Article pubs.acs.org/Macromolecules
Helix-Sense-Selective and Enantiomer-Selective Living Polymerization of Phenyl Isocyanide Induced by Reusable Chiral Lactide Using Achiral Palladium Initiator Jia-Li Chen, Li Yang, Qian Wang, Zhi-Qiang Jiang, Na Liu,* Jun Yin, Yunsheng Ding, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Advanced Functional Materials and Devices, Anhui Province, Hefei 230009, China S Supporting Information *
ABSTRACT: Polymerization of phenyl isocyanide using achiral Pd(II) initiator with the presence of chiral L- or D-lactide (L-LA or D-LA) as additive was found to proceed in helix-sense-selective manner. The polymerization of achiral phenyl isocyanide, 4isocyanobenzoyl-2-aminoisobutyric acid decyl ester (1) by this method produced optically active helical poly-1m(L), whose chirality was solely come from the helical conformation without containing of any other chiral atoms. The added chiral LA can be facilely recovered and reused in the helix-sense-selective polymerizations without significantly loss of its chiral induction, and the chiral economy of the polymerization is high. When enantiomerically pure phenyl isocyanide bearing an R- or S-alanine pendent with a long n-decyl chain (1r or 1s) were polymerized by this method, the polymerization was found to proceed in a highly enantiomer-selective manner with one of the enantiomers preferentially polymerized over the antipode by a factor of 3.6. Single-handed helical polyisocyanides can be achieved when the chirality of the monomer was appropriately matched with the added LA.
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INTRODUCTION Naturally occurring biopolymers such as proteins and DNA commonly have helical conformations, which are essential for their sophisticated and fundamental functions.1 Since the discovery of helical structures in biopolymers, researchers have endeavored to develop artificial helical polymers with controlled helical sense.2 Representative examples of artificial helical polymers including steric restricted polyacrylates3 and poly(aryl vinyl)s,4 polyisocyanides,5 polyisocyanate,6 polycarbodiimide,7 and polyacetylenes.8 Among them, helical polyisocyanide with controlled helical sense have been the focus of intense research efforts in recent years due to their interesting rigid helical structures and various applications.5,9 Optically active helical polyisocyanides were usually obtained through the polymerization of chiral isocyanide monomers with achiral initiator or catalyst.10 An efficient alternative is the polymerization of achiral isocyanide monomers with chiral initiator, catalyst, or cocatalyst.11 However, optically active helical polyisocyanides prepared by these methods usually have more than one chiral sources, that is, chiral pendants, chiral chain end, and a one-handed predominant helical main chain. This fact often causes complications in the understanding of their chiral behaviors and functions. Thus, development of novel synthetic method for facile synthesis of optical active helical polyisocyanides whose chirality is solely come from the helical main chains is of great interesting. Polymerization of achiral © 2015 American Chemical Society
isocyanides with Ni(II) complex in the presence of an optically active amine as cocatalyst resulted in the formation of optically active helical polyisocyanide with a chiral amino group on the polymer chain end.11a Nickel initiator containing chiral acetate ligands produced helical polyisocyanides with predominant one-handed helix, while the molecular weight of the resulting polymers can be very low.11b,c Chiral induction is an effective method to obtain optically active polyisocyanides with an excess of one-handed helical conformation. Nevertheless, specific functional pendants are usually required to facilitate the acid−base interaction or host−guest complexation.12 To the best of our knowledge, there are only few examples of optically active polyisocyanides have been reported whose chirality was solely arise from their helical backbone, without the coexistence of any other chiral moieties such as chiral side groups, chiral end group, or asymmetric carbons in their main chain.13 Partially inspired by the stereospecific reactions of enzymes in living system, the enantiomer-selective polymerizations of optically inactive racemic monomers have attracted considerable researches attention in recent years.14 This polymerization is a kind of kinetic resolution, in which one enantiomer Received: September 26, 2015 Revised: October 20, 2015 Published: October 28, 2015 7737
DOI: 10.1021/acs.macromol.5b02124 Macromolecules 2015, 48, 7737−7746
Article
Macromolecules
Scheme 1. Helix-Sense-Selective Polymerization of Achiral Phenyl Isocyanide Using Pd(II) Initiator in the Presence of L-LA and D-LA
palladium(II) chloride as initiator was performed with the presence of L-LA ([1]0 = 0.2 M, [1]0/[Pd]0 = 30−150, [LLA]0/[1]0 = 5). It was found that all the polymerizations were proceed smoothly in THF at 50 °C, afforded well-defined poly1m(L)s in high yields with controlled Mns and narrow Mw/Mns as determined by size exclusion chromatography (SEC) with equivalent to their polystyrene (PSt) standard. The Mn of the isolated polymers showed a linear correlation with the initial feed ratio of monomer to initiator, and all the polymers possessing narrow Mw/Mn (
