One-Step Synthesis of Lignin-Based Triblock Copolymers as High-Temperature and UV-Blocking Thermoplastic Elastomers

Synthesis of Pyridin-2(1H)-imines via the Transformation of Conjugated Ynones

Conjugated ynones represent an important class of reactive species, useful synthetic intermediates, and synthons. However, the reactivity and synthetic applications of ynones are usually focused on the transformation of mono- or dual-functional groups. Herein, we developed a straightforward synthesis of pyridin-2(1H)-imines from the transformation of conjugated ynones. This cascade process probably began with a Michael addition of ynones and 2-aminopyridines, further underwent an intramolecular cyclization to generate the N,O-bidentate intermediates, and finally reacted with sulfonyl azides giving the pyridin-2(1H)-imines with accompanying loss of diazo.

Topology Controlled All-(Meth)acrylic Thermoplastic Elastomers by Multi-Functional Lewis Pairs-Mediated Polymerization

It remains challenging to synthesize all-(meth)acrylic triblock thermoplastic elastomers (TPEs), due to the drastically different reactivities between the acrylates and methacrylates and inevitable occurrence of side reactions during polymerization of acrylates. By taking advantage of the easy structural modulation features of N-heterocyclic olefins (NHOs), we design and synthesize strong nucleophilic tetraphenylethylene-based NHOs varying in the number (i.e. mono-, dual- and tetra-) of initiating functional groups. Its combination with bulky organoaluminum [iBuAl(BHT)2] (BHT=bis(2,6-di-tBu-4-methylphenoxy)) constructs Lewis pair (LP) to realize the living polymerization of both acrylates and methacrylates, furnishing polyacrylates with ultrahigh molecular weight (Mn up to 2174 kg ⋅ mol-1) within 4 min. Moreover, these NHO-based LPs enable us to not only realize the control over the polymers' topology (i.e. linear and star), but also achieve triblock star copolymers in one-step manner. Mechanical studies reveal that the star triblock TPEs exhibit better mechanical properties (elongation at break up to 1863 % and tensile strength up to 19.1 MPa) in comparison with the linear analogs. Moreover, the presence of tetraphenylethylene group in the NHOs entitled the triblock TPEs with excellent AIE properties in both solution and solid state.

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.

Wholly sustainable graft copolymers derived from cellulose, lignin, and hemicellulose for high-performance elastomers, adhesives, and UV-blocking materials

Sustainable elastomers derived from renewable biobased resources with excellent mechanical properties and varied functions are highly pursued to substitute traditional petroleum-based polymers yet challenging due to their limited macroscopic performance. In this work, we designed a series of wholly biobased cellulose-graft-poly(vanillin acrylate-co-tetrahydrofurfuryl acrylate) (Cell-g-P(VA-co-THFA) copolymer elastomers with cellulose as the rigid backbone, sustainable VA derived from lignin and soft THFA derived from hemicellulose as the hard and soft segments in the rubbery side chains. Moreover, the grafted side chains can be cross-linked to introduce an additional dynamic network structure via Schiff-base chemistry between the aldehyde and amino groups. The mechanical properties of Cell-g-P(VA-co-THFA) copolymer elastomers, including tensile strength, extensibility, elasticity, and toughness can be facilely manipulated by the VA/THFA feed ratio, cellulose content, and cross-linking density. These Cell-g-P(VA-co-THFA) copolymer elastomers are thermally stable and possess outstanding adhesion behavior and prominent UV-shielding performance. Besides dramatically enhanced mechanical properties, the cross-linked Cell-g-P(VA-co-THFA) counterparts exhibit remarkable shape memory behavior. This work provides a robust and convenient strategy for developing strong and versatile sustainable elastomers with different application demands by integrating different biomass feedstocks via elaborate molecular design.

Organocatalyzed Photo-Controlled Synthesis of Ultrahigh-Molecular-Weight Fluorinated Alternating Copolymers

Ultrahigh-molecular-weight (UHMW) polymers with tailored structures are highly desirable for the outstanding properties. In this work, we developed a novel photoorganocatalyzed controlled radical alternating copolymerizations of fluoroalkyl maleimide and diverse vinyl comonomers, enabling efficient preparation of fluorinated copolymers of predetermined UHMWs and well-defined structures at high conversions. Versatility of this method was demonstrated by expanding to controlled terpolymerization, which allows facial access toward fluorinated terpolymers of UHMWs and functional pendants. The obtained copolymers exhibited attractive physical properties and furnished thermoplastic, anticorrosive and (super)hydrophobic attributes as coatings on different substrates. Molecular simulations provided insights into the coating morphology, which unveiled a fluorous protective layer on the top surface with polar groups attached to the bottom substrate, resulting in good adhesion and hydrophobicity, simultaneously. This synthetic method and customized copolymers shed light on the design of high-performance coatings by macromolecular engineering.

