2 ] 2 [3], R x Al(OR) 3-x [4,5], and (tetraphenylporphyrinato)Al(OR) [6]. The polymerizations with lanthanide compounds such as yttrium alkoxides [7, 8, 9] and Cp * 2 SmMe(THF) (Cp * 2 = pentamethylcyclopentadienyl, THF = tetrahydrofuran) [10] and with bulky titanium bis(phenolate) [11] also give polymers with narrow MWD's.We have reported that combinations of tert-butyllithium (t-BuLi) and bulky aluminum bisphenoxides are efficient initiating systems for highly stereospecific living polymerization of methacrylates [12][13][14][15][16][17][18][19][20][21]. For example, polymerizations of primary alkyl methacrylates with t-BuLi/bis(2,6-di-tert-butylphenoxy)methylaluminum[MeAl(ODBP) 2 ] (Al/Li 2) in toluene at low temperature give highly heterotactic polymers with narrow MWD's. The existence of excess amounts of MeAl(ODBP) 2 over t-BuLi is essential for the heterotactic polymerization,

Synthesis of polycaprolactone: a review

“…The proposed Scheme 9 ROP of e-CL catalysed by LDA. 34 Scheme 10 ROP of e-CL catalysed by cyclopentadienyl sodium. 36 Scheme 11 ROP of e-CL using bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide]-alcohol system, adapted from Zhong et al 43 mechanism for the following steps is shown in Scheme 12.…”

“…At low catalyst concentration, not only one, but both of the Al-alkyl bonds participated in the polymerisation process. Different Al complexes based on salicylaldimines (ESIw Table 1.3, entries [32][33][34][35] with different substituents on the imino group, showed an increasing activity in the substituent order C 6 F 5 -44 2,6-i PrC 6 H 3 -44 tert-butyl-4 adamantyl-. 68 The effect of different substituents on the two phenyl rings of the ligand (e.g.…”

Synthesis and Characterization of Zirconium and Hafnium Aryloxide Compounds and Their Reactivity Towards Lactide and ϵ‐Caprolactone Polymerization

“…[5][6][7][8] However, the study of polyesters generated from ring-opening polymerization using non-tin metal alkoxides as catalysts has just started to attract researchers' attention. Among those metal alkoxides, aluminum, [9][10][11][12][13][14][15] lithium, [16,17] titanium, [18][19][20][21] and some lanthanide metals [22][23][24][25][26][27] have been studied by different groups. We have previously examined the ring-opening polymerization of ε-caprolactone and lactide with aluminum alkoxides supported by monoanionic bidentate ketiminate ligands.…”

A New Role for CO2: Controlling Agent of the Anionic Ring-Opening Polymerization of Cyclic Esters

“…Besides manipulating the active species responsible for propagation and fine-tuning their reactivity, another approach has been to selectively activate the monomers as a means to favor propagation over scrambling reactions. Using bis­(2,6-di- tert -butyl­phenoxy)­ethylaluminum to activate ε-caprolactone, Kitayama et al could, for instance, successfully anionically polymerize ε-caprolactone under controlled/living conditions . Recently, Waymouth and co-workers demonstrated the ability of dithioureas to selectively activate the carbonyl function of both lactones and lactides, also observed that propagation is vastly favored over transesterification, and succeeded in their attempt to obtain a truly living polymerization of cyclic esters. , In this article, we describe a totally novel strategy based on the use of CO 2 along with simple lithium alkoxides as a means to bring about the controlled/living anionic ring-opening polymerization of cyclic esters.…”