Synthesis Of Styrene/Butyl Acrylate Copolymer Via RAFT Semibatch Miniemulsion Polymerization And Characterization

Conventional radical polymerization is featured by slow initiation, fast propagation and extremely fast termination. In such a system, once an individual radical is born, the time period for propagation is only around 1 second before termination. It is impossible to synthesize the block copolymer with such an approach. In 1990s, controlled / "living" radical polymerization (CLRP) is invented. The radical polymerization gains living features of "fast" initiation and "slow" propagation by introducing a dormant agent in CLRP. CLRP can effectively control molecular weight and its distribution, chain architecture and composition profiles along polymer chain and become the most important living polymerization technique after living anionic polymerization was invented. Compared with the living anionic polymerization, CLRP has a much larger variety of monomer, mild polymerization conditions, and can be carried out in water system.However, CLRP is a kind of living polymerization with pitfalls: the irreversible termination is still occur in such a system. The irreversible termination brings about a series of problems: a low polymerization rate and high purities in product when the molecular weight of the targeted polymer is high (>100,000). The major commercial challenge of CLRP is how to increase the polymerization rate and how the purities influence on the product properties.Block copolymer is the most promising product prepared by CLRP. On the other hand, reversible addition-fragmentation transfer (RAFT) miniemulsion polymerization is considered the most promising process for commercial applications of CLRP. In this thesis, a series of poly(styrene-b-butyl acrylate-b-styrene) with various molecular weights and compositions was prepared via RAFT miniemulsion polymerization. The molecular weight distribution (MWD), composition distribution (CD), and living chain distribution (LCD) were characterized by GPC with UV-RI dual detectors. Results showed that MWD was dramatically broadened during the process synthesized the middle block. In some cases, a shoulder peak appeared. The high molecular weight polymer had a higher BA composition and living chain fraction. The reason for these observations is that the smaller particles would polymerize faster than the larger ones. Even though the living chain fraction of high molecular weight section in diblock copolymer (intermediate products) was the highest, the styrene propagation in the synthesis of the third block mainly occurred in the middle molecular weight section. It is likely that the particles which contribute those high molecular weight polymers have a low monomer concentration due to its lower styrene composition in polymer. The resulted SBAS had broad MWD, CD and low living chain fraction. The investigation of structure-mechanical properties disclosed that the high fraction of dead polymer could reduce much the elongation at break. The modulus and tensile strength were mainly determined by styrene fraction in polymer. To reduce the dead chain fraction and to balance the polymerization rates among the particles with different sizes would be a key to optimize the product properties. Using oil-soluble AIBN initiator, oligomer RAFT agent and mixture of NaHCO₃ and Na₂CO₃ as pH buffer in the case of KPS as a initiator would be beneficial for such an aim.