Regulating Step-growth Polymerization And Polymer Chain Structure Via Flow Mixing

Step-growth polymerization is carried out step by step through the reaction between functional groups on the molecular chain,which is the most important method to prepare various special polymers.In the process of step-growth polymerization,the viscosity of the reaction system keeps increasing,which is not only prone to uneven mixing,resulting in the non-stoichiometry of reaction groups in local areas and side reactions of end-capping,but also will hinder the molecular diffusion,making it difficult for the polymerization reaction to proceed further to obtain high-molecularweight polymerization products.Intensifying flow mixing can increase the movement of molecular chains and improve the uniformity of reaction group distribution,thereby promoting the step-growth polymerization reaction.However,the sensitivities of diffusion,spatial distribution and orientation of rigid and flexible molecules with different chain lengths to the flow mixing are different,which greatly affects the polymerization process and the final product quality.Focusing on the mechanism of flow mixing on the kinetics of step-growth polymerization and the molecular chain structure of products,this paper first takes the synthesis of linear copolyurea with 4,4’-diphenylmethane diisocyanate(MDI),shortchain diamine of m-phenylenediamine(MPD)and long-chain diamine of polyoxypropylene diamine(D2000)as monomers as object,and the effects of macromixing and micromixing on the intensification of reaction process of MDI/MPD/D2000 system and the molecular chain structure of copolyurea are systematically studied,and the affecting rules of flow mixing on the polymerization of monomers with different molecular chain lengths are revealed.Then,taking the polycondensation reaction of preparing poly(p-phenylene-cis-benzobisoxazole)(PBO)as the research object,the effect of flow mixing on the polymerization of rigid chain molecules is investigated,and the intensified mechanism of mixing efficiency and flow pattern on the polycondensation reaction of rigid chain molecules is explored.The detailed results are as follows:1.For the ternary copolymerization system of MDI/MPD/D2000,because the intrinsic reaction rate of D2000/MDI is much faster than that of MPD/MDI,MDI reacts with D2000 first to produce-(D2000-MDI)n-sequence,and then reacts with MPD to produce-(MPD-MDI)n-sequence.Changing the ratio of raw diamines can adjust the composition,sequence structure and mechanical properties of prepared copolyurea.With the increase of short-chain monomer of MPD,the content of MPD-MDI-MPD sequence of copolyurea increases,and the microphase size and the degree of microphase separation both decrease,resulting in a significant increase in tensile modulus and tensile strength.With the increase of long-chain monomer of D2000,the content of D2000-MDI-D2000 sequence increases,which greatly lengthens the flexible molecular chain and increases the degree of microphase separation,therefore the prepared copolyurea shows better flexibility.2.In a paddle impeller reactor(PIR),the effect of stirring speed on the polymerization of MDI/MPD/D2000 system is studied.The results show that the reaction between long-chain monomer of D2000 and MDI is easier to change from reaction control to diffusion control than that between short-chain monomer of MPD and MDI.Enhanced macromixing can significantly reduce the diffusion-hindered effect of the D2000/MDI reaction without affecting the MPD/MDI reaction,thereby promoting the D2000/MDI reaction and changing the sequence structure and properties of the copolyurea prepared from MDI/MPD/D2000 system correspondingly.With the stirring speed increasing from 100 rpm to 800 rpm,the content of MPD-MDI-MPD sequence decreases from 57 mol%to 15 mol%,while the contents of MPD-MDI-D2000 and D2000-MDI-D2000 sequence increase from 39 mol%and 4 mol%to 64 mol%and 21 mol%,respectively.As a result,the Shore A hardness,tensile strength and elongation of copolyurea change from 81,56 MPa and 65%to 68,40 MPa and 188%,respectively.3.Compared with PIR,microchannel mixer(MC)and dynamic mixer(DM)can shorten the micromixing time by 1～2 orders of magnitude.For MDI/MPD/D2000(molar ratio of 2/1/1)copolymerization reaction system,due to the efficient micromixing,the copolyurea with molecular weight higher than 4.0×104 g·mol-1 is synthesized in MC in less than 0.1 s.With a longer residence time,the highest molecular weight of synthetic copolyurea in DM is 6.8×104 g·mol-1,which is 40%higher than in PIR,and the acceleration of stirring speed promotes the reaction of low-molecularweight oligomer of-(MDI-D2000)n-,leading to a relative decrease in the content of MPD-MDI-MPD sequence.4.For the reaction of preparing rigid chain molecule of PBO,in the initial stage,with the improvement of mixing efficiency,the translational and rotational diffusion coefficients of rigid molecules greatly increase,accelerating the apparent reaction rate significantly.But as the oligomeric chains grow longer,both diffusion coefficients decrease sharply and the effect of the mixing efficiency on the enhancement of diffusion becomes less significant.Especially,at the late stage,the rotational diffusion coefficient decreases to zero which restraines the polycondensation to proceed further.In the rotational flow,the rigid polymer molecules are more likely to be in parallel orientation than in the elongational flow.Therefore,increasing the rotational flow can more effectively intensify the rigid-chain molecular polymerization reaction and increase the molecular weight of PBO products.