Yasuhiro Kohsaka

Size‐Complementary Rotaxane Cross‐Linking for the Stabilization and Degradation of a Supramolecular Network

As cross-linked polymers are widely used fundamental materials, their chemical recycling becomes a quite important issue. A promising strategies is to utilize reversible crosslinking based on dynamic covalent chemistry (DCC). 3] For example, Chen et al. reported the efficient thermal reversibility of cross-linking by Diels–Alder reaction. On the other hand, supramolecular gels formed by intermolecular interactions, such as hydrogen bonds, can be efficiently de-crosslinked by specific stimuli without destruction of covalent bonds. 3d, 5–7] However, supramolecular gels are naturally stable only under limited conditions that are capable of keeping the intermolecular interactions strong. A polyrotaxane network (PRN) is a supramolecular gel stabilized by not only intermolecular interactions but also mechanical restriction. We previously reported a reversibly cross-linkable PRN based on DCC, consisting of poly(crown ether) backbone and bis(ammonium) cross-linker possessing a disulfide bond in its center (Figure 1a). The work gave a new impulse for the chemical recycling of cross-linked polymers, but it suffered from the disadvantage that it is a sluggish reaction. Therefore, we have developed a novel approach that enables the efficient de-cross-linking of PRNs without any cleavage of covalent bonds. Recently, we have found a novel procedure in which the axle component as a cross-linker has an end group the size of which is complementary to the macrocycle cavity placed on the trunk polymer (Figure 1b). The end groups provide an energy barrier to slow the dissociation, thereby kinetically stabilizing the rotaxane skeleton. Thus, the PRN stabilizes the network structure under normal conditions, but it can be de-cross-linked when certain conditions, such as those that accelerate the dissociation of the rotaxane skeletons, are satisfied. Because the decross-linking can be achieved without breaking the covalent bonds, the PRN is selectively degraded so as to not damage the trunk polymer. Furthermore, the stability and de-crosslinking capability of PRNs can be adjusted by the size of the end groups of the axle components. Herein we describe the concept of novel de-cross-linkable network polymers that utilize the size-complementary effect of the rotaxane crosslinks. First, we investigated a model for the size-complementary effect using crown ether/ammonium salt [2]rotaxanes 1 (Scheme 1). According to previous reports, roxaxanes 1 with suitable end groups (R = cyclohexyl, tBu, 4-tBuC6H4) were sufficiently stable to maintain its threaded structure owing to both the bulky end groups and hydrogen bonds between dibenzo [24]crown-8 ether (DB24C8) and the ammonium group. However, 1 dissociated into two parts, axle and wheel, when the hydrogen bonds were disturbed by a stimulus, in accordance with reported results (R = 4tBuC6H4). [12] Details are summarized in Table 1. As we envisioned, the dissociation rate depends on both the size of the end group and the kind of the external stimulus, as discussed below. An investigation into the dissociation behavior of 1 in dimethylsulfoxide (DMSO), a polar solvent that disturbed the hydrogen bonds, showed the decomposition of only 1a among three derivatives. The dissociation rate of 1a to DB24C8 and 2a obeyed first-order kinetics. The half-life (t1/2) was estiFigure 1. Strategy for de-cross-linking a PRN using a) a reversible cleavage of disulfide bond and b) characteristics of a rotaxane crosslink consisting of size-complementary components.

Synthesis and post-polymerization reaction of end-clickable stereoregular polymethacrylates via termination of stereospecific living anionic polymerization

Poly(methyl methacrylate)s with high stereoregularity and clickable end-groups were synthesized via terminating reactions with α-(halomethyl)acrylates in stereospecific living anionic polymerization. The terminating reaction was efficient and tolerant to the reaction conditions to such an extent that almost quantitative end-functionalization was achieved in isotactic- and syndiotactic-specific polymerization systems. The terminating reactions were also achieved in polymerizations of vinyl methacrylate and trimethylsilyl methacrylate. For the polymerization of butyl acrylate, however, the termination efficiency was limited to less than 69%. Furthermore, the quantitative end-functionalization of the incorporated CC double bond at ω-end was achieved with various thiols catalyzed by Et3N. The base-catalysed thiol–ene reaction of the stereoregular poly(vinyl methacrylate) with a ω-end CC double bond selectively proceeded to retain vinyl ester functions, and the subsequent hydrolysis afforded ω-functional stereoregular poly(methacrylic acid). A combination of the terminating agent with a protected lithium amide afforded stereoregular poly(methyl methacrylates) with orthogonally clickable α- and ω-ends.

Stereoregular poly(methyl methacrylate) with double-clickable ω-end: synthesis and click reaction

Isotactic and syndiotactic poly(methyl methacrylate)s with orthogonally double-clickable terminal ends, that is, α,β-unsaturated esters for Michael addition-type thiol–ene reactions and azide or alkynyl groups for azide–alkyne click reactions, were prepared via a terminating reaction of stereospecific anionic polymerization with propargyl and 2-chloroethyl α-(chloromethyl)acrylates. The subsequent polymer modification via a double click reaction proceeded quantitatively in a one-pot system under ambient conditions. The facile and almost quantitative double-end-functionalization would open a new material design based on stereoregular PMMAs with controlled molecular weights.

