Termination Mechanism in the Radical Polymerization of Methyl Methacrylate and Styrene Determined by the Reaction of Structurally Well-Defined Polymer End Radicals

A novel method to determine the termination mechanism of radical polymerization, i.e., the selectivity between disproportionation (Disp) and combination (Comb), is developed. The method relies on product analyses of the reaction of polymer-end radicals, which are generated from structurally well-controlled living polymers, and the analyses of molecular weight and end-group structure of the product polymers by GPC, mass spectroscopy, and ¹H NMR unambiguously determined the contribution of two competing pathways. The termination mechanism in the polymerization of methyl methacrylate (MMA) and styrene was investigated as a proof of principle of the method by using the corresponding polymers prepared by organotellurium-mediated radical polymerization. The ratios of Disp and Comb (<i>D</i>/<i>C</i>) of poly­(methyl methacrylate) (PMMA) or polystyrene (PSt) end radicals at 25 °C were 73/27 or 15/85, respectively, and the results agreed well with the previous reports. The contribution of the Comb increased at higher temperature in both cases, though the temperature dependence was less pronounced in PSt radicals (<i>D</i>/<i>C</i> = 67/37 and 13/87 at 100 °C for PMMA and PSt, respectively). Thermodynamic parameters were determined as ΔΔ<i>G</i>^‡_d/c = (−6.9 ± 0.3) – <i>T</i> × (−14.4 ± 1.0) × 10^–3 (kJ mol^–1) for PMMA and ΔΔ<i>G</i>^‡_d/c = (−2.0 ± 0.5) – <i>T</i> × (−20.8 ± 1.5) × 10^–3 (kJ mol^–1) for PSt, in which ΔΔ<i>G</i>^‡_d/c and <i>T</i> are difference in Gibbs energy undergoing Disp and Comb, and temperature in Kelvin, respectively, by carrying out the same experiments between −20 to +100 °C. The parameters reveal that Comb is enthalpically less favored but entropically more favored than Disp in both cases. The effects of molecular weight (chain length) were also investigated, and the <i>D</i>/<i>C</i> ratio became constant when the molecular weight of polymers was more than about 3000

Controlled Copolymerization of 1‑Octene and (Meth)acrylates via Organotellurium-Mediated Living Radical Polymerization (TERP)

Copolymerization of 1-octene and (meth)­acrylates, such as methyl acrylate, trifluoroethyl acrylate (TFEA), methyl methacrylate, and trifluoroethyl methacrylate, under organotellurium-mediated living radical polymerization (TERP) conditions was investigated. Polymerization under thermal conditions gave copolymers with considerably broad molecular distributions (polydispersity index [PDI] > 1.45), whereas that under photoirradiation greatly increased the PDI control. Structurally well-controlled copolymers with number-average molecular weights (<i>M</i>_n) of 3000–18 000 and low PDIs (1.22–1.45) were obtained. Addition of Brønsted acids, such as 1,3-C₆H₄[C­(CF₃)₂OH]₂ and hexafluoroisopropanol, increased the insertion of 1-octene into the copolymer. The molar fraction of 1-octene (MF_oct) reached ∼0.5 in the copolymerization using TFEA as an acrylate monomer and excess amount of 1-octene in the presence of the acid. The copolymer was used as a macro-chain-transfer agent for the synthesis of block copolymers. This is the first example of the use of this type of copolymer as a macro-chain-transfer agent in the controlled synthesis of block copolymers

Modular Synthesis of Mid-Chain-Functionalized Polymers by Photoinduced Diene- and Styrene-Assisted Radical Coupling Reaction of Polymer-End Radicals

Photoirradiation of structurally well-defined “living” polymers prepared by organotellurium-mediated living radical polymerization in the presence of dienes or styrenes induced selective polymer-end coupling reaction with the concomitant insertion of the dienes or styrenes with >90% coupling efficiency. The number of inserted dienes or styrenes could be highly controlled to two molecules when acrylic polymers were used. Therefore, various mid-chain-functionalized polymers with well-controlled molecular and macromolecular structure in terms of their molecular weight, molecular weight distribution, functionality, and position were successfully synthesized by employing functionalized dienes or styrenes. The method was applied to the facile synthesis of mid-chain-functionalized telechelic polymers and a 4-miktoarm star polymer with a well-controlled structure

Synthesis and Characterization of [<i>n</i>]CPP (<i>n</i> = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization

