Effect Of Vinyl Ether-based Macromonomer On Synthesis And Properties Of Polycarboxylate Superplasticizer And Study On The Copolymerization Kinetics

Polycarboxylate superplasticizer?PCE?is a kind of comb-like graft copolymer with amphiphilic properties.It has good dispersion effect on the surface of cement particles owing to the adsorption and steric hindrance effect.Due to its excellent application performance and unique molecular structure characteristics,it has been widely used in concrete engineering as a novel type of concrete admixture with high performance.The synthesis of PCE is a typical reaction of free radical copolymerization,which generally reacted between unsaturated polyether macromonomer and unsaturated carboxylic acid monomers.The types of macromonomer with different structural characteristics will profound impact on the reaction conditions and the properties of the products.In this paper,the polyether macromonomers which has been widely used in the market were summarized.It is found that they are all vinyl alcohol-based macromonomers,and their starting agents are all vinyl alcohol compounds.On this basis,according to the structure improvement of starting agent,a novel kind of polyether macromonomer with vinyl ether compound as starting agents was proposed,which is called vinyl ether-based macromonomer.The structure of double bond substituents in these two kinds of macromonomers is obviously different,which has a great influence on their properties.According to the designed molecular structure of vinyl ether-based macromonomer,three kinds of vinyl ether compounds were synthesized by using special guide tube continuous reactor and self-prepared metal complex catalyst,which were used as starting materials for macromonomer production.Then,three kinds of vinyl ether-based macromonomer were synthesized by the ethoxylation of starting agents,and the factors such as catalyst,condition and reaction time were tested and optimized.It was determined that the best reaction condition for preparing vinyl ether-based macromonomer was metal sodium catalyst,the reaction temperature was controlled below 110?,and the chip temperature was not more than 40?.Then the prepared macromonomer and acrylic acid?AA?were copolymerized to prepare PCE copolymer.At the same time,two kinds of vinyl alcohol-based macromonomers that can be purchased on the market were used for comparative experiments with the same compositions.The difference of reaction temperature and dropping time between two kinds of macromonomer that reacted with AA were mainly compared.The experimental results show that vinyl ether-based macromonomer has higher reaction activity.When the conversion rate was similar,the copolymerization reaction of three vinyl ether-based macromonomers could be completed in lower reaction temperature?5??and shorter reaction time?0.5-1hours?compared with the reaction conditions of vinyl alcohol-based macromonomer?35?,3-4 hours?.The copolymerization reaction of vinyl ether-based macromonomer was more gentle and simple,which can greatly improve the production efficiency.The homogeneity,interfacial adsorption properties and application properties of the copolymers were compared,the results show that the properties of PCE copolymer synthesized by vinyl ether-based macromonomer could reach or even surpassed the product that synthesized by vinyl alcohol-based macromonomer,this indicate that the vinyl ether-based macromonomer has good application advantages.In order to better explain the high activity of vinyl ether-based macromonomers in the synthesis process,ethylene glycol monovinyl polyethylene glycol?EPEG?was taken as the representative of vinyl ether-based macromonomer.The composition of copolymer at low conversion was analyzed combined with GPC test results,linear?Fineman-Ross,YBR,kelen-Tüdos?and nonlinear least squares?NLLS?methods and Q-e equation were used to calculate the reactivity ratios of different monomers,the result is r_1?EPEG?=0.0080-0.014,r_2?AA?=4.5-4.6.The results show that the activity of vinyl ether-based macromonomer was higher than that of vinyl alcohol-based macromonomer,and it is easier to react with AA radical to form copolymer.Through the analysis of the copolymerization curve in the system of EPEG-AA,it is concluded that the copolymerization of EPEG-AA is non-ideal copolymerization,and the macromonomer can enter into the main chain of polyacrylic acid in the form of embedding to form comb-like graft copolymer.At the same time,because the activity of vinyl ether-based macromonomer is closer to that of AA,the block distribution of macromonomer in the copolymer is more uniform,the conversion rate of reaction is higher,the composition of copolymer is closer to the initial monomer ratio,so the molecular structure is more easier to be controlled reasonable,and the application performance of the copolymer is better.Though the synthesis,characterization and application research of vinyl ether-based macromonomer,it has been confirmed that this kind of macromonomer has higher polymerization activity.The analytical structure calculation was carried out to better identify the fundamental factors affecting the macromonomer activity from the perspective of molecular structure.The molecular model was established by Gaussian09 software.The molecular structures of different macromonomers were simulated at B3LYP/6-31G?d?level.The electronic distribution of C=C double bond orbital was observed.The results show that the deflection trend of electron cloud on the C=C double bond of vinyl ether-based macromonomer is larger,and the double bond is easier to be broken and then the addition reaction will occur.The fundamental reason for this change is that the double bond substituent of vinyl ether-based macromonomer is ether bond?-O-?formed by O atom.Its electronic effect is different from that of alkyl group of double bond substituent of vinyl alcohol-based macromonomer.It shows stronger electron pushing effect,which makes the electrons concentrate on 1-C atom and increases the potential difference between C atoms on double bond.The final experimental conditions were consistent with the calculation results of reactivity ratio and the analysis conclusion of electron cloud distribution in molecules.The relationship between the structural characteristics of vinyl ether-based macromonomer and high polymerization activity is demonstrated.The reason for the performance advantage of copolymers synthesized by such macromonomers is reasonably explained,and the organic unity of theory and experiment is realized.Based on the combination of theory and experiment,the high activity characteristics of vinyl ether-based macromonomer have been explained,which proved that it had significant advantages in large-scale production,and provided theoretical basis for more extensive application in the future.