Modified Microemulsion Polymerization Of Methacrylates And Rich-syndiotacticity Of The Resulting Polymers

The objects of this research is modified microemulsion polymerization of methacrylates. On the basis of our researches before, various poly(methacrylate)s were prepared by modified microemulsion polymerization procedure. The reaction process and the characteristics of the resulting polymers were studied; the effects of the reaction conditions on polymers properties, i.e. rich-syndiotacticity et.al, were considered and the possible mechanism of rich-syndiotacticity was also discussed.The obvious drawbacks of traditional microemulsion polymerization, such as high surfactant amounts and low polymer contents, have limited it's further applications. Nano-sized (15-40nm) poly (methacrylate)s latexes with low surfactant content (1-6 wt%) and high polymer content (15-30 wt%) were easily prepared by a modified microemulsion polymerization procedure.A series of modified microemulsion polymerization of different methacrylates show that the monomer post-addition speed is the major factor effectting the polymer molecular weight. In a certain range of post-addition speed, the molecular weight of polymer increased with increasing speed of monomer post-additionMost of the poly(methacrylate)s prepared by modified microemulsion polymerization , i.e. poly(methyl methacrylate), poly(ethyl methacrylate), poly(iso-butyl methacrylate), poly(cyclohexyl methacrylate), poly(benzyl methacrylate) , were rich in syndiotacticity and their glass transition temperatures were also higher than that reported by literature. Modified microemulsion polymerizations of various methacrylates were proceeded at different temperatures. It can be concluded that the polymer tacticities were mainly influenced by reaction temperature. The syndiotacticity of the polymers increased as the reaction temperature decreased for all the systems we studied.This rich-syndiotacticity may be caused by both the restricted volume effect of nanoparticles and the activation enthalpy effect during the propagation process. It is considered that in the small volume of a latex particle formed in microemulsion, the propagating polymer chains must have more gauche conformations than in its unperturbed state, especially near the surface of the particle, the path of its random walk would be forced to fold back into the particle. Therefore, the polymer formed in a microemulsion was conformationally restricted and the potential energy was higher. To compensate this effect, the propagating chain would be liable to have smaller root-mean-square end-to-end distance and lower potential energy during the propagation process. On the other hand, the difference in activation enthalpy (ΔH^?) and that in activation entropy (ΔS^?) between the isotactic and syndiotactic specific propagations in modified-microemulsion polymerization systems were calculated. In general, syndiotactic propagation was more favored by activation energy in a modified microemulsion polymerization system as we took both enthalpy and entropy into account. According to this calculation, chain propagating were restricted in a certain degree during the propagation process and the resulting polymers were rich in syndiotacticity.For the monomers whose polymerization carried out below the glass transition temperatures of the resulting polymers, such as methyl methacrylate, ethyl methacrylate and cyclohexyl methacrylate, polymer propagating chains were restricted during the reaction process, and the products were rich in syndiotacticity and higher in glass transition temperatures. For the monomers whose glass transition temperatures of the resulting polymers were in the range of the changed reaction temperatures, i.e. iso-butyl methacrylate and benzyl methacrylate, when the polymerization was carried out above the glass transition temperatures of products, the syndiotacticity of the resulting polymers would decrease obviously because of the less restricted volume effect on the propagation chains. For the monomers whose polymerization were carried out far above the glass transition temperatures of the products, i.e. hexyl metharylate, the resulting polymers have almost the same Tgs with bulk polymerized poly(hexyl metharylate) and the difference of their tacticities is not obvious, as the restricted volume effect on the propagation chains was further weakened.Tacticities of the polymers seems to be affected by the monomer structure. Monomers with bulkier side groups would result in polymers with richer syndiotacticities. The influence of the bulkier side group on the main chain may be weakened if there are other alkyl groups between them. It seems that the influences of the bulkier alkyl side groups and the more rigid phenyl groups on the main chain were different and requires separate discussions.