Elsevier

Polymer

Volume 189, 17 February 2020, 122201
Polymer

Organocatalyzed controlled radical polymerization with alkyl bromide initiator via in situ halogen exchange under thermal condition

https://doi.org/10.1016/j.polymer.2020.122201Get rights and content

Highlights

  • Alkyl bromides can be used as initiator in organic catalyzed controlled radical polymerization under thermal condition.

  • The reversible halogen exchange of Br with I at the polymer chain end promotes the polymerization.

  • Ammonium bromide acts as co-catalyst to generate radical from alkyl iodides.

Abstract

Alkyl bromide (R–Br) was employed as a starting compound (precursor) to initiate the bulk controlled radical polymerization of methyl methacrylate (MMA) under thermal condition (60 °C) in the presence of tetra-n-octylammonium iodide (Oct4NI) and 2,2′-azobisisoheptonitrile (V65). Alkyl iodide (R–I) and tetraoctylammonium bromide (Oct4NBr) were in situ generated by the reaction of R–Br with Oct4NI and then R–I was employed as an initiating dormant species for the polymerization. Carbon-iodine (C–I) bonds were cleaved and radicals were generated by the catalyst of Oct4NI and Oct4NBr. The chain end groups of the obtained polymers contain both I and Br, which was proved by the results of 1H nuclear magnetic resonance spectra, and elemental analysis. The reversible halogen exchange of Br with I at the polymer chain end promotes the polymerization. The introducing of V65 not only increased the polymerization speed, but also ensured high monomer conversion. Low-polydispersity (Mw/Mn = 1.1–1.5) polymers were obtained with high conversions (e.g., 70–90%) in the polymerizations of methyl methacrylate, styrene, acrylonitrile, and functional methacrylates. The attractive feature was that alkyl bromide was able to initiate the controlled radical polymerization of methyl methacrylate under thermal condition in the presence of organic catalyst. Besides, this new system presented desirable features of high monomer versatility and the accessibility to a wide range of polymer structural designs for use in a range of applications.

Introduction

Controlled radical polymerization (CRP), also termed as reversible deactivation radical polymerization, has matured into one of the most powerful methodologies for the synthesis of polymers with targeted molecular weight, complex architecture, high end group functionality, and narrow molecular weight distribution [[1], [2], [3], [4], [5]]. Atom transfer radical polymerization (ATRP), one of the CRP that uses alkyl halogen as initiator, has been intensively investigated in the past decades [[6], [7], [8], [9]]. CRP utilizes a reversible generation of a propagating radical (Polymer) from a dormant species (Polymer-X), where X is a capping agent including chlorine, bromine and iodine (Scheme 1a) [[10], [11], [12], [13]]. The polymer-X is predominant in the reaction system due to the higher reaction rate constant of deactivation process than the one of activation process, resulting in the significant suppression of the termination process. By repeating the activation/deactivation cycles, all the polymer chains bear the same possibility to grow, yielding well-distributed polymers (low polydispersity index, PDI) [14]. Most ATRP used a redox-action transition metal complex in its lower oxidation state as the activator, such as copper (Ⅰ) was commonly used as the central metal ion [15,16].

Nowadays, CRP initiated by non-metallic activators has attracted much interesting [17,18]. Overcoming the challenge of metal contamination in traditional ATRP systems, one important method is the use of photocatalyst [[19], [20], [21], [22], [23]]. Recently, metal-free ATRP processes, mediated by light and catalyzed by an organic-based photoredox catalyst, have been reported [24]. However, these processes operated only by the irradiation of light [25,26]. Goto and his coworkers have developed metal-free CRP that can be operated under thermal condition without light. They used iodine as a capping agent and organic molecules as catalysts, yielding a polymer iodide (polymer-I) possessing an iodine at the growing chain end (Scheme 1b) [27,28]. The catalysts include amine, phenol, soluble iodide salts with organic cations (such as ammomnium) [[29], [30], [31], [32]]. The polymerization is easy to handle, and amenable to a wide range of monomers. The limitation of this method is the used only alkyl iodides (R–I) as initiators, which are unstable and expensive compared to alkyl bromides (R–Br).

