Investigation On Self-Stabilized Precipitation Homopolymerization/Copolymerizati Of Maleimide And Its Application

Maleimide（MI）is a typical 1,2 disubstituted cyclic monomer with symmetric structure,which has been widely used in biomedicine,thermosetting resin,photosensitive materials and other fields.The hydrogen atom attached to nitrogen in the imide ring can be replaced by various functional groups,leading to the formation of N-substituted maleimide with different structure and properties.As a dienophile,MI can react with conjugated diolefins by reversible Diels-Alder reaction and react with nucleophiles through Michael addition,generating stable carbon-sulfur bonds,carbon-nitrogen bonds,etc.by click reaction.The unsubstituted MI is weakly acidic in solution,and the-NH group can react with a variety of functional groups such as cyanate group and epoxy group.In spite of appreciable steric hindrance,MI is prone to homopolymerize and copolymerize with other monomers due to release of strain in the five-membered ring via opening of the double bond.In addition,the rigid five-membered ring also makes the as-prepared polymer possess high mechanical properties and thermal stability.Although polymerization of N-substituted maleimide have been extensively studied,the preparation and application of unsubstituted MI-based homopolymer and copolymers have been rarely researched.In this paper,the self-stabilized precipitation polymerization（2SP）system was extended to the polymerization of MI.Firstly,homopolymer microspheres of MI were prepared by 2SP polymerization,and the nucleation and growth mechanism of the microspheres were investigated in detail.Secondly,the reaction kinetics of MI copolymer with diisocyanate was investigated and the surface of MI copolymer microspheres was modified by diisocyanate,which was applied in catalyst immobilization.Finally,MI-based microspheres with different morphologies,sizes and chemical compositions were simply and efficiently prepared by 2SP process.The imide groups on the surface of the microspheres provided a platform for further chemical modification,and their applications in the field of thermosetting resins were investigated.The main contents are as follows:1.Monodispersed MI homopolymer microspheres（PMI）with high density of imide groups were prepared by 2SP polymerization in ester solvent,which possess excellent thermal stability and high T_g due to the rigid five-membered ring of imide groups and the intermolecular hydrogen bonding.By simply adjusting the monomer and initiator concentration,crosslinker dosage and temperature,monodisperse spherical PMI microspheres were successfully obtained with tunable diameters of 0.24-1.08μm.To elucidate the nucleation,stabilization and growth mechanism,the size,surface morphology and molecular weight of the as-prepared PMI microspheres were monitored over the polymerization process.Furthermore,the intermolecular hydrogen-bonding interactions between imide and carbonyl groups might also exert an important influence on the stabilization and growth of the PMI particles,which was verified by 2SP polymerization of MI in the presence of urea and succinimide.During the radical polymerization of MI,resonance-stabilized succinimidyl radical was formed by intramolecular tautomerization.The obtained nitrogen and oxygen radicals are generally less reactive in propagating than a carbon radical and their formation was a monomolecular chain termination reaction,which deactivated and terminated to yield PMI with relatively low molecular weight.Meanwhile,monomer and initiator concentration and temperature had less effect on the molecular weight of PMI.2.SMI microspheres were prepared via 2SP copolymerization of St and MI.The reaction kinetics of the isocyanate group of hexamethylene-diisocyanate（HDI）/toluene diisocyanate（TDI）with SMI was investigated using dibutyltin dilaurate（DBTL）as catalyst.Due to the high reactivity,the increase of catalyst concentration has little effect on the reaction rate constant of TDI.However,the reaction rate constant of HDI and SMI is greatly increased with the increase of catalyst concentration.Then,St/MAH/MI was copolymerized to prepare PSMM microspheres with surface imide and anhydride groups in MEK/n-heptane mixed solvent.The effects of reaction medium,crosslinker concentration,monomer feed ratio and monomer concentration on microsphere morphology,chemical composition and yield were investigated,and the particle size of PSMM microsphere was regulated from 0.52μm to 0.96μm.PSMM-HDI microspheres with highly reactive surface isocyanate groups were directly formed through reaction with HDI.After covalent immobilization of Ti O₂ nanoparticles and hydrolysis of surface anhydride groups,PSMM-HDI@Ti O₂ particles with remarkably improved dispersity and adsorptive property were finally fabricated.The synergistic effect of adsorption-photocatalytic degradation greatly improved the photocatalytic degradation efficiency of PSMM-HDI@Ti O₂.Furthermore,methylene blue（MB）was selected as a model dye to verify the photocatalytic degradation ability of PSMM-HDI@Ti O₂.The effects of mass ratio of PSMM microspheres to Ti O₂,dye concentration and catalyst dosage on degradation efficiency were investigated in detail.The results showed that when the mass ratio of PSMM-HDI to Ti O₂ was 10/4,the degradation efficiency of MB could reach 100%within 90 min.3.Hollow polymer microspheres（HPGM）were prepared by precipitation copolymerization of GMA and MI with PMS as template particles.The effects of solvent composition,monomer feed ratio,core-shell mass ratio and template particle size on the morphology,size,shell thickness,chemical composition and hollow structure of HPGM were investigated systematically.Then,HPGM was used as fillers to modify EP resin.The epoxy groups on the surface of HPGM are able to avoid agglomeration and increase the dispersity of the particles in EP resin.Besides,the plenty of imide groups on the particle surface could further react with the epoxy groups of EP,resulting in a lower curing temperature and better compatibility.The embedded HPGM could introduce imcrovoids to the resin matrix,which could remarkably toughen and reduce dielectric constant of the resultant composites.The maximum tensile and impact strength of HPGM/EP resin could reach 91.8 MPa and 33.3 k J/m² when the content of HPGM was 2 wt%.The composite resin possessed lowest dielectric constant and dielectric loss（3.63 and 0.031 at 10⁷ Hz）with 4 wt%content of HPGM,which have great potential in the field of electronic communications and aerospace.4.Porous polymer microspheres（PMI-POSS）with imide groups were prepared successfully by 2SP copolymerization of MI and octa（vinyl）-silsesquioxanes（OV-POSS）with rigid cage skeleton.The influence of monomer feed ratio and DVB contents on the particle size,chemical composition and specific surface were investigated systematically.The specific surface of PMI-POSS can reach up to 186.5 m²/g with the increase of OV-POSS and DVB.GMA was copolymerized with MI and OV-POSS by 2SP to prepare PGM-POSS microspheres,which were used to modify EP resin.Compared with PMI-POSS,the epoxy groups on the surface of PGM-POSS microspheres effectively improved the compatibility with EP resin.The low polarity of OV-POSS and the porous structure of PGM-POSS significantly reduce the dielectric constant and dielectric loss of EP resin.When the content of PGM-POSS was10%,the dielectric constant and dielectric loss of PGM-POSS/EP resin reduced to 2.62 and 0.021 at 10⁷ Hz,respectively.The impact strength and tensile strength of PGM-POSS/EP resin was also improved effectively.