Polymer brushes based on N-methacryloxysuccinimide as platform for versatile post-polymerization modification
Graphical abstract
Introduction
Post-polymerization modifications, widely known as polymer analogous reactions, are a common way to introduce a wide range of functional groups to already prepared polymer chains [1], [2]. This approach is used since several decades, but it became even more promising with the emergence of living/controlled radical polymerizations such as nitroxide-mediated radical (NMR), reversible addition–fragmentation chain transfer (RAFT) and atom transfer radical (ATRP) polymerizations [3], [4], [5], [6]. Those controlled polymerization techniques have significantly improved tolerance to various functional groups compared to the conventional polymerization methods, so they allow the development of well-defined polymers bearing various groups able to undergo analogous reactions. In addition, the discovery of several selective couple reactions, grouped under the term “click”-chemistry, provided the base of rapid and significant growth in application of the post-polymerization reactions.
From the numerous polymer analogous reactions known to date, the modification of active ester polymers is one of the oldest methods, developed in the early 1970's [7], [8]. Up to date, huge number of active ester monomers were developed and polymerized by the whole spectrum of available polymerization techniques [9], [10]. Their polymerizability and modification reactivity is under continuous study and different strategies are used with varied success. Some years ago, ester-actived polymers became also widely usable for the coupling of various biofunctionalities [11], [12]. The direct (co-)polymerization of monomers with active ester functionality (N-hydroxysuccinimide (NHS) ester monomers, pentafluorophenyl (PFP) ester monomers, p-nitrophenyl (PNP) ester monomers, etc.) [9] is the most common strategy for preparing corresponding polymer as it prevents an extra functionalization step. This approach also provides a kind of polymer platform to undergo an immediate reaction step with various functional groups. The active ester pendant group of these co- and homopolymers is capable of undergoing fast and straightforward substitution reactions, from which the most common is the amidation reaction with various aliphatic and aromatic amines to realize corresponding poly(meth)acrylamides [13], [14], [15].
In this context hydrolytically stable active esters [16] such as N-hydroxysuccinimide (meth)acrylate became particularly popular in order to overcome the difficulties in ATRP polymerization of (meth)acrylamides. The polymerization of (meth)acrylamides in typical ATRP conditions exhibit much lower equilibrium constant than with acrylates/methacrylates or styrene. This can be attributed to occurring complexation of the polymerization catalyst with the growing polymer chain and/or nucleophilic displacement of the terminal halogen atom by the amide group. Both side reactions lead to termination of the polymerization and loss of kinetic control [17], [18]. Overall, the polymerization kinetics for N-hydroxysuccinimide methacrylate in solution are well studied, utilizing different reaction conditions (solvent, catalyst, ligand, reaction temperature, etc.) [19]. By tuning the reaction rate and polymerization temperature dependency it is possible to limit the high concentration of free radicals leading to increased termination from radical-radical coupling reactions and prevent loss of control over the molecular weight distribution of the final polymer. Further, the post-polymerization modification of those polymers is well developed and optimized [11], [20], [21]. This also includes reaction with various functional moieties, from which biofunctionalization is particularly popular in last decades.
In opposite to the well-studied polymerization N-hydroxysuccinimide (meth)acrylate in solution, there is limited number of works dealing with active ester derived polymers covalently tethered over surface [22], [23], [24], [25], [26], and even fewer of them focuses on N-hydroxysuccinimide (meth)acrylates-based polymers [27], [28], [29]. The majority of works utilizes the “grafting-to” approach, covalently bonding already synthesized ester polymers to activated surfaces [30]. Furthermore, two-step post-polymerization process is used to introduce in the first step succinimide ester moieties to various types of parent polymer brushes and in the second step the modification of the active ester groups with the desired functionality [12], [31], [32], [33], [34]. However, direct polymerization of N-hydroxysuccinimide esters initiated from a covalently attached surface initiator is rarely done. Free-radical polymerization from immobilized AIBN initiator results in polymer films with thickness approaching 80 nm [35]. Poly-N-hydroxysuccinimide methacrylate brushes were grown by surface initiated ATRP (SI-ATRP) over magnetite [36], [37] and quartz [38] flat surface, and the monomer was also polymerized as a short second block over surface-tethered polymer chains via SI-ATRP [28], [29]. Additionally, no direct kinetic studies from the surface were reported for SI-ATRP of N-hydroxysuccinimide methacrylate, while all kinetic data originating from polymer chains obtained from sacrificial initiator in solution [38]. Considering previous studies, attempting SI-ATRP, rarely succeed to achieve thicknesses greater than 5 nm even at prolonged polymerization times (up to 24 h) [28], [38], as well as estimating kinetic control over the brush growth.
