Study On The Polymerization And Material Properties Catalyzed By N-Heterocyclic Olefin-based Lewis Pairs

Polymer materials have been widely used in all aspects of production and life due to their unique properties,and the precise synthesis of polymers is crucial to improving the performance of polymer materials,which requires the development of a polymerization technique with both activity,controllability and selectivity.Lewis pair polymerization（LPP）has achieved some important results in the last decade,but there are still some problems and challenges,such as:（1）the relationship between the structure of polymers and high performance polymeric materials needs to be studied in depth,（2）it is difficult to introduce functionality in Lewis pair catalysts,which is not helpful to the expansion and improvement of the properties of polymer materials,（3）polymer topologies produced by Lewis pair polymerization are not yet rich enough.To address these problems,we developed three types of Lewis pair catalyst systems in this thesis,and the different material applications were studied acccording to the characteristics of different catalytic systems.In Chapter Ⅱ,we achieved the living/controlled polymerization of trifluoroethyl methacrylate（TFEMA）using Lewis pairs composed of N-heterocyclic olefins（NHOs）with an initiation center and organic aluminum compound Me Al（BHT）2,and the living polymerization of TFEMA was demonstrated by a series of experiments,including polymer number-average molecular weight（Mn）in agreement with the theoretical molecular weight,molecular weight distribution（D）maintained in a narrow range,linear increase in Mn with increasing monomer conversion,and successful chain extension experiments.Based on the living/controlled polymerization of TFEMA,we introduced Lewis pair polymerization into polymerization-induced self-assembly（LP-PISA）,and obtained nanofiber morphologies in a large experimental window using PTFEMA as solvophilic blocks and poly（perfluorooctylethyl methacrylate）（PHDFDMA）as solvophobic blocks.We have realized ultra-fast synthesizing PTFEMA-b-PHDFDMA diblock copolymer by sequential addition monomers,and by adjusting the length of solvophilic and solvophobic blocks and solid content,the diameter of the nanofiber morphologie can be adjusted in the range of 11.7-25.1 nm,and further increasing the length of the solvophilic block obtained a mixture of fusiform and fiber-like micelles as well as pure fusiform micelles,which helped us better understand the mechanism of fiber morphology formation.In Chapter Ⅲ,we further optimized the"one-pot,two-step"process used in Chapter II to achieve PISA by rapidly synthesizing micelles with different morphologies in a"one-pot,single-step"process.The combination of strong nucleophilic NHO as Lewis base with bulky organoaluminum Lewis acid can synergistically and efficiently copolymerize the mixture of acrylate DMAEA and fluorinated methacrylate TFEMA into self-assembled block copolymers in single-step process using compound sequence control（CSC）strategy.The structural characterization of copolymers by ¹H,¹³C NMR,gel permeation chromatography（GPC）,diffusion-ordered spectroscopy（DOSY）and differential scanning calorimetry（DSC）analyses indicated the formation of well-defined di-BCPs.Kinetics studies also manifested the capability of such LP-PISA strategy in synthesizing diblock copolymers in one-step manner.By adjusting the degree of polymerization of solvophobic block,a series of di-BCPs with different morphologies（i.e.spheres,worms and vesicles）can be produced by this LP-PISA strategy,which can be further confirmed by TEM and DLS measurements.Moreover,by using appropriate Lewis pair and PHDFDMA with liquid-crystalline properties as solvophobic blocks,it only took2 min for this LP-PISA strategy to synthesize diblock copolymers nanofibers in single-step manner.These results demonstrate that LP-PISA is capable of synthesizing polymeric nanoparticles with different morphologies from monomer mixtures in a rapid and single-step process,which might provide a way to realize the large-scale commercial applications of PISA in the future.In Chapter Ⅳ,we have designed and synthesized a novel dual-initiating NHO as Lewis base,which is more efficient than the NHO with one initiation center for the synthesis of block copolymers and combined with two commercial,cheap,and simple electrolyte salts,Li TFSI and Li CF3SO3 as Lewis acids,we realized rapid and selective ring-opening polymerization of common lactones（ε-CL,δ-VL,and rac-LA）,producing polymers with predicted molecular weight,narrow molecular weight distribution,high initiation efficiency.The livingness of these new Lewis pair catalytic systems had been verified by successful chain extension experiments and the synthesis of well-defined triblock copolymers in one-pot two-step procedure due to the dual-initiating feature of Lewis base.Mechanistic studies,including stoichiometric NMR reaction,MALDI-TOF MS,detailed polymerization and kinetic studies,enabled us to propose a possible polymerization mechanism that the NHO nucleophilic attacked the Li TFSI-activated monomer,followed by its ring-opening to form oxygen anion without H-transfer to attack the incoming monomer activated by Li TFSI.Thanks to the conductive feature of Lewis acids,we conducted ionic conductive elastomers（ICEs）in-situ without the procedure of purification and subsequent addition of electrolyte salts and these materials exhibited excellent mechanical properties and ionic conductivities at room temperature,suggesting their potential applications as flexible conductive substrates and touch sensors.In Chapter Ⅴ,we designed and synthesized three novel tetraphenylethylene-based NHOs as Lewis bases.Among them,the single-initiating center and double-initiating center NHOs can only yield linear polymers,while the four initiating center NHO can synthesize star polymers with predicted molecular weights,relative low dispersity distribution,and high to near quantitative initiation efficiencies.In particular,when using NHO with four initiating centers as Lewis base,ultra-high molecular weight（UHMW）star PMEA（Mw up to 4006 kg/mol）and PMMA（Mw=2455 kg/mol）could be obtained within 4 and 200 min,respectively,which we believe will be a highly efficient and versatile catalytic system for the synthesis of UHMW polymers.Benefiting from the introduction of tetraphenylethylene into the NHO,produced polymers exhibited typical aggregation-induced emission（AIE）properties.Taking advantage of the compound sequence control（CSC）strategy of Lewis pair polymerization,well-defined triblock copolymers and star copolymers could be obtained in single-step manner,respectively and we have systematically investigated the mechanical properties of these copolymers as thermoplastic elastomers（TPEs）.Star TPEs exhibited superior mechanical properties to the triblock TPEs,with elongation at break between 517-1684%,tensile strength between1.7-12.5 MPa,and elastic recovery as high as 94%.Due to the low cost of the monomers and the facile polymer synthesis process,it may be used as a shaped material for the anti-counterfeit application on a large scale in the future.