Biased Lewis Pairs: A General Catalytic Approach to Ether‐Ester Block Copolymers with Unlimited Ordering of Sequences

Abstract

Polymerizing epoxides after cyclic esters remains am ajor challenge,t hough their blockc opolymers have been extensively studied and used for decades.R eported here is as imple catalytic approach based on am etal-free Lewis pair that addresses the challenge.W hen the Lewis acid is used in excess of ab ase,s elective (transesterification-free) polymerization of epoxides occurs in the presence of esters,w hile selectivity towardcyclic esters is achieved by an oppositely biased catalyst. Hence,o ne-pot blockc opolymerization can be performed in both ester-first and ether-first orders with selectivity being switchable at any stage,y ielding ether-ester-type block copolymers with unlimited orderingo fs equences as well as widely variable compositions and architectures.The selectivity can also be switched back and forth several times to generate am ultiblock copolymer.E xperimental and calculational results indicate that the selectivity originates mainly from the state of catalyst-activated hydroxy species.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

N-Heterocyclic carbene/Lewis acid-mediated ring-opening polymerization of propylene oxide. Part 2: Toward dihydroxytelechelic polyethers using triethylborane

“…of Et 3 B is used for the formation of AH complex and the monomer is activated by the residual Et 3 B (2 equiv. ), or vice versa[27,37]. In fact, when the amount of [Et 3 B] ≤ [I i Pr], the ROP of PO indeed occurred even at a slow polymerization rate (entries 2-3, Table1, [Et 3 B]/[I i Pr] = 0.6, 1, respectively) as shown previously[26].…”

“…-initiated population was observed by SEC, owing to the decrease in concentration of BnOH.Mechanism. Et 3 B is believed to play an activating role for the monomer and to complex the base, similarly to the metal-based i-Bu 3 Al used as strong Lewis acid in the ROP of epoxides[1][2][3][4][5][6] or metal-free Et 3 B in the Lewis pair-mediated ROP of epoxides and its copolymerization with other cyclic monomers[25][26][27][37][38][39][40][41].For a molar ratio of [Et 3 B]/[I i Pr] equals to 3/1, Et 3 B would play a dual role by being involved both in the formation of an activated-hydroxyl complex (AH) and an activated monomer (AM). Interestingly, H 2 O is yielding twice the molar mass in agreement with a double initiation (Scheme 2).The specific amount of Et 3 B used for the formation of two complexes is not shown in the proposed scheme as there is an open discussion about the specific ratio of Et 3 B in these two functions, which means whether 1 equiv.…”

A Synthetic Method for Site‐Specific Functionalized Polypeptides: Metal‐Free, Highly Active, and Selective at Room Temperature

“…[16,17] Especially, the bifunctional thiourea/R 3 Ns ystems explored for ROP by Waymouth et al in 2005, [14] in which ag eneral base motif (R 3 N) was used to activate the ROH initiator/chain-end and athiourea to activate the monomer and stabilize the incipient charged tetrahedral intermediate,h ave been widely used for the synthesis of sustainable polymer materials without metallic contaminants.Very recently,anelegant bicomponent metal-free system based on triethylborane (Et 3 B)/phosphazene base (tBuP 2 )w as developed by Zhao,L iu, and coworkers for the copolymerization of cyclic ethers and lactones.B yp recisely controlling the Lewis acid (Et 3 B)/base (tBuP 2 )c atalysts ratio,t he propagating active species can selectively incorporate cyclic ethers or lactones to polymer chains,r esulting in clean ether-ester block copolymers with unlimited sequences. [18] Therefore,a sa na lternative to organometallic and enzymatic catalysis,o rganocatalysis provides promising approaches to the preparation of clean polymers.…”

Ethoxylation of Phenols Catalyzed by Metal‐Free Lewis Pairs: Living/Controlled Polymerization in a Slow‐Initiation Mode†

“…The bicomponent cooperative catalysis leads to significantly enhanced polymerization efficiency, even at room temperature (RT), and strict selectivity for epoxide ROP against transesterification and chain transfer to monomer. [ 26,29‐32 ] Especially, the combination of an organobase and excess triethylborane (Et 3 B) allows chemoselective and well‐controlled ROP of epoxides using carboxylic acids as the initiators. [ 13 ] Interestingly, the results of the ROP conform well to a living/controlled polymerization despite the fact that the chain initiation is much slower than the propagation, which is distinct from classic living/controlled polymerizations.…”