Chapter 7 Nitroxide-Mediated Polymerization 7.1 Introduction Controlled radical polymerization (CRP) under radical initiation conditions belongs to priority areas in the development of the synthetic chemistry of polymers of the last years [1–16]. Nitroxide-mediated polymerization (NMP) was invented by Solomon [1, 13]. Since this discovery, nitroxide-mediated radical polymerization is a power- ful method to synthesize well-defined macromolecular architectures with precisely controlled topologies, compositions, microstructures, and functionalities [3–5]. The most common mechanisms for reversible activation in polymerization reactions are schematically illustrated in Scheme 7.1. Persistent radical effect (PRE) occurs when two radicals are generated at the same time, at the same rate, and one is more persistent than the other, the self-termination reactions are lowered, leading to an unusually high selectivity for the cross-coupling reaction [10]. The effect has been investigated for the preparation of macromolecules with a narrow molar mass distribution through radical polymerization. Nitroxide-mediated polymerization is widely applied in industrial polymer syn- theses as a method for production of large-tonnage polymers and is employed to manufacture new pigments, sealants, emulsion stabilizers, and block copolymers, etc., with a various set of properties. NMP has also paved an avenue for complex macromolecular architectures (statistical, block, graft) in the fields of nanoscience and nanotechnology [5, 9, 12] and references cited therein. A brief summary of NMP developments in both the patent and open literature during the period of the early Scheme 7.1 Mechanisms for reversible activation in polymerization reactions [6] © Springer Nature Switzerland AG 2020 161 G. I. Likhtenshtein, Nitroxides, Springer Series in Materials Science 292, https://doi.org/10.1007/978-3-030-34822-9_7 162 7 Nitroxide-Mediated Polymerization 1980–2000 was presented in [11]. Various important aspects in this area, such as syn- thesis of nitroxides and alkoxyamines, fundamentals to applications in materials sci- ence, kinetic aspects of NMP,recent developments in NMP,nitroxide-mediated poly- merization in dispersed media, NMP of methacrylic esters, complex macromolecular architectures prepared by NMP,surface-initiated NMP,from nanoporous materials to microelectronics, NMP under homogenous conditions, and NMP-derived materials for biomedical applications were presented and discussed in separate chapters of the book [12]. In this chapter, fundamentals and recent developments in the NMP area are briefly summarized and illustrated by particular examples. 7.2 Mechanism of the Nitroxide-Mediated Polymerization The basic idea of the NMP method (Scheme 7.1) is that the initiation of the “living” free polymerization (LEP) process occurs as a result of the dissociation of a special compound (A-S) into two radicals, active (AR·) and stable (SR·) ones. Persisted radical effect (PRE) can also occur for radicals generated from the decomposition of two different initiators affording the active radical and a persistent radical. The active radical involves into the polymer chain initiation and self-recombination reactions. The recombination of a stable radical is thermodynamically forbidden, and, in this case, it can only participate in reversible cross-recombinations with the partner radical (AR·) and with a polymer radical Rn·. The perfect candidates for the role of initiator of LFP were found to be alkoxyamines. These compounds cleave into transient alkyl and persistent aminoxyl radicals which then combine and regenerate the parent compounds. Simultaneously, the alkyl species self-terminate, and this causes a continuous buildup of excess aminoxyl. Hence, the back-reaction to the alkoxyamine (e.g., R1R2NOR) accel- erates, and the self-termination slows down in time. The cross-recombination of · · · macroradical (Rn ) with R1R2NO leads to a very low concentration of AR , limiting the self-termination reaction of the alkyl radical, and allowing the controlled growth of the macroradical. The first work on alkoxyamines R1R2NOR was reported by Jones and Major in 1927 [17]. The radical reactivity of these compounds was discovered by Kovtun et al. in 1974 [18].TheformationofR1R2NOR in recombination of corresponding radicals was also demonstrated. In NMP, the principle stages are [9]: (1) Initiation can be either performed in a bicomponent system or in a unimolecular initiation process. (2) The nitroxide can then cross-recombine with the propagating active chain to give a nitroxide- terminated radical as dormant species. (3) This macroalkoxyamine can undergo reversible C–ON bond homolysis at elevated reaction temperatures and releases