Radical cations and dications of [<i>n</i>]­cyclo-<i>p</i>-phenylenes ([<i>n</i>]­CPPs, <i>n</i> = 5, 6, 10, and 12), which are the models of those of linear oligo-<i>p</i>-phenylenes without a terminus, were synthesized as hexafluoro­antimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF₆ or SbF₅. The radical cations, [<i>n</i>]­CPP^•+, and dications, [<i>n</i>]­CPP²⁺, exhibited remarkable batho­chromic shifts in their UV–vis–NIR absorption bands, suggesting that [<i>n</i>]­CPP^•+ and larger [<i>n</i>]­CPP²⁺ exhibit longer polyene character than the shorter analogues. The larger batho­chromic shift was consistent with the narrower HOMO–SOMO and HOMO–LUMO gaps in larger [<i>n</i>]­CPP^•+ and [<i>n</i>]­CPP²⁺, respectively. In [<i>n</i>]­CPP^•+, the spins and charges were equally and fully delocalized over the <i>p</i>-phenylene rings of the CPPs, as noted by ESR. ¹H NMR revealed that the hydrogen of [<i>n</i>]­CPP²⁺ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [<i>n</i>]­CPP²⁺. The single resonances observed in all [<i>n</i>]­CPP²⁺ strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]­CPP by VT-NMR experiment and theoretical calculation

Selective Synthesis and Crystal Structure of [10]Cycloparaphenylene

[10]Cycloparaphenylene ([10]CPP) was selectively synthesized in four steps in 13% overall yield from commercially available 4,4′-diiodobiphenyl by using mono-I–Sn exchange, Sn–Pt transmetalation, I–Pd exchange, and subsequent oxidative coupling reactions. The single-crystal X-ray structure of [10]CPP is described

Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops

Three N-substituted tetracyclo­(2,7-carbazole)­s were synthesized to investigate the inner regions of nanohoops. One compound has a 5,5-dimethylnonane bridge between two neighboring <i>anti</i>-carbazoles, which can be used as covalently bonded “methane probes”. These probes near the ring center are strongly shielded by local ring currents and exhibit a singlet at δ = −2.70 ppm in ¹H NMR. To visualize local and macrocyclic ring currents separately, we drew nucleus-independent chemical shift contour maps of tetracyclo­(9-methyl-2,7-carbazole) and [<i>n</i>]­cycloparaphenylenes (CPPs). Local ring currents make the interior diatropic, and paratropic regions exist only outside the ring. Macrocyclic ring currents in [5]­CPP to [7]­CPP generate deshielding cones, which are typical of antiaromatic [4<i>n</i>]­annulenes

Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops

Three N-substituted tetracyclo­(2,7-carbazole)­s were synthesized to investigate the inner regions of nanohoops. One compound has a 5,5-dimethylnonane bridge between two neighboring <i>anti</i>-carbazoles, which can be used as covalently bonded “methane probes”. These probes near the ring center are strongly shielded by local ring currents and exhibit a singlet at δ = −2.70 ppm in ¹H NMR. To visualize local and macrocyclic ring currents separately, we drew nucleus-independent chemical shift contour maps of tetracyclo­(9-methyl-2,7-carbazole) and [<i>n</i>]­cycloparaphenylenes (CPPs). Local ring currents make the interior diatropic, and paratropic regions exist only outside the ring. Macrocyclic ring currents in [5]­CPP to [7]­CPP generate deshielding cones, which are typical of antiaromatic [4<i>n</i>]­annulenes

In-Plane Aromaticity in Cycloparaphenylene Dications: A Magnetic Circular Dichroism and Theoretical Study

The electronic structures of [8]­cyclo­para­phenylene dication ([8]­CPP²⁺) and radical cation ([8]­CPP^•+) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV–vis–NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]­CPP²⁺ shows in-plane aromaticity with a (4<i>n</i> + 2) π-electron system (<i>n</i> = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]­CPP²⁺ but also all [<i>n</i>]­CPP (<i>n</i> = 5–10) dications and dianions exhibit in-plane aromaticity

Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [<i>n</i>]­cycloparaphenylene dications ([<i>n</i>]­CPP²⁺, <i>n</i> = 5–9), which have unique in-plane aromaticity. The fluorescence peak positions of the [<i>n</i>]­CPP²⁺s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with <i>n</i> > 5. The fluorescence quantum yield of [6]­CPP²⁺ was the highest among the [<i>n</i>]­CPP²⁺s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [<i>n</i>]­CPP²⁺s were smaller than those of neutral [<i>n</i>]­CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [<i>n</i>]­CPP²⁺s undergo smaller structural changes upon S₀–S₁ transition than [<i>n</i>]­CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S₀–S₁ transition of smaller [<i>n</i>]­CPP²⁺s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]­CPP²⁺ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap

Radical Ions of Cycloparaphenylenes: Size Dependence Contrary to the Neutral Molecules

Cycloparaphenylenes (CPPs) have attracted wide attention because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals. For application of CPPs, information on their charged states (radical cation and radical anion) is essential. Here, we measured absorption spectra of the radical cations and the radical anions of CPPs with various ring sizes over a wide spectral region by means of radiation chemical methods. The peak position of the near-IR bands for both the radical cation and the radical anion shifted to lower energies with an increase in the ring size. This trend is contrary to what is observed for transitions between the HOMO and LUMO of the neutral CPP. The observed spectra of the CPP radical ions were reasonably assigned based on time-dependent density functional theory. These results indicate that the next HOMO and the next LUMO levels are important in the electronic transitions of radical ions