Preparation Of High Molecular Weight Polypropylene Glycol In Microchannel Reactor

Polyether polyol is an important oligomer synthesized by the ring-opening polymerization(ROP)of epoxides and is mainly used in the production of polyurethane foams as well as in surfactants and lubricants,widely applied in aerospace,construction,textile and automobile etc.However,there is not only a large risk of reaction heat,but also a long reaction time in its traditional production process by using the semi-continuous reactor.The microchannel reactor(MCR)has the characteristics of high heat transfer efficiency and process safety,but the viscous material and high reaction heat release make it difficult to produce the high molecular weight polyether polyols.In this work,using double metal complex(DMC)as catalyst,the bulk polymerization and the solution polymerization of propylene oxide(PO)were carried out in MCR by single feed and segmented feed respectively.A series of high molecular weight polypropylene glycol(PPG)products were prepared,and degradation reactions of the products under different conditions were explored,which provides a theoretical reference for PPG’s storage and transportation quality stability.The main conclusions can be summarized into four points:1.For bulk polymerization,single feed is only suitable for preparing PPG with molecular weight less than 3000 g/mol,and the molecular weight distribution(MWD)is relatively narrow,while segmented feed can increase the molecular weight to about 5000 g/mol,but more high molecular weight trailing will appear in their gel permeation chromatography(GPC)spectrogram,making the MWD wider.The increase of viscosity is the main reason to restrict the preparation of high molecular weight PPG by MCR.2.PPG products with Mn=2～8 kg/mol were prepared by solution polymerization of PO in MCR with n-hexane to reduce the viscosity of materials.It was found that the molecular weight of the product was basically equal to the theoretical molecular weight when the residence time was long enough.When the monomer feed flow rate increases,the MWD first widens and then narrows.When the tube length is longer,the molecular weight is higher and the MWD is wider.The induction period of polymerization was shortened with the increase of temperature.It is easier to prepare high molecular weight PPG with segmented feed than with single feed,but the MWD became wider.These tendencies can be explained from the viscosity change of polymerization system and its effect on the ratio of chain transfer to chain propagation rate.3.The solution polymerization of PO in MCR was carried out also in toluene,a solvent with higher boiling point.It was found that the PPG conversion and molecular weight prepared by using toluene as solvent were lower,and the MWD was wider.The feed velocity had little effect on the molecular weight and MWD of the polymer.The influence of temperature on molecular weight of PPG with toluene as solvent was small,but MWD was wider.The smaller diameter of the solvent mixing coil will enhance the secondary flow in the tube,which can increase the molecular weight of the product,but will form distinct double peaks in GPC spectrogram.4.It was found that the molecular weight of the produced PPG will decrease,and MWD will become narrower when the product placed in room temperature and air environment for a period of time.In view of this phenomenon,the stability of PPG products was explored through the degradation experiment of artificially strengthening storage environment.High molecular weight PPG was found to degrade more easily.The higher the temperature is,the faster the degradation rate of polyether is.The degradation reaction is the first order reaction at different temperatures in the air environment,and the apparent activation energy is 18.9 kJ/mol.The degradation rate of the polyether under nitrogen atmosphere is slower,and it is still a first-order reaction which reaction rate constant is 1/3 of that in air environment at the same temperature,while the degradation in oxygen atmosphere is close to third-order reaction.It can be concluded that oxidation reaction is the main cause of PPG degradation,and there is also the possibility of thermal degradation.The degradation rate of polyether is lower when the concentration of residual catalyst is higher.Based on fully understanding the mechanism of ROP of PO catalyzed by DMC and microscopic mixing characteristics of polymer fluid in the MCR,the preparation of high molecular weight PPG was realized,which provides an important theoretical basis for the safe,rapid and continuous synthesis of PPG in MCR and its subsequent storage and use.