the stable nitroxide radical as well as the active polymer chain. Detail schemes of the reaction mechanism in NMP and initiation mechanism of uni- and bimolecular initiators were presented in review [9]. The development of alkoxyamines as initia- tors permits controlled homopolymerization of acrylates and acrylamides. Data on 7.2 Mechanism of the Nitroxide-Mediated Polymerization 163 TEMPO derivatives, alkoxyamines including alkoxyamines bearing stereo centers, and light-sensitive alkoxyamines used in DNP were also described in the review. Steps involved in the TEMPO-mediated nitroxide-mediated polymerization are as follows: (1) The TEMPO/monomer adduct is formed with a source of radicals from a conventional initiator like benzoyl peroxide. (2) With an increase in temperature, the equilibrium is shifted to the addition of monomer, and the radical concentra- tion is controlled by the persistent radical effect. (3) Propagation continues and the growing polymer is capped by TEMPO and the cycle continues. The recent review [5] also provided information on the key components of NMP such as TEMPO derivatives used as nitroxides, alkoxyamines derivatives showing improved bond hydrolysis, alkoxyamines bearing stereo centers, nitroxides and alkoxyamines used for the polymerization (RDRP), of methyl methacrylate (MMA), and functional alkoxyamines. Spin traps are agents that are commonly used in electron spin resonance spec- troscopy (ESR) as a tool to convert transient radicals into a stable form. In polymer- izations, spin traps act as radical scavengers, and hence, mostly inhibit chain growth. Nitroxides and nitrons can be radical spin trap scavengers and used in ESR studies to quantify radical initiation [19–22]. A nitrone, for example, possesses higher affinity toward reacting with oxygen-centered radicals, acrylates, styrenic, and tertiary carbon-centered radicals [20, 21]. In enhanced spin capturing polymerization (ESCP), (macro)radical spin trapping by nitrons is in competition with conventional macroradical chain growth, leading to an effective chain length control of the polymerization, namely trapping macroradicals and shorting the resid- ual polymer chains. The trapped species constitutes a macronitroxide, which by itself is able to trap a further transient radical in a radical recombination event. Thus, the polymer product consists of a coupling product of two polymers, which is bridged by an alkoxyamine functionality in a typical example of the nitroxide-mediated polymerization, the preparation of ABA-type block copolymers via tandem enhanced spin capturing polymerization (ESCP) [23]. Midchain −1 alkoxyamine functional polystyrenes (Mn = 6200, 12,500, and 19,900 g mol ) were chain extended with styrene as well as tert-butyl acrylate at elevated tempera- ture NMP conditions (T = 110 °C), generating a tandem ESCP-NMP sequence. It 164 7 Nitroxide-Mediated Polymerization was found that the efficiency of the block copolymer formation process decreases with an increasing chain length of the ESCP precursor macromolecules. Photopolymerization reactions are recognized as powerful tool for modification of coatings, inks, photoresists, dual-cure systems, and others [24, 25]. Nitroxide- mediated photopolymerization (PNMP) is characterized by important advantages, e.g., thin films can be polymerized very rapidly by UV irradiation, and PNMP is an ecological alternative to thermal processes and is able to modify and pattern surfaces. During PNMP, the photolysis of an alkoxyamine bearing a chromophore, for example, leads to a reversible equilibrium between the alkoxyamine and the generated nitroxide and alkyl radicals, providing a controlled radical photopolymerization [26]. In the alkoxyamines with a closer proximity between the chromophore and the C–O bond, the homolysis of the alkoxyamine is facilitated due to an energy transfer from the chromophore (“antenna”) to the weak C–O bond. In work [27], the thermally driven polymerizations were performed via the cleavage of the alkoxyamine functionality; whereas, the photochemically induced polymerizations were carried out either by nitroxide-mediated photopolymerization (NMP2) or by a mechanism, depending on the structure of the light-sensitive alkoxyamine employed. With each alkoxyamine, as initiators of thermally and photoinduced polymerizations, block copolymers were obtained, and the system was applied to the post-modification of polymer coatings for application in patterning and photografting. The photoradical polymerization of vinyl acetate was performed using 4- methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate (BAI) [28]. It was found that in this condition: (1) The polymerization proceeds by the living mechanism based on lin- ear