Utilizing “grafting-from” active ester polymer brushes has many advantages over other polymer “grafted-to” architectures, such as the generating of a much larger volume of functional groups along the polymer chain length or avoiding consumption of large amounts of expensive and difficult to prepare monomers. Therefore, the more detailed study of both homo- and copolymerization kinetics of active ester monomers is of great interest, as well as studying their post-polymerization modification.
The aim of our work was to develop a protocol for SI-ATRP of N-hydroxysuccinimide methacrylate in dimethylformamide (DMF) close to ambient temperature. Copper(I) bromide/copper(II) bromide/2,2′-bipyridyl catalytic system was utilized and the monomer concentration in the polymerization feed was adjusted in order to achieve linear polymer brush growth and obtain a final layer with reasonable thickness. Additionally, the optimized polymerization conditions were applied to realize block copolymer brushes with oligo(ethylene glycol) methyl ether methacrylate (MeOEGMA) as comonomer to achieve poly(MeOEGMA-block-N-hydroxysuccinimide methacrylate) hierarchical brushes with different lengths of the second active ester containing block. The polymerization kinetics for homopolymerization and block copolymerization were followed by spectroscopic ellipsometry (SE). Further, we performed post-polymerization modification of the ester side groups with various primary amines, which allowed us to tune the functionalization of the final layers. The resulting polymer layers were characterized in details by infrared reflection-absorption (IRRAS) and X-ray photoelectron (XPS) spectroscopies, contact angle goniometry and corresponding surface energy analysis.
Section snippets
Materials
N-Hydroxysuccinimide, 1-amino-2-propanol, 5-aminofluorescein (isomer I), propargyl amine, 4-dimethylaminopyridine (DMAP), 2,2′-bipyridyl, copper(I) bromide, copper(II) bromide, oligo(ethylene glycol) methyl ether methacrylate (MeOEGMA, Mw = 300 g/mol), chloramine T, potassium iodide and copper(I) iodide (all purchased from Sigma-Aldrich), 2-fluoroethyl amine hydrochloride, 2,2-difluoroethyl amine and 2,2,2-trifluoroethyl amine (Fluorochem Limited), NH2-PEG-N3 (Mw = 5000 g/mol, Rapp Polymers
Spectroscopic ellipsometry (SE)
The dry thicknesses of the obtained polymer layers after the polymerization and modification steps were measured using a J.A. Woollam M-2000X Spectroscopic Ellipsometer (J.A. Woollam, USA). All data were acquired in the wavelength range λ = 245–1000 nm at angles of incidence of 60, 65 and 70°. The experimental data were fitted in the CompleteEASE software, using Cauchy dispersion relation for the film layers.
Grazing angle attenuated total reflection fourier-transform infrared spectroscopy (GAATR-FTIR)
The infrared spectra of all films were recorded in the dry state using a Nicolet Nexus
Controlled homopolymerization of poly(MSI) brush via SI-ATRP
As the need of new polymer compositions containing ever widening range of functional groups is continuously increasing in last decades, the conventional methods of direct polymerization are in some cases found less adequate. In the case of ATRP the polymerization system is relatively tolerant to a wide variety of functional groups, but for monomers like the acryl- and methacrylamides the control of the process is hindered by competing complexation of the catalyst to the amide nitrogen. In
Conclusions
In this contribution we reported a successful surface-initiated atom transfer radical polymerization of a methacrylate-derived active ester monomer (MSI), as well as its block copolymerization capability. We observed a linear increase in film thickness and that at low polymerization temperatures pointing to a controlled polymerization kinetics. The controlled character of the polymerization is further proven by chain extension experiments leading to the conclusion that different surface
CRediT authorship contribution statement
Radoslava Sivkova: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. Jan Svoboda: Investigation, Methodology, Formal analysis, Writing – review & editing. Jiří Pánek: Investigation, Methodology. Dietmar Appelhans: Conceptualization, Writing – review & editing. Ognen Pop-Georgievski: Conceptualization, Methodology, Supervision, Writing – review & editing, Funding acquisition.
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.
Acknowledgement
This work was supported by the Czech Science Foundation (GACR) (Contract No. 20-07313S). J.P. acknowledges the support from the Czech Academy of Sciences within the Strategy AV21 framework (VP26-Breakthrough future technologies). The research stay of R.S. at IPF was supported by the Czech Academy of Sciences under Contract No. MSM200501903.
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