A Bacterial Chitinase Acts as Catalyst for Synthesis of the N‐Linked Oligosaccharide Core Trisaccharide by Employing a Sugar Oxazoline Substrate

A chitinolytic enzyme, chitinase A1 from Bacillus circulans WL‐12, was found to catalyze a glycosyl‐transferring reaction to form the N‐linked oligosaccharide core structure, Man(β1‐4)‐GlcNAc(β1‐4)‐GlcNAc, by employing Man(β1‐4)‐GlcNAc‐oxazoline as glycosyl donor. When the reaction was carried out in the presence of 20 v/v% acetone, the trisaccharide was obtained in 32% yield. It has been shown for the first time that a chitinase behaves like an endo‐β‐N‐acetylglucosaminidase in spite of low structural similarity between them.     

Thermo-responsive mending of polymers crosslinked by thermally reversible covalent bond: Polymers from bisfuranic terminated poly(ethylene adipate) and tris-maleimide

Remending properties of a network polymer with reversible reactivity are described. The network structure is constructed by a Diels-Alder (DA) reaction between furyl-telechelic poly(ethylene adipate) (PEAF₂) and a tris-maleimide, M₃. When a film sample was cut into two pieces and the cut surfaces were kept in contact with each other at 60 °C, rejoining of the cut pieces was observed. This mending was induced by the reversible cross-linking reaction bridging the cut surfaces. At the cut front, the “weak” DA adducts are selectively dissociated sacrificially to release the stress so as to protect the chemical structure of the prepolymer and the linker against the scission or degradation. The dissociated furan and maleimide readily reconnect by forward DA reaction to mend the material. The remending was also observed for the samples kept at room temperature after melting at 60 °C. So, the PEAF₂ network polymer is a thermo-responsive mendable material in which crack healing is induced by a prompt thermal stimulus.     

Flow Synthesis of Plasmonic Gold Nanoshells via a Microreactor

Gold nanoshells with tunable surface plasmon resonances are a promising material for optical and biomedical applications. They are produced through seed‐mediated growth, in which gold nanoparticles (AuNPs) are seeded on the core particle surface followed by growth of the gold seeds into a shell. However, synthetic gold nanoshell production is typically a multistep, time‐consuming batch‐type process, and a simple and scalable process remains a challenge. In the present study, a continuous flow process for the seed‐mediated growth of silica–gold nanoshells is established by exploiting the excellent mixing performance of a microreactor. In the AuNP‐seeding step, the reduction of gold ions in the presence of core particles in the microreactor enables the one‐step flow synthesis of gold‐decorated silica particles through heterogeneous nucleation. Flow shell growth is also realized using the microreactor by selecting an appropriate reducing agent. Because self‐nucleation in the bulk solution phase is suppressed in the microreactor system, no washing is needed after each step, thus enabling the connection of the microreactors for the seeding and shell growth steps into a sequential flow process to synthesize gold nanoshells. The established system is simple and robust, thus making it a promising technology for producing gold nanoshells in an industrial setting.     

Transition-metal-free controlled polymerization for poly(p-aryleneethynylene)s

A transition-metal-free controlled polymerization for the attainment of poly(p-aryleneethynylene)s is developed. The polymerization of 1-pentafluorophenyl-4-[(trimethylsilyl)ethynyl]benzene with a catalytic amount of fluoride anions proceeds in a chain-growth-like manner to afford polymers with controlled molecular weights and low polydispersity indexes. The mechanism involves a pentacoordinated fluorosilicate as a key intermediate. The anionic “living” nature of this process is applied to block copolymerization and also surface-terminated polymerization.     

Enantio‐ and Stereoselective Cyclopolymerization of Hexa‐1,5‐diene Catalyzed by Zirconium Complexes Possessing Optically Active Bis(phenolato) Ligands

Enantio‐ and stereoselective cyclopolymerization of hexa‐1,5‐diene was achieved by enantiomerically pure dichloro zirconium(IV) pre‐catalysts 2 possessing chiral [OSSO]‐type bis(phenolate) ligands (−)‐ 1 and (+)‐ 1 in combination with dried methylaluminoxane (dMAO) as an activator. The corresponding activities were recorded with quite high values up to 1,960 g mmol( 2 )^–1 h^–1, which are extremely larger than those of the related complexes. The microstructure analysis for the PMCPs furnished by pre‐catalysts (Λ,S,S)‐ 2 and (Δ,R,R)‐ 2 showed good isotacticity factors (α = 75−78%) and relatively high proportions of trans‐cyclopentane rings (σ = 14−21%). These enantiomeric PMCPs exhibited large specific optical rotations ([α]_D = +28 to +32° from (Λ,S,S)‐ 2 , −26 to −34° from (Δ,R,R)‐ 2 ).

     

Chain contraction and nonlinear stress damping in primitive chain network simulations

Doi and Edwards (DE) proposed that the relaxation of entangled linear polymers under large deformation occurs in two steps: the fast chain contraction (via the longitudinal Rouse mode of the chain backbone) and the slow orientational relaxation (due to reptation). The DE model assumes these relaxation processes to be independent and decoupled. However, this decoupling is invalid for a generalized convective constraint release (CCR) mechanism that releases the entanglement on every occasion of the contraction of surrounding chains. Indeed, the decoupling does not occur in the sliplink models where the entanglement is represented by the binary interaction (hooking) of chains. Thus, we conducted primitive chain network simulations based on a multichain sliplink model to investigate the chain contraction under step shear. The simulation quantitatively reproduced experimental features of the nonlinear relaxation modulus G(t,γ). Namely, G(t,γ) was cast in the time-strain separable form, G(t,γ)=h(γ)G(t) with h(γ)=damping function and G(t)=linear modulus, but this rigorous separability was valid only at times t comparable to the terminal relaxation time, although a deviation from this form was rather small (within ±10%) at t>τ(R) (longest Rouse relaxation time). A molecular origin of this delicate failure of time-strain separability at t～τ(R) was examined for the chain contour length, subchain length, and subchain stretch. These quantities were found to relax in three steps, the fast, intermediate, and terminal steps, governed by the local force balance between the subchains, the longitudinal Rouse relaxation, and the reptation, respectively. The contributions of the terminal reptative mode to the chain length relaxation as well as the subchain length/stretch relaxation, not considered in the original DE model, emerged because the sliplinks (entanglement) were removed via the generalized CCR mechanism explained above and the reformation of the sliplinks was slow at around the chain center compared to the more rapidly fluctuating chain end. The number of monomers in the subchain were kept larger at the chain center than at the chain end because of the slow entanglement reformation at the center, thereby reducing the tension of the stretched subchain at the chain center compared to the DE prediction. This reduction of the tension at the chain center prevented completion of the length equilibration of subchains at t～τ(R) (which contradicts to the DE prediction), and it forces the equilibration to complete through the reptative mode at t?τ(R). The delicate failure of time-strain separability seen for G(t,γ) at t～τ(R) reflects this retarded length equilibration.