JOURNAL OF ORIGINAL ARTICLE WWW.POLYMERCHEMISTRY.ORG POLYMER SCIENCE Synthesis of n-Alkyl Methacrylate Polymers with Pendant Carbazole Moieties and Their Derivatives Yuriy Bandera,1,2 Tucker M. McFarlane,1,2,3 Mary K. Burdette,1,2 Marek Jurca,1,2 Oleksandr Klep,1,2 Stephen H. Foulger 1,2,4 1Center for Optical Materials Science and Engineering Technologies, Advanced Materials Research Laboratories, Clemson University, 91 Technology Drive, Anderson, South Carolina 29625 2Department of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634 3Sonoco Institute of Packaging Design and Graphics at Clemson University, Clemson University, Clemson, South Carolina 29634 4Department of Bioengineering, Clemson University, Clemson, South Carolina 29634 Correspondence to: S. H. Foulger (E-mail: [email protected]) Received 3 October 2018; Accepted 6 November 2018; published online 3 December 2018 DOI: 10.1002/pola.29285 ABSTRACT: New methacrylate monomers with carbazole moie- their effect on the energy profile, thermal, dielectric, and ties as pendant groups were synthesized by multistep synthe- photophysical properties when compared to the parent poly- ses starting from carbazoles with biphenyl substituents in the mer poly(2-(9H-carbazol-9-yl)ethyl methacrylate). According to aromatic ring. The corresponding polymers were prepared the obtained results, these compounds may be well suited for using a free-radical polymerization. The novel polymers memory resistor devices. © 2018 Wiley Periodicals, Inc. contain N-alkylated carbazoles mono- or bi-substituted with J. Polym. Sci., Part A: Polym. Chem. 2019, 57,70–76 biphenyl groups in the aromatic ring. N-alkyl chains in poly- mers vary by length and structure. All new polymers were KEYWORDS: dielectric properties; radical polymerization; struc- synthesized to evaluate the structural changes in terms of ture-property relations INTRODUCTION A widely studied semiconducting polymer is exhibit multiple types of switching behavior.14,15,28 Polymers poly(N-vinyl carbazole) (PVK), which has seen uses in a wide with pendant carbazole groups require proper face-to-face variety of organic electronic devices including organic light alignment of the carbazole moieties for efficient charge trans- – – emitting diodes (OLEDs),1 9 photovoltaics,10 13 and memory fer, and due to the steric hindrance of the carbazole groups 14–20 devices. PVK was found to utilize pendant carbazole caused by their proximity to the backbone of the polymer, groups as the main charge carrier and has been shown to have PVK has been shown to have a single conductivity state as the 21,22 high hole mobility. It initially found prominence as a highly carbazole groups have a frustrated realignment.15 However, photosensitive organic conductor and was extensively studied changes to the flexibility of the carbazole groups through by the Xerox Corporation primarily due to its role in electro- modifications of the chain length that separate the electroni- photography leading to further understanding of the charge cally active group from the polymer backbone enhances the transport properties and excimer formation.23–27 The charge freedom of the carbazole group and allows for conformational transport of PVK is based on the pendant carbazole groups, changes under electric fields resulting in multiple resistivity thus their alignment and spatial proximity is key for efficient states.15 2-(9H-carbazol-9-yl)ethyl methacrylate (PEMA) charge mobility. The importance of the carbazole spacing was (cf. Fig. 1 C2) is one such polymer that offers sufficient flexi- demonstrated by chemically altering the polymer to adjust the alignment of the carbazole groups resulting in varied electrical bility to the carbazole groups such that they are capable of fi properties.24 Therefore, conformational changes in the undergoing realignment under an electric eld and transition- carbazole groups allow for the alteration of conductivity in the ing from a low conductivity OFF state to a high conductivity 15 polymer leading to the development of resistive memory appli- ON state. Due to its low volatility and propensity to remain cations using pendant carbazole groups.14–16 in the ON state when exposed to a reverse bias, as well as over large time scales, this material is used to create write- With this in mind, acrylate polymers have been developed, once-read-many devices which limits its possible applications. which are based on PVK that have pendant carbazoles and However, this approach to enhancing conformational Additional supporting information may be found in the online version of this article. © 2018 Wiley Periodicals, Inc. 70 JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019, 57,70–76 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ORIGINAL ARTICLE O O O O O O O O O O O O 7 9 7 N N N N N C2 8a 8b9 15 FIGURE 1 Monomers synthesized in the effort: (C2) 2-(9H-carbazol-9-yl)ethyl methacrylate (8a) 9-(3-([1,10-biphenyl]-4-yl)-9H-carbazol-9-yl) nonyl