Strong and ultrafast stimulus-healable lignin-based composite elastomers with excellent adhesion properties

With the increased environmental issues, advanced high-performance and multifunctional polymeric materials derived from biomass have tremendous attention due to the great potential to replace their traditional petroleum-based counterparts. In this work, a series of lignin graft copolymers, lignin-graft-poly(n-butyl acrylate-co-acrylic acid) (Lig-g-P(BA-co-AA)), were rationally prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. These lignin-based copolymers demonstrate good thermal stability and tunable glass transition temperature (Tg) values. The mechanical performance, including tensile strength, extensibility, Young's modulus, and toughness can be facilely adjusted by the BA/AA feed ratio and lignin content during polymerization. Owing to the extraordinary photothermal conversion ability of lignin, the Lig-B550 copolymer, containing 11.8 wt% lignin content, shows excellent stimulus-healing behavior within 1 min with a 97.1 % healing efficiency under near-infrared (NIR) laser irradiation. Moreover, the Lig-g-P(BA-co-AA) copolymers exhibit remarkable adhesion property, broadening their potential applications in the adhesive area. This grafting strategy is versatile and efficient, conferring the resultant lignin-based composite elastomers with dramatically enhanced mechanical properties and unprecedented photothermal behavior, which can inspire the further development of strong lignin-based sustainable elastomers.

Fully biobased sustainable elastomers derived from chitin, lignin, and plant oil via grafting strategy and Schiff-base chemistry

With the dramatically increased environmental problems, the rational design of sustainable polymers from renewable feedstocks opens new avenues to reduce the huge pollution impact. The major challenge for sustainable polymers is the decreased mechanical performance compared to that of petroleum-based materials. In this work, fully biobased sustainable elastomers were developed by integrating renewable chitin, lignin, and plant oil into one macromolecule, in which chitin was chosen as the rigid backbone, while a lignin-derived monomer vanillin acrylate (VA) and a plant oil-based monomer lauryl acrylate (LA) were selected as the hard and soft segments for the grafted side chains. A series of Chitin-graft-poly(vanillin acrylate-co-lauryl acrylate) (Chitin-g-P(VA-co-LA)) copolymers with varied feed ratios and chitin contents were synthesized by using reversible addition-fragmentation chain transfer (RAFT) polymerization as an effective grafting strategy. In addition, a dynamic cross-linked network was incorporated via Schiff-base reaction to improve the macroscopic behavior of such kind of chitin graft elastomers. These sustainable elastomers are mechanically strong and show excellent reprocessablity, as well as outstanding UV-blocking property. This strategy is versatile and can inspire the further development of fully biobased sustainable materials from natural resources.

Recyclable cyclic bio-based acrylic polymer via pairwise monomer enchainment by a trifunctional Lewis pair

The existing catalyst/initiator systems and methodologies used for the synthesis of polymers can access only a few cyclic polymers composed entirely of a single monomer type, and the synthesis of such authentic cyclic polar vinyl polymers (acrylics) devoid of any foreign motifs remains a challenge. Here we report that a tethered B-P-B trifunctional, intramolecular frustrated Lewis pair catalyst enables the synthesis of an authentic cyclic acrylic polymer, cyclic poly(γ-methyl-α-methylene-γ-butyrolactone) (c-PMMBL), from the bio-based monomer MMBL. Detailed studies have revealed an initiation and propagation mechanism through pairwise monomer enchainment enabled by the cooperative and synergistic initiator/catalyst sites of the trifunctional catalyst. We propose that macrocyclic intermediates and transition states comprising two catalyst molecules are involved in the catalyst-regulated ring expansion and eventual cyclization, forming authentic c-PMMBL rings and concurrently regenerating the catalyst. The cyclic topology of the c-PMMBL polymers imparts an ~50 °C higher onset decomposition temperature and a much narrower degradation window compared with their linear counterparts of similar molecular weight and dispersity, while maintaining high chemical recyclability.