Photodegradable cross-linked polymer derived from a vinylic rotaxane cross-linker possessing aromatic disulfide axle

A new concept for photodegradable cross-linked polymers utilizing characteristics of rotaxane cross-links and aromatic disulfides is proposed. The cross-linked polymer is obtained by the radical polymerization of a vinyl monomer in the presence of a [3]rotaxane-type cross-linker having two radically polymerizable groups. The [3]rotaxane-type cross-linker was prepared in 93 % yield by the typical rotaxane-forming reaction using a dumbbell-shaped aromatic disulfide possessing a bis(ammonium salt) moiety and a crown ether wheel tethered by a hydroxymethyl group (96 %) and the subsequent vinyl group-endowment (80 %). The radical polymerization of methyl methacrylate (MMA) in the presence of the cross-linker (0.1 mol %) at 60 °C afforded solvent-insoluble polymer in 90 % yield. When the polymer was swollen to a gel in dimethylformamide (DMF) and a small part of the gel was UV-irradiated, the gel was promptly solubilized, probably via the photochemical scission of the S–S linkage of the interlocked aromatic disulfide, causing the efficient decomposition of the rotaxane cross-links. The recovered poly(methyl methacrylate) bearing a small amount of crown ether moiety has a molecular weight of Mn 170 kg/mol (Mw/Mn 2.1) that indicated the occurrence of the site-selective photodegradation.

α-(Aminomethyl)acrylate: polymerization and spontaneous post-polymerization modification of β-amino acid ester for a pH/temperature-responsive material

Ethyl α-(aminomethyl)acrylate, a β-amino acid ester carrying a conjugated vinylidene group at the α-position, was radically polymerized. The polymerization was found to involve subsequent ester–amide exchange reaction between the amino pendants of the polymer and an ester group of the monomer, affording acrylamide-bearing units in 11–15% contents. The obtained polymer exhibited pH/temperature responsiveness in aqueous media.

An efficient synthetic entry to rotaxanes utilising reversible cleavage of aromatic disulphide bonds

Reversible cleavage of an aromatic disulphide bond of a dumbbell-shaped molecule having two sec-ammonium salt moieties enabled the insertion of the disulphide linkage through a crown ether to give the corresponding [2]- and [3]rotaxanes. The formation of the rotaxanes proceeded much more rapidly than the case of an aliphatic disulphide system.

Polymerization of α-(halomethyl)acrylates through sequential nucleophilic attack of dithiols using a combination of addition–elimination and click reactions

Polymerization of α-(halomethyl)acrylates and dithiols was achieved by the combination of SN2′ (addition–elimination) and subsequent thiol–ene click reactions. A Bu3P catalyst and an excess amount of a base that does not yield an acidic salt, e.g., 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or K2CO3, were key for achieving Mn > 104.

Synthesis of Thermo-Responsive Polymer via Radical (Co)polymerization of N,N-Dimethyl-α-(hydroxymethyl)acrylamide with N,N-Diethylacrylamide

α-Functionalized acrylamides have not been considered as an effective monomer design due to their poor polymerizability, although the analogues, α-functionalized acrylates, are attractive monomers of which polymers exhibit characteristic properties. In this article, we report the first example of radical polymerization of α-functionalized N,N-disubstituted acrylamide affording thermo-responsive hydrophilic polymers. N,N-dimethyl-α-(hydroxymethyl)acrylamide (DMαHAA) was (co)polymerized with N,N-diethylacrylamide (DEAA). Although the homopolymerization did not afford a polymeric product, the copolymerizations with various feed ratios yielded a series of the copolymers containing 0%–65% of DMαHAA units. The obtained copolymers exhibited a lower critical solution temperature (LCST) in water; the cloud points (Tcs) were linearly elevated as the contents of DMαHAA units from 32 to 64 °C, indicating that DMαHAA functioned as a more hydrophilic monomer than DEAA. The linear relationship between Tc and DMαHAA content suggests that the homopolymer, poly(DMαHAA), should have Tc at ca. 80 °C, although it is not available by direct radical homopolymerization.

Conjugate substitution and addition of α-substituted acrylate: a highly efficient, facile, convenient, and versatile approach to fabricate degradable polymers by dynamic covalent chemistry

Polycondensation of bis[α-(halomethyl)acrylate] and dithiols, dicarboxylic acids, primary monoamines, and bisphenols via conjugate substitution yielded a series of poly(conjugated ester)s with acryloyl groups in the backbones. In the case of a dithiol monomer, a polar solvent promoted the conjugate addition (Michael addition) of a mercapto chain end into the acryloyl group in the resulting backbone to cause gelation. Therefore, kinetic control of the polymerization by the selection of nonpolar solvents and end capping with the mercapto group in the case of polar dithiols was necessary to prepare linear polymers. The monomer, bis[α-(halomethyl)acrylate], was also effective in achieving chain extension of common polyesters with phenolic hydroxyl end groups. These poly(conjugated ester)s underwent main chain scission when they were treated with benzyl mercaptan in the presence of a basic catalyst through a thiol exchange reaction via conjugate addition and substitution.

Anionic polymerization of ethyl acrylate initiated by tetrabutylammonium azide: direct synthesis of end-clickable polyacrylate

Tetrabutylammonium azide was found to initiate the anionic polymerization of ethyl acrylate (EA) in the presence of alkylaluminum bisphenoxides in toluene to afford azide-end polymers with narrow molecular weight distributions in quantitative yields. The resulting polymers showed high enough reactivity for the copper-catalyzed azide–alkyne cycloaddition.