Effect Of Monomer Sequence Distribution On The CO₂-Philicity Of A Well-defined Ternary Copolymer: Poly?Vinyl Acetate-co-vinyl Butyrate-co-vinyl Butyl Ether?

Because of their numerous applications in the field of CO₂ systems,such as interface engineering as a stabilizer to improve the stability of the interface,petroleum engineering as a thickening agent to improve oil recovery,as well as gas separation membrane separation technology to improve the separation efficiency of gas,great efforts have been devoted to developing the environmental CO₂-philic polymers.In order to show better performance in these fields,many efforts have been made to design environmentally friendly high performance CO₂-philic polymers in recent years.However,these CO₂-philic polymers generally have poor solubility in CO₂,which greatly restricts the prospect of the application of supercritical carbon dioxide.Therefore,it is necessary to design high soluble CO₂ polymer and to build the relationship between the structure of CO₂-philic polymer and its influence to solubility.It is the requisite way to develop high solubility CO₂-philic polymer in the future.The work aims to interpret the phase behavior of multi-polymers in super critical CO₂ via a spatially dependent structural parameter,the monomer sequence distribution,rather than traditional statistical factors.Well-defined statistical copolymers based on three CO₂-philic monomers,vinyl acetate,vinyl butyrate and vinyl butyl ether,were synthesized with reversible addition-fragmentation chain transfer free-radical polymerization by combining their kinetic behaviors and pairwise reactivity ratios into the Alfrey–Goldfinger–Skeist model.The corresponding cloud point pressures become nonlinear with the polymer composition and possess upper critical solution pressure behavior.Additionally,their diad and triad monomer sequence distributions were calculated by simple theoretical arithmetic.While analyzing the cloud point pressure measurements,we found that both the differences in composition and symmetrical structure of the monomer triads could impact the CO₂-philicity of triads,which were close to the properties of the products,and thus,the variations in the triad sequence distribution could reasonably predict the phase behavior of the products.The details are as follows:Firstly,we perform the RAFT homopolymer and copolymer experiments based on three CO₂-philic monomer:VAc,VBu and VBE,and the theoretical research through the traditional dynamic analysis method.For the polymerization system containing VAc and VBu monomer with Rhodixan A1,the reaction is first order kinetics,molecular weight with increasing conversion rate and molecular weight distribution is narrow,these results indicated that the polymerization characteristics is controlled;but for the homo-polymerization of VBE in the same polymerization conditions has no chain growth.However,when the VBE as a comonomer into the VAc and VBu system,the system can be polymerized.Kinetic studies illustrate that the semilogarithmic kinetic plots of the reactions containing VBE are nonlinear because of the higher consumption rates of VBu and VAc than of VBE;therefore,when VBE is predominant in the overall monomer feedstock,the rate of polymerization would slow dramatically.Further,we used three classical methods,the Fineman–Ross,Kelen–Tu?dos,and nonlinear least-square methods,to determine the reactivity ratios of the pairwise combinations of the three CO₂-philic monomers.By combining the kinetic behavior studies and the measured reactivity ratios into the ternary monomeric Alfrey–Goldfinger–Skeist model,the compositional drift along the polymer backbone containing less-activated monomers was successfully predicted,and the feed monomer ratio for desired monomer compositions in products could also be determined,and the actual measured data and the theoretical model of the match.Then,we synthesized two kinds of well-defined CO₂-philic copolymers,the first kind of CO₂-philic polymers with the proportion of VAc:VBu of fixed 1:1 and VBE variable;second types of CO₂-philic polymers with fixed VBE content 20%,VAc and VBu as variables.In addition,the cloud point pressures of a series of well-defined products with precise compositions and narrow dispersions were measured.And in the polymer with a molecular weight of 6800-7200 g/mol,the polymer with a monomer component ratio?VAc:VBu:VBE?of 41.8:40.4:17.8 has the largest cloud point pressure.Additionally,the corresponding diad and triad monomer sequence distributions were calculated with average monomer feed and reactivity ratios by simple theoretical mathematics.Moreover,when comparing the probability variation trends to the phase behavior,we could deduce that symmetrical‘‘three-dimensional''spatial structural triads containing VBE such as‘321'‘312'and‘322'would have a greater CO₂-philicity,benefiting from a combination of two factors:weaker polymer–polymer interaction and stronger polymer–CO₂ interaction.It could be concluded that the monomer sequence distribution plays a vital role in the phase behavior of multi-polymers in sc-CO₂.