Synthesis of Well-Defined Novel Reactive Block Polymers Containing a Poly(1,4-divinylbenzene) Segment by Living Anionic Polymerization

In order to synthesize a variety of block polymers having poly­(1,4-divinylbenzene) (PDVB) segments, the living anionic block polymerizations of DVB with styrene, 2-vinylpyridine (2VP), <i>tert</i>-butyl methacrylate (^tBMA), methyl methacrylate (MMA), <i>N</i>-(4-vinylbenzylidene)­cyclohexylamine (<b>1</b>), 2-(4′-vinylphenyl)-4,4-dimethyl-2-oxazoline (<b>2</b>), or 2,6-di-<i>tert</i>-butyl-4-methylphenyl 4-vinylbenzoate (<b>3</b>) were conducted in THF at −78 °C with the anionic initiator bearing K⁺ in the presence of a 10-fold excess of potassium <i>tert</i>-butoxide. With the sequential addition of DVB and each of these monomers, the following block polymers having PDVB segments were successfully synthesized: PS-<i>b</i>-PDVB, P2VP-<i>b</i>-PDVB, PDVB-<i>b</i>-P2VP, PDVB-<i>b</i>-P^tBMA, PDVB-<i>b</i>-P­(<b>1</b>), PDVB-<i>b</i>-P­(<b>2</b>), PDVB-<i>b</i>-P­(<b>3</b>), PS-<i>b</i>-PDVB-<i>b</i>-P^tBMA, PS-<i>b</i>-P2VP-<i>b</i>-PDVB-<i>b</i>-P^tBMA, and PS-<i>b</i>-PDVB-<i>b</i>-P2VP-<i>b</i>-P^tBMA. The resulting polymers are all novel block polymers with well-defined structures (predictable molecular weights and compositions and narrow molecular weight distributions) and possess reactive PDVB segments capable of undergoing several postreactions. Based on the results of such sequential block polymerizations, the anionic random copolymerization of DVB and 2VP, the polymerizability with (C₄H₉)₂Mg, and some other addition reactions, it was found that the comparable reactivity of the chain-end anions follows the sequence of PS^– > PDVB^– > P2VP^– > P^tBMA^–. Accordingly, the reactivity of the corresponding monomers increases as follows: styrene < DVB < 2VP < ^tBMA

Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer–inorganic composites with diaryl­bibenzo­furanone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C–C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer–inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation

Multicolor Mechanochromic Polymer Blends That Can Discriminate between Stretching and Grinding

Mechanochromic polymers, which react to mechanical force by changing color, are expected to find applications in smart materials such as damage sensors. Although numerous types of mechanochromic polymers have been reported so far, developing mechanochromic polymers that can recognize different mechanical stimuli remains a formidable challenge. Materials that not only change their color in response to a mechanical stimulus but also detect its nature should be of great importance for practical applications. In this paper, we report our preliminary findings on multicolor mechanochromic polymer blends that can discriminate between two different mechanical stimuli, i.e., stretching and grinding, by simply blending two mechanochromic polymers with different architectures. The rational design and blending of two mechanochromic polymers with radical-type mechanochromophores embedded separately in positions adjacent to soft or hard domains made it possible to achieve multicolor mechanochromism in response to different stimuli. Electron paramagnetic resonance and solid-state UV–vis measurements supported the mechanism proposed for this discrimination

Precise Synthesis of Miktoarm Star Polymers by Using a New Dual-Functionalized 1,1-Diphenylethylene Derivative in Conjunction with Living Anionic Polymerization System

The general utility of a 1,1-diphenylethylene (DPE) derivative substituted with trimethylsilyl- and <i>tert</i>-butyldimethylsilyl-protected hydroxyl functionalities, as a new dual-functionalized core agent in conjunction with a living anionic polymerization system, has been demonstrated by the successful synthesis of various well-defined 3-arm ABC and 4-arm ABCD μ-star polymers. Two different protected hydroxyl functionalities were progressively deprotected to generate hydroxyl groups, followed by conversion to α-phenyl acrylate (PA) functions at separate stages, and the PA functions were reacted with appropriate living anionic polymers to result in the above μ-stars. In order to further synthesize μ-star polymers with five or more arms, a new iterative methodology using a functional DPE anion derived from the above DPE derivative has been developed. The reaction system of this methodology is designed in such a way that the PA function used as the reaction site is regenerated after the introduction of an arm segment in each reaction sequence, and this sequence, consisting of “arm introduction and regeneration of the PA reaction site”, is repeatable. With this methodology, a series of new well-defined μ-star polymers up to a 5-arm ABCDE type, composed of all different methacrylate-based polymer segments, were successfully synthesized for the first time

Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer–inorganic composites with diaryl­bibenzo­furanone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C–C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer–inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation

Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer–inorganic composites with diaryl­bibenzo­furanone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C–C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer–inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation

Thermally Healable and Reprocessable Bis(hindered amino)disulfide-Cross-Linked Polymethacrylate Networks

