| α-olefins and maleic anhydride can undergo self-stable precipitation polymerization(2SP)reactions in a specific solvent environment.The reaction can easily generate monodisperse polymer microspheres,which have important applications in the fields of medicine and industry.Although a large number of experimental studies have been carried out on this polymerization reaction,the regulation mechanism of molecular weight and the selection of specific solvents have always been difficult problems that hinder the further application of this reaction.To solve these two problems,it is necessary to investigate the chain growth process in the early stage of the reaction.The size of the characterization needs to be reduced to the microscopic level(below 100 nm),and the chain growth speed during the reaction process is extremely fast,which are difficult to conduct an experiment to observe the process.On the other hand,molecular dynamics(MD)simulation,as a new technical means,has achieved many research results in small-scale conformational analysis and ultrafast evolution processes.Therefore,this subject designed an MD scheme to simulate the evolution of polymer conformation in the early stage of 2SP polymerization and verified the applicability of the algorithm by experiments.Using this algorithm,the relation between the evolution of polymer conformation and molecular weight and the relation between the evolution of polymer conformation and solvent were investigated.The main contents and results of this research are as follows:(i)First,to verify the feasibility of the scheme,1-pentene and cyclopentene were chosen as model compounds of C5 olefins,and 1-pentene-maleic anhydride and cyclopentene-maleic anhydride polymerization and simulation were employed under the same temperature,concentration,and feeding ratio.For 1-pentene-maleic anhydride,the number-average molecular weight(Mn),weight-average molecular weight(Mw),and polydispersity coefficient(PDI)simulated by this scheme are 6001,7531,and 1.25,respectively,and the simulated glass transition temperature(Tg)was 349 K.The Mn,Mw,and PDI obtained by the experiments were 5550,7660,and 1.38,respectively,and the experimental Tg was 344 K.The simulated values were consistent with the experimental values.For cyclopentene-maleic anhydride,the Mn,Mw,and DPI simulated by this scheme were 4581,5028,and 1.1,respectively,and the simulated Tg is 328 K.The Mn,Mw,and PDI obtained by the experiments are4517,5150,and 1.14,respectively,the experimental Tg is 327 K.The simulated values are very close to the experimental values.It is found that the simulated and experimental values are in good agreement with molecular weight and Tg,which are the two critical factors characterizing polymer structure.These show that this scheme is suitable for C5 olefin–maleic anhydride 2SP polymerization and the scheme can be used to predict the conformational evolution of C5olefin-maleic anhydride polymers.(ii)The evolution of the conformation during the growth of the molecular chain in the early stage of 2SP polymerization has a great influence on the molecular weight of the reaction product.The mechanism of the inter-chain entanglement was revealed by monitoring the snapshots of the molecular conformation with different degrees of polymerization and analyzing the data of molecular end distance and gyration radius.It was found that "intramolecular cyclization entanglement" was generated in the conformation evolution of C5 monoene polymerization systems.When a spherical chain conformation was formed by the "intramolecular cyclization entanglement",the chain growth stopped.Therefore,the molecule weight of C5 monoene-maleic anhydride is small.For C5 diene-maleic anhydride,it was found that spherical chains are rare,the chains are mostly dendritic and rod-shaped,and both dendritic chains and rod-shaped chains can continue to grow to higher molecular weight polymers.To investigate the intrinsic structural factors for the formation of spherical chains,rod-like chains,and dendritic chains,the distributions of the bond length and twist angle of the main chain of the molecular chain at different degrees of polymerization were compared.It was found that the bond length had little effect on the chain morphology,and the uniformity of the torsion angle was the molecular structure factor for the formation of spherical and rod-like chains.This provides a guideline for finding new molecules with self-stabilizing polymerization characteristics.(iii)The matching of the reaction solvent and the polymer is critical for whether the self-stable precipitation polymerization can occur.The study of the relation between the solvent and the polymer is essential for explaining the mechanism of 2SP polymerization.Using the designed scheme,the effects of solvent polarity,Flory-Huggins interaction parameter χ and free volume on the chain conformation were investigated.It was found that the effect of solvent polarity and free volume ratio on the entanglement behavior of C5 monoenemaleic anhydride has no obvious principle.After studying the Flory-Huggins interaction parameter χ of C5 monoolefin-maleic anhydride copolymer and solvents,we found that spherical chains were formed when χ was at the range of 0.08 to 0.28.2SP polymerization can occur in the solvents whose value is within the range.Spherical chains were not formed when χ≥0.37 or χ≤0.07,and the experiments verified that 2SP polymerization can not occur within the range.This rule can reduce the number of experiments for solvent selection through simulation and save raw materials.In summary,it was validated that the new MD simulation scheme is suitable for simulating the molecular structure of polymers produced by 2SP polymerization.The microscopic mechanism that affects the molecular weight of C5 olefin-maleic anhydride polymers was demonstrated by this scheme.The general rule of selection of solvent is also obtained by this scheme.Therefore,the research results in this paper contribute to the development and utilization of C5 olefin-maleic anhydride polymers and provide a guideline for designing new 2SP polymerization systems. |
