Green And Surfactant-Free Emulsion Polymerization Of Amphiphilic Copolymers In Aqueous Phase

Green chemistry is a new field that seeks to reduce the environmental pollution from source. It contains 12 principles. One important principle is to avoid using organic solvents and auxiliaries. In our group, we dedicate our work in synthesizing polymers (homopolymer and copolymer), using environmentally friendly solvent water as that only solvent, and avoid using auxiliaries such as surfactant, which is always utilized in conventional emulsion polymerization.In this paper, hydrophobic monomer and hydrophilic monomer were copolymerized successfully in surfactant-free emulsion polymerization under the assistant of ultrasound. Further, controlled radical polymerization has been introduced into our experiment to obtain well-defined amphiphilic block copolymers in surfactant-free emulsion polymerization. The main work is described as below.1. The hydrophilic monomer acrylamide (AM) and hydrophobic monomer styrene (St) have been directly copolymerized in a surfactant-free aqueous emulsion with the assistance of powerful ultrasound. Fourier Transform Infrared spectrocopy (FT-IR), nuclear magnetic resonance (NMR), and differential scanning calorimetry (DSC) measurements revealed that copolymers of AM and St were obtained. Elemental analysis was used to calculate the composition of the copolymer. Size exclusion chromatography (SEC) measurement showed that the molecular weight (Mw) of the copolymer is 1.86×10~5 g/mol and the polydispersity index (PDI) = 2.31. The self-assembly behavior in different solvents was investigated utilizing laser light scattering (LLS), atomic force microscopy (AFM), and transmission electron microscopy (TEM); the copolymer film showed amphiphilicity, as measured by contact angle goniometry.2. Amphiphilic polymeric particles with hydrophobic cores and hydrophilic shells were prepared via living radical emulsion polymerization of styrene using a water-soluble poly(acrylamide)-based macro-RAFT agent in aqueous solution in the absence of any surfactants. Firstly, the homopolymerization of acrylamide (AM) was carried out in aqueous phase by reversible addition-fragmentation chain transfer radical polymerization (RAFT) using a trithiocarbonate as a chain transfer agent. Then the PAM-based macro-RAFT agent has been used as a water-soluble macromolecular chain transfer agent in the batch emulsion polymerization of Styrene (St) free of surfactants. The RAFT controlled growth of hydrophobic block led to the formation of well-defined poly(acrylamide)-co-polystyrene amphiphilic copolymer, which was able to work as a polymeric stabilizer (self-stability). Finally, very stable latex was prepared, having no visible phase separation for several months. FT-IR and ~1H-NMR measurements showed that the product was the block copolymer PAM-co-PS in the form of stable latex. Atomic force microscope (AFM), transmission electron microscope (TEM) and dynamic light scattering (DLS) studies indicated that the nanoparticles have a narrow particle size distribution and the average particle hydrodynamic radius was kept in the diameter of 58 run. Core-shell structure of the copolymer was also recorded by TEM. The mechanism of the self-stability of polymer particles during the polymerization in the absence of surfactants was studied. This facile method might also be extended to various hydrophobic and hydrophilic monomers, to induce more complex polymer architectures. Another hydrophilic monomer poly(acrylic acid) would be used instead of PAA as hydrophilic macro RAFT agent to investigate the effects of the length of hydrophilic chains on the emulsion stability and nanostructure of final products, as well as the living characterization of the polymerization in the presence of the macro RAFT agent, besides of its role as emulsion stabilizer.In addition, environmental induced structure conversion of amphiphilic block copolymer film is also being investigated.3. Acrylic acid was polymerized directly in aqueous phase in the presence of a water-soluble trithiocarbonate as RAFT agent, and well-defined poly (acrylic acid) (PAA) with low polydispersity index (PDI) and high molecular weight was successfully prepared. The polymerization exhibits living free radical polymerization characteristics: controlled molecular weight, low polydispersity and well-suited linear growth of molecular weight with conversion at different ratios of monomer toRAFT agent. Moreover, no transfer to solvent or polymer was observed even at high conversion of high ratio of monomer to RAFT agent. The number-average molecular weight of 2.3×10~5 g/mol of PAA can be obtained, and the generated PAA can work as a macro RAFT agent to form more complex architectures with sequentially polymerization.