Mechano-Mediated Atom Transfer Radical Polymerization Piezoelectrically Catalyzed By ZnO Nanomaterials

Mechanochemistry refers to the method of applying mechanical energy to condensed matter such as solid and liquid by means of shearing,friction,impact and extrusion to induce changes in its structure and physical and chemical properties,thus triggering chemical reactions.Compared with the traditional thermochemical reaction,mechanochemistry used mechanical energy rather than heat energy to overcome the reaction barrier,the reaction can be completed without harsh conditions such as high temperature and pressure.Among them,ultrasonic mediated mechanical atom transfer radical polymerization（mechano-ATRP）was a new controlled radical polymerization technology developed gradually in recent years.Compared with controllable radical polymerization induced by light or heat,mechano-ATRP was considered as an important method to replace traditional polymerization because of its advantages of strong and uniform force,fast polymerization rate,low polymerization temperature,strong ability of regulating polymerization and easy to prepare polymer products in large quantities.Piezoelectric materials were often used as piezoelectric catalysts for mechano-ATRP because they can realize mechano-induced electron transfer process and efficiently convert mechanical energy into chemical energy for polymerization reaction.Compared with BTO,ZnO nanomaterials not only have piezoelectric properties,but also show semiconductor properties,which can effectively avoid the defects of poor electrical conductivity and low electron transfer efficiency of BTO.Therefore,ZnO can achieve high catalytic activity at a lower load,and effectively reduce the difficulty of separation and purification of polymerization products.Although there have been many successful cases based on ZnO piezoelectric catalyzed mechano-ATRP,there are still some shortcomings as follows:（1）The effect of ZnO nanomaterials with different morphologies on polymerization was not clear;（2）The work and mechanism in green solvent aqueous system need further study;（3）Efficient catalysis and separation recovery of nano-piezoelectric materials.In view of the above problems,we have carried out relevant studies,and the main results are as follows:（1）Three kinds of 1D ZnO nanorods（1D ZnO NRs）with different aspect ratio were prepared by chemical bath deposition method,named NR-1（aspect ratio 6）,NR-2（aspect ratio 10）and NR-3（aspect ratio 13）respectively.Mechano-ATRP piezoelectric catalytic performance of methyl methacrylate was catalyzed by NR-1,NR-2,NR-3 and 0D zinc oxide nanoparticles（0D ZnO NPs,named NP）was compared.The polymerization efficiency was as follows:NR-3>NR-2>NP>NR-1.The specific surface area and piezoelectric constant d₃₃ of the nanomaterials were characterized and their synergistic effect on polymerization was proved.In previous ATRP of acrylonitrile with low activity,the polymerization rate of acrylonitrile was low（>50 h,<60%）due to the outer-sphere electron transfer（OSET）reaction between carbon radical and copper.However,the OSET reaction was effectively inhibited by less copper salt loading in the system,and a high monomer conversion（67%）was achieved in a short time（6 h）.（2）ZnO NRs with aspect ratio of 11 were prepared and mechano-ATRP in aqueous phase was carried out.The effect of copper content and water volume was studied.It was found that the polymerization effect was better under the condition of 1000 ppm Cu Br₂ and 1:2 monomer/water volume ratio.The monomer conversion reached 61%after 1h,and the polymer with narrow molecular weight distribution（M_w/M_n～1.2）was obtained.The addition of isopropanol（IPA）as hydroxyl radical catcher proved that hydroxyl radical can improved the polymerization efficiency in mechano-ATRP.（3）Considering the separation of piezoelectric materials,ZnO nanoarrays（ZnO NAs）with close and uniform arrangement were grown on 15ⅹ15mm ITO conductive glass by hydrothermal method,and applied to mechano-ATRP with aqueous phase.The results showed that ZnO NAs could achieved a higher polymerization rate（67%）within2 h with a tiny loading（0.01 wt%）,which was higher than that of dispersed ZnO NRs（55%）.More importantly,ZnO NAs can be easily separated from the system because they grow on the substrate and can be taken out and recycled directly after the reaction.After three cycles,it still maintains 89%catalytic capacity and show good structural stability,which solved the separation and recycling problems of nanomaterials to a certain extent.