Study On Soap-free Emulsion Polymerization Of Styrenes Mediated By Cyclodextrin

The polymerizations are generally carried out in organic solvents or surfactants if the organic monomers have the poor water-solubility. In order to purify the final product, these solvents must be removed ultimately, which results in the tedious post-processing, high costs and the waste of resources. Particularly organic solvents produce pollution to the environment. Therefore, it is a valuable research topic in both theory and practical applications that how to improve the polymerization process, reduce costs, make poor water-soluble monomers be polymerized directly in the aqueous phase. Cyclodextrins (CD) exhibit a torus-shaped structure with a hydrophobic cavity and a hydrophilic outer side. Because of this structure, they are able to enclose smaller hydrophobic molecules to form host-guest compounds. With the solutions of CD, inclused insoluble molecules become disperse in solution by the treatment, in which, the guest molecules are not modificated chemically, due to there is no formation of covalent bonds between them and the CD. The polymerization mediated by CD is a good way to solve the problems mentioned above. However the research history of this new polymerization is short still. The attention is focused on the description of experimental phenomena and the exploration of reaction conditions and it is lack of a detailed study on polymerization mechanism of the mediating reaction of CD at present stage. Some experimental phenomenon can not be understood suitablely. In this article, several basic aspects about CDs, such as the unique structure and properties and their applications in several important fields, were introduced. As an emphasis, applications of CD in polymer synthesis were summarized. Take the soap-free emulsion polymerization of styrene mediated by randomly methylatedβ-cyclodextrin (RAMEB) as an example, the inclusion ratio, inclusion rate and the thermal stability of RAMEB/styrene inclusion complex were studied both experimentally and theoretically, as well as the effect of RAMEB on soap-free emulsion polymerization of styrene. Main contents and results are as follows:1. The inclusion complex of styrene and RAMEB was synthesized by solvent method and characterized by UV, TG-DSC, and ¹HNMR spectrum. The thermal stability of the inclusion complex is almost the same as pure RAMEB, and it is much better than the pure styrene. Obsvervation by molar ratio method showed that the 1:1 inclusion complex was formed between RAMEB and styrene. The inclusion process finished within 4 hours at room temperature. The inclusion ability of RAMEB with styrene is stronger than that ofβ-CD. The stability constant of inclusion reaction decreases with the temperature increases, and the inclusion reaction is exothermic. In addition, the standard molar Gibbs energies, enthalpies, and entropies were derived for the inclusion process according to the K^θat different temperatures. Entropy change is negative but the absolute valueâ–³S^θis smaller, which means that hydrophobic force is not the main driver of inclusion reaction. Styrene in the cavity of RAMEB maintains a certain freedom degree of movement and rotation. Enthalpy change is negative and the absolute valueâ–³H^θis larger. The results indicate that the association of the guest styrene molecule with RAMEB is intriduced mainly by enthalpy changes and that the Van der Waals interaction and the dipole-dipole interaction were the main binding forces in the complexation.2. Using styrene as monomer, K₂S₂O₈ as initiator, the soap-free emulsion polymerization with and without the presence of RAMEB were investigated. Result shows that the addition of RAMEB can greatly accelerate the polymerization. With increase of the amount of RAMEB, the conversion of monomers also increases. As the ratio of RAMEB/monomer reaches 0.15, the monomer conversion may reach about 91% ultimately, which is much higher than that (63%) in absence of RAMEB. However the monomer conversion decreases with the further addition of RAMEB. The presence of RAMEB also affects the size and distribution of the polystyrene products. With increase the amount of RAMEB, the average size of the polystyrene particles becomes smaller and the polydisperty of size distribution decreases initially and then increases after a minimum, when the ratio of RAMEB/monomer reaches 0.15. The addition of RAMEB improves the stability of polymer emulsion greatly. Kinetic analysis showes that the addition of RAMEB has a very significant effect on the apparent reactivity of the monomer. As the ratio of RAMEB/monomer reaches 0.05, the monomer reactivity increase by 60%.When the RAMEB/ monomer ratio is increased to 0.15, the monomer reactivity is 3 times of that in absence of RAMEB. However, as the further increase of RAMEB/monomer ratio, the monomer reactivity decreases. It may be understood that, with the overfull increase of RAMEB, it has steric hindrance to the monomers and reduces the activity of monomers.