In previous studies, the use of R–Br as initiator was shown to proceed under thermal condition [[33], [34], [35]]. In the presence of sodium iodide (NaI) and via in situ halogen exchange, the R–Br was transferred to R–I and then was activated by tetrabutylammonium iodide (BNI). Meanwhile, we also used excess amount of BNI as a catalyst, as well as an iodinating agent (without using NaI) in the same system, which is totally a metal-free process. One equivalent of BNI was used to transfer R–Br to R–I and the other one equivalent of BNI worked as a polymerization catalyst, resulting in a relatively large deviation of the experimental and the theoretical molecular weight. To our best of knowledge, bromoalkyl ATRP initiator activation by inorganic salts under 90 °C has been studied [36]. Unfortunately, chain growth was controlled only when using LiI as activator. These results inspired us to develop a metal-free CRP by alkyl bromide initiator under thermal condition.

In this work, we used tetra-n-octylammonium iodide (Oct4NI, ammonium iodide) as an iodinating agent, as well as, organic catalyst. Oct4NI has longer alkyl chain and better solubility in various methacrylates than BNI, leading to higher iodinating efficiency. Ethyl 2-bromophenylacetate (EPh-Br) was employed as a starting compound (precursor). Notably, the addition of 2,2′-azobisisoheptonitrile (V65) ensured a more efficient controlled radical polymerization (Scheme 2). The generated tetra-n-octylammonium bromide (Oct4NBr) from Oct4NI acts as an efficiency catalyst as well. Carbon-iodine (C–I) bonds were cleaved and radicals were generated, and then the polymer chain extended. The chain end group of the obtained polymers contained both I and Br. The avoid using of NaI and other metal-contain compounds indicated this polymerization process is a metal-free reaction. The studied monomers were methyl methacrylate (MMA), styrene (St), acrylonitrile (AN), and several functional methacrylates (Fig. 1). We studied the homopolymerizations of these monomers and a chain-end functional polymer, demonstrating the high monomer versatility and the accessibility to a wide range of polymer architectural design.

Section snippets

Materials

Methyl methacrylate (MMA) (˃99.8%, Tokyo Chemical Industry (TCI)), styrene (St) (˃99%, TCI), acrylonitrile (AN) (>99%, TCI), benzyl methacrylate (BzMA) (˃98%, TCI), N,N-dimethylaminoethyl methacrylate (DMAEMA) (˃98.5%, TCI), 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate (TMSPMA) (˃98%, TCI), 2-hydroxyethyl methacrylate (HEMA) (˃95%, TCI), Glycidyl Methacrylate (GMA) (˃95%, TCI), butyl methacrylate (BMA) (>99.0%, TCI), and poly(ethylene glycol) methacrylate (PEGMA) (average molecular

Study of the halogen exchange of Br with I

We studied the transformation of EPh-Br with Oct4NI to generate EPh-I in low-mass systems (Fig. 2, open triangle). EPh-Br (80 mM, 1.0 equiv) and Oct4NI (80 mM, 1.0 equiv) were dissolved in a mixed solvent (dielectric constant ε = 7.89) of 30% acetone-d6 (ε = 20.7) and 70% toluene-d8 (ε = 2.4), which has a similar polarity with MMA (ε = 7.9). The reaction of EPh-Br and Oct4NI gradually produced EPh-I and Oct4NBr. After 1 h, 41% EPh-Br was transferred to EPh-I, and finally achieved reversible

Conclusions

This investigation has experimentally demonstrated the effective halogen exchange of alkyl bromide (R–Br) with Oct4NI to generate alkyl iodide (R–I) and Oct4NBr. Based on the result, R–Br was introduced as a starting precursor, and Oct4NI was employed as catalyst in the polymerization of MMA by the assistant of 2,2′-azobisisoheptonitrile (V65). The selective of R group of alkyl bromides was important. Controlled radical polymerization of MMA was obtained by using proper amount of Oct4NI and

CRediT authorship contribution statement

Bogeng Guo: Investigation, Software, Resources. Linxi Hou: Project administration, Funding acquisition, Conceptualization. Yan Li: Software, Writing - review & editing. Longqiang Xiao: Conceptualization, Methodology, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the start-up talent project (GXRC-18041), ‘111’ program of Fuzhou University, Fuzhou University Testing Fund of precious apparatus (2019T023), and Hunan Provincial Natural Science Foundation of China (Grant No. 2019JJ50652). We thank Prof. Atsushi Goto (Nanyang Technological, Singapore) for their kind help in the preparation of this manuscript.

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