The Oxidative Polymerization And Mechanism Of 2, 6-Dimethyl Phenol In Water

The solution polymerization of 2,6-dimethylphenol(DMP) is widely used in industrial production of poly(2,6-dimethyl-1,4-phenylene oxide)(PPO),an important engineering plastics.However,both a solvent recovery process and an explosionproof reactor are required.The use of water as the reaction medium is environmentally friendly and PPO can be separated easily due to its insolubility in water.Therefore,the oxidative polymerization of DMP to form PPO using water as the reaction medium would be of significance.The oxidative polymerization of DMP in water and mechanism were studied in this paper.PPO with high molecular weight was prepared in water and a novel one-pot synthetic method for preparing PPO/PS alloy in reactor containing aqueous medium was proposed based on green chemistry.The solubility of DMP in water was studied and it was found that DMP was dissolved in water under the polymerization condition.The change of molecular weight,and particle size and distribution during the oxidative polymerization of DMP in water with time was studied.At the beginning of the oxidative polymerization of DMP,the oligomers with the hydrophilic phenoxy anion at the end of oligomer chains were formed rapidly in water.When the critical molecular weight was reached,the oligomers precipitated from the water and original particles with a size of 20-50 nm were formed,in which the hydrophobic parts with benzene ring aggregated inside of the particles and the hydrophilic groups with phenoxy anion pointed to the aqueous phase.With the development of the oxidative polymerization,the molecular weight increased,while the concentration of the phenoxy anion and density of electric charge on the surface of the original particles decreased,and the repulsion force between particles weakened.Therefore,the stability of particles in water decreased,and the original particles coagulated and the stable primary particles with a size of 0.1μm formed at the same time.With the further development of the polymerization,the density of electric charge on the surface of the primary particles decreased gradually, resulting in the further coagulation of the primary particles and the formation of the final particles with a size of 10μm.The number of the primary particles gradually decreased with the polymerization time,while that of the particles with a size of 10μm increased and finally the particle size was mainly about 10μm at the end of the polymerization.A three-stage mechanism of the particle formation mechanism during the oxidative polymerization of DMP in water was proposed,that is,the particle nucleation,the first coagulation,and the second coagulation.During the oxidative polymerization of DMP the increase of particle size mainly depended on the particle coagulation,and the driving force of the particle coagulation was the decrease of the concentration of the hydrophilic phenoxy anion with the polymerization time.The initial oxidative polymerization kinetics of DMP in water with K₃Fe(CN)₆ was investigated by the electrochemical method.It was found that the initial polymerization rate of DMP increased with the increase of DMP and K₃Fe(CN)₆ concentration,and the reaction orders with respect to the concentration of DMP and K₃Fe(CN)₆ were 1 and 2,respectively.The initial polymerization rate of DMP increased with the increase of NaOH concentration due to the decrease of the oxidative potential of DMP,and the reaction order with respect to NaOH concentration was 2.The initial polymerization rate of DMP increased with the increase of temperature.The initial polymerization rate constant of DMP was calculated and the apparent activation energy of the oxidative polymerization of DMP was calculated to be 47.7 kJ·mol^-1 according to Arrhenius equation.The initial polymerization kinetics of DMP under the action of K₃Fe(CN)₆ was proposed according to the reaction order and Arrhenius equation.The effects of the species and content of swelling agent on the molecular weight of PPO synthesized in water were studied.It was found that the molecular weight of PPO increased significantly after introducing a certain amount of swelling agent during the oxidative polymerization of DMP.T_g of PPO/swelling agent mixture decreased with the increase of the content of swelling agent and the relation between T_g of PPO/swelling agent mixture and the content of swelling agent obeyed Fox equation.The relation between the molecular weight of PPO and T_g of PPO/swelling agent mixture was correlated and it was revealed that T_g played an important role in the molecular weight of PPO synthesized in water,which was probably because swelling agent could swell polymer particles under the action of agitation and lower its T_g,resulting in the increase of the mobility of oligomer chains and facilitation of the oxidative polymerization at the later period.The morphology of the polymer particle synthesized in water was observed by transmission electron microscope and it was found that after the introduction of toluene the final polymer particle showed compact cobblestone morphology.Toluene made primary particles fuse together and the profile of primary particles was so ambiguous that could not be seen.Considering the defect of solution polymerization of DMP,the low molecular weight of PPO synthesized in water and difficulty in processing of PPO,a novel one-pot synthetic method for preparing PPO/PS alloy in reactor was proposed based on green chemistry.During the oxidative polymerization of DMP,the reactive styrene was used as swelling agent to improve the molecular weight of PPO.After the oxidative polymerization of DMP,styrene was in-situ polymerized under the initiation of dibenzoyl peroxide(BPO) and dicumyl peroxide(DCP),finally thermodynamically compatible PPO/PS alloy was prepared.When styrene content was 50 wt%,for the synthesized PPO/PS alloy the yield and the weight-average molecular weight were determined to be 95%and 1.7×10⁵ for PPO,93%and 2.0×10⁵ for PS,respectively.The influence of the concentration of DMP and catalyst,molar ratio of N/Cu, oxygen partial pressure,NaOH concentration and temperature on the initial polymerization rate(R₀) of DMP was studied.It was found that the initial polymerization rate of DMP increased with the concentration of DMP and catalyst. The initial polymerization rate increased with the molar ratio of N/Cu at first and then decreased.The reaction order with respect to the oxygen partial pressure was 0.1, therefore the oxygen partial pressure had litle influence on the initial polymerization rate.The initial polymerization rate increased with NaOH concentration and reached its maximum value at a concentration of 0.50 mol/L.1/R₀ was in direct proportion to 1/[DMP]₀,which indicated that the initial polymerization kinetics of DMP obeyed Michaelis-Menten model.The dissociation rate constant of the intermediate complex (k₂) and Michaelis-Menten constant(K_m) at various temperatures were calculated,and both k₂ and K_m increased with temperature.The heterogeneous oxidative polymerization kinetics of DMP with copper-EDTA in water was studied.It was found that the oxygen-uptake rate increased with the increase of DMP concentration and temperature.The change of oxidative polymerization rate and molecular weight of PPO with time was calculated, and it was found that the molecular weight of PPO increased gradually at the earlier stage and rapidly at the later stage,which was in accordance with the traditional stepwise polymerization.The polymerization rate increased with time at the initial stage of polymerization and then decreased under low temperature.The polymerization rate calculated based on the oxygen-uptake disagreed with the result estimated by Michaelis-Menten kinetic model,indicating that Michaelis-Menten kinetic model applied to the polymerization of DMP in organic solvents was not appropriate to describe the full course of the polymerization of DMP in water.In order to reflect the catalyst deactivation during the oxidative polymerization and difference of the reactivity between monomer,the oligomer soluble in water,and the oligomer unsoluble in water,a new heterogeneous kinetic model was proposed according to Flory statistical theory.It was found that the polymerization rate estimated by the new model was consistent with the calculated data.