A facile approach to polymethacrylate networks that contain thermally exchangeable bis­(2,2,6,6-tetramethylpiperidin-1-yl)­disulfide (BiTEMPS) cross-linkers is reported, and the easily inducible healability and reprocessability of the obtained networks are discussed. The free radical polymerization of BiTEMPS cross-linkers and hexyl methacrylate (HMA) monomers afforded insoluble and colorless networks of poly­(hexyl methacrylate) (PHMA) films, whose structures were characterized after de-cross-linking via thermal BiTEMPS exchange reactions with added low-molecular-weight BiTEMPS. Swelling experiments and stress-relaxation measurements at elevated temperatures revealed the contribution of BiTEMPS as a polymer chain exchanger both in the gels and in the bulk. The BiTEMPS-cross-linked PHMA networks showed damage healability and repeatable reprocessability in the bulk by simple hot pressing at 120 °C under mild pressure (∼70 kPa). These results should grant facile access to various vinyl polymer networks with on-demand malleability

High Anionic Polymerizability of Benzofulvene: New Exo-Methylene Hydrocarbon Monomer

The anionic polymerization of benzofulvene (BF) quantitatively proceeded with various initiators, such as <i>sec</i>-BuLi, diphenylmethyllithium, diphenylmethylpotassium, triphenylmethyllithium, and triphenylmethylpotassium, in THF at −78 °C to give polymers having predicted molecular weights based on the molar ratios between BF and the initiators and narrow molecular weight distributions (MWD, <i>M</i>_w/<i>M</i>_n < 1.1). Although the initiation efficiencies were not quantitative, BF could also be polymerized with benzylmagnesium chloride, potassium <i>tert-</i>butoxide, and the anionic living poly­(<i>tert</i>-butyl methacrylate), indicating the high anionic polymerizability. The planar conformation and the polarized electron density of BF obtained by the density functional theory (DFT) calculation supported the observed anionic polymerizability higher than that of 2-phenyl-1,3-butadiene, the acyclic analogue of BF. A series of new block copolymers, polystyrene-<i>b</i>-poly­(BF), poly­(2-vinylpyridine)-<i>b</i>-poly­(BF), and poly­(<i>tert</i>-butyl methacrylate)-<i>b</i>-poly­(BF), were synthesized by the sequential anionic copolymerization of BF and the comonomers. On the other hand, no polymerization of styrene and 2-vinylpyridine took place with the living poly­(BF), indicating the low nucleophilicity of the propagating indenyl anion formed from BF. BF readily underwent the free-radical polymerization with α,α′-azobis­(isobutyronitrile), while the observed cationic polymerizability of BF was quite low. ¹H and ¹³C NMR analyses revealed that the repeating units of poly­(BF) consisted of a 1,2-addition unit and a 1,4-addition unit without a 3,4-addition unit regardless of the polymerization conditions. The exo-methylene moiety (CH₂) of BF always participated in the addition polymerization. The addition modes were dependent on the polymerization temperature and not on the solvent or the countercation of the anionic initiator. For instance, polymer obtained with <i>sec</i>-BuLi in THF at 40 °C contained 72% of the 1,4-addition unit and 28% of the 1,2-addition unit. Therefore, BF acted as a polymerizable 1,3-diene possessing a fixed transoid framework

Successive Synthesis of Miktoarm Star Polymers Having up to Seven Arms by a New Iterative Methodology Based on Living Anionic Polymerization Using a Trifunctional Lithium Reagent

A new stepwise iterative methodology based on living anionic polymerization using a trifunctional lithium reagent substituted with trimethylsilyl (TMS), <i>tert</i>-butyldimethylsilyl (TBDMS), and tetrahydropyranyl (THP) ethers of protected hydroxyl functionalities has been developed in order to obtain synthetically challenging many-armed μ-star polymers. In each reaction sequence of the new methodology, these three ether functions were selectively deprotected in turn under carefully selected conditions as designed, followed by conversion to three α-phenyl acrylate (PA) reaction sites step by step at different reaction stages. They were used for the introduction of two different arm segments and the reintroduction of the above same three ethers. This reaction sequence was repeated three times to successively synthesize 3-arm ABC, 4-arm ABCD, 5-arm ABCDE, 6-arm ABCDEF, and 7-arm ABCDEFG μ-star polymers with well-defined structures. Herein, the A, B, C, D, E, F, and G arms were poly­(cyclohexyl methacrylate), polystyrene, poly­(4-methoxystyrene), poly­(4-methylstyrene), poly­(methyl methacrylate), poly­(ethyl methacrylate), and poly­(2-methoxyethyl methacrylate) segments, respectively. The trifunctional lithium reagent was also demonstrated to satisfactorily function as a convenient and useful core agent access to the general synthesis of 4-arm ABCD and 6-arm A₂B₂C₂ μ-star polymers

Enhancing Mechanochemical Activation in the Bulk State by Designing Polymer Architectures

Mechanoresponsive polymers can have attractive functions; however, the relationship between polymer architecture and mechanoresponsiveness in the bulk state is still poorly understood. Here, we designed well-defined linear and star polymers with a mechanophore at the center of each architecture, and investigated the effect of molecular weight and branched structures on mechanoresponsiveness in the solid state. Diarylbibenzofuranone, which can undergo homolytic cleavage of the central C–C bond by mechanical force to form blue-colored radicals, was used as a mechanophore because the cleaved radicals could be evaluated quantitatively using electron paramagnetic resonance measurements. We confirmed that longer polymer chains induce mechanochemical activation more effectively and found that, in the bulk state, the star polymers have higher sensitivity to mechanical stress compared with a linear polymer having similar molecular weight arm segment