methacrylate (8b) 11-(3-([1,10-biphenyl]-4-yl)-9H-carbazol-9-yl)undecyl methacrylate (9) 9-(3,6-di([1,10-biphenyl]-4-yl)-9H-carbazol- 9-yl)nonyl methacrylate (15) 2-(2-(2-(3-([1,10-Biphenyl]-4-yl)-9H-carbazol-9-yl)ethoxy)ethoxy)ethyl methacrylate. flexibility has shown promise in creating polymers with multi- polymerization with AIBN in chlorobenzene at 60–65 C with ple conductive states.14 yields ranging from 28% to 66% (cf. Supporting Information for details). Lengthening or oxygenating the n-alkyl chain and The current effort explores the role that linkage length and adding one or more biphenyl groups to the carbazole pendant pendant “bulkiness” has on the thermophysical and electrical group on the parent polymer were investigated.33 Structure– properties of four polymers with pendant carbazole moieties property relationships, such as glass transition temperature relative to PEMA. These polymers are developed through a (Tg), decomposition temperature, activation energy, electronic synthetic approach utilizing previously reported routes.29–32 band gap, absorbance, and photoluminescence were studied Specifically, carbazole derivatized n-alkyl methacrylate poly- to interpret the full effect of these manipulations when com- mers are synthesized and studied with side chain lengths pared to the properties of C2, poly(9-(9H-carbazol-9-yl)nonyl ranging from n =2ton = 11 and coupled with mono- or dual- methacrylate) (PNMA, the unsubstituted version of 10a and substituted biphenyl groups on the pendant carbazole group 11), and poly(9-(9H-carbazol-9-yl)undecyl methacrylate) with molar volumes of the derivatized carbazole moieties (PUMA, the unsubstituted version of 10b).14 ranging from 266.5 or 391.1 cm3. Thermal Properties Perhaps the most drastic change observed when lengthening RESULTS AND DISCUSSION or oxygenating the n-alkyl chain as well as adding one or Figure 1 presents poly(2-(9H-carbazol-9-yl)ethyl methacry- more biphenyl groups to the carbazole moiety occurred in the late) (C2) and the monomers synthesized in the current study. thermal properties of the polymer when compared to C2. The Monomers were synthesized with a multistep route utilizing glass transition temperatures (and molecular weights) for modified Suzuki coupling reactions followed by a variety of polymers 10a,b, 11, and 16 are shown in Table 1. When nucleophilic substitution reactions.29–32 Polymers with vari- increasing the length of the chain from 9 methylene bridges ous molecular weights were obtained through free-radical (10a) to 11 methylene bridges (10b), the Tg is lowered by TABLE 1 Molecular Weight, Glass Transition, and Decomposition Temperature Characterization a b c d Compound Mn (g/mol) Tg ( C) Tdecomp ( C) Change in Structure from C2 10a 11,900 73 405 Alkyl chain increased by 7 CH2 groups and biphenyl group attached to carbazole moiety 10b 11,456 59 386 Alkyl chain increased by 9 CH2 groups and biphenyl group attached to carbazole moiety 11 21,489 95 445 Alkyl chain increased by 7 CH2 groups and double biphenyl group attached to carbazole moiety 16 16,231 78 398 Oxygenation of alkyl chain and biphenyl group attached to carbazole moiety C2 – 125 306 – a c Molecular weight (Mn) was characterized by gel permeation chromatog- Decomposition temperature (Tdecomp) was found via thermogravimetric raphy (GPC) with chloroform as the eluent. analysis (TGA). b d Glass transition temperature (Tg) was found via differential scanning Tabulated structural changes in monomer when compared to C2. calorimetry (DSC). JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019, 57,70–76 71 JOURNAL OF ORIGINAL ARTICLE WWW.POLYMERCHEMISTRY.ORG POLYMER SCIENCE 14 C due to the extra flexibility of the chain that the addi- motion of the end group, a beta (β) transition associated with tional methylene bridges provide. The longer n-alkyl chain the motion of the side chains, and an alpha (α) transition asso- allows the carbazole moiety more freedom of movement with ciated with the motion of the entire polymer occurring near a negligible increase to the free volume. However, when Tg of the glass transition temperature. However, previous work has PNMA is compared to its counterpart (10a), the Tg of PNMA shown that the alpha transition merges with the beta transi- is much greater (14 C) than that of 10a (73 C) due to the tion at high temperature and/or when the side chain becomes addition of the bulky biphenyl group to the carbazole moi- longer in n-alkyl methacrylate polymers.34 Therefore, these 14 ety. Similarly, the Tg of PUMA is much greater than that of polymers only exhibit a high temperature β transition and a 10b, again, due to the addition of the biphenyl group.14 As low temperature γ transition denoted by peaks in the tan(δ). expected, increasing the number of biphenyl substituents on The response of 10a observed in Figure 2 was representative the carbazole moiety from one (10a) to two (11) greatly of the other polymers presented in this work (cf.