Contrasting the Charge Carrier Mobility of Isotactic, Syndiotactic, and Atactic Poly((N-carbazolylethylthio)propyl methacrylate)

Abstract

Isotactic nonconjugated pendant electroactive polymers (NCPEPs) have recently shown potential to achieve comparable charge carrier mobilities with conjugated polymers. Here we report the broader influence of tacticity in NCPEPs, using poly­((N-carbazolylethylthio)­propyl methacrylate) (PCzETPMA) as a model polymer. We utilized the thiol–ene reaction as an efficient postpolymerization functionalization method to achieve pendant polymers with high isotacticity and syndiotacticity. We found that a stereoregular isotactic polymer showed ∼100 times increased hole mobility (μh) as compared to both atactic and low molecular weight syndiotactic PCzETPMA, achieving μh of 2.19 × 10–4 cm2 V–1 s–1 after annealing at 120 °C. High molecular weight syndiotactic PCzETPMA gave ∼10 times higher μh than its atactic counterpart, comparable to isotactic PCzETPMA after annealing at 150 °C. Importantly, high molecular weight syndiotactic PCzETPMA showed a dramatic increase in μh to 1.82 × 10–3 cm2 V–1 s–1 when measured after annealing at 210 °C, which surpassed the well-known conjugated polymer poly­(3-hexylthiophene) (P3HT) (μh = 4.51 × 10–4 cm2 V–1 s–1). MD simulations indicated short-range π–π stacked ordering in the case of stereoregular isotactic and syndiotactic polymers. This work is the first report of charge carrier mobilities in syndiotactic NCPEPs and demonstrates that the tacticity, annealing conditions, and molecular weight of NCPEPs can strongly affect μh.

Synthesis of Block Copolymers Containing Stereoregular Pendant Electroactive Blocks

“…Within the entire family of copolymers B9 f , the only higher molar mass copolymer with a dominant isotactic PCzETPMA-block gives the highest μ h upon annealing of 2.33 × 10 −5 cm 2 /V•s, which is consistent with our previous findings that higher levels of stereoregularity and higher molar masses correspond to increased μ h in NCPEPhomopolymers. 15,17 While the previously reported isotactic PCzETPMA homopolymer gave a superior μ h of 2.19 × 10 −4 cm 2 /V•s upon annealing, molar mass differences between the polymers have to be considered, where M n = 46.5 kg/mol was reported for the homopolymer and M n = 28.2 kg/mol for the copolymer (B9 f ) in this study. 15 Given the noticeable impact of molar mass on μ h when going from B1 f −B4 f to B5 f −B9 f , the even higher molar mass of the homopolymer could serve as one explanation for its superior mobility.…”

“…After growth of the second block, the GPC traces show a wave-shaped pattern with a negative RI associated with the PAMA block (which is consistent with previous studies on PAMA homopolymers) that immediately transitions into the PS-peak with a positive RI without leveling out on the baseline in between. 15 After functionalization of the PAMA-block, the copolymers show just one peak with a positive RI at a shifted molar mass compared to the PS-peak.…”

“…PAMA functionalized with N-carbazolylethanethiol was chosen as the electroactive block for consistency with our study on analogous NCPEP homopolymers. 15 The carbazole pendant groups are attached via a six-atom spacer based on our previous study on isotactic poly(N-carbazolylalkyl acrylates) reporting a six-atom spacer to give the highest μ h . 16 Introduction of the pendant group was achieved via postpolymerization functionalization of the PAMA-block in photochemical thiol−ene reactions.…”

“…Synthesis of the PAMA-blocks was guided by our previous findings for stereoregular PAMA homopolymers showing that anionic polymerization in toluene at low temperatures yields highly isotactic PAMA as well as work by Brownstein et al demonstrating that addition of as little as 2.5 vol % THF to the toluene leads to atactic PAMA. 15,25 Using these methods, we synthesized nine PS-b-PAMA block copolymers indicated as B1 u −B9 u (Table 1), where the subscript u indicates an unfunctionalized PAMA block. Here, we varied the molar mass, the tacticity of the PAMA-block, and the relative ratio of the PS-and PAMA-blocks.…”

“…This is consistent with our reported results for the functionalization of PAMA homopolymers. 15 While B1 u −B4 u copolymers with either a dominant PS or a dominant PAMA block were synthesized, it proved more difficult to realize copolymers with a dominant PAMA block in the higher molar mass range, especially under conditions required for atactic PAMA. Therefore, PS is the dominant block in B5 u and B6 u ; however, the relative ratio of the blocks was varied to range from a clearly dominant PS-block to two blocks of roughly the same length with B6 u .…”