RAFT Emulsion Polymerization Of Butadiene And Controllable Synthesis Of Block Copolymer Of Butadiene And Styrene

Both ab initio and seeded emulsion RAFT polymerization of butadiene were systematically investigated in terms of polymerization kinetic, controlling over chain structures, gelation point, and morphology of latex particles, aiming at the controllable synthesis of polybutadiene homopolymer and polystyrene/polybutadiene block copolymers. The obtained new understandings and achievements were as follows.RAFT ab initio emulsion polymerization of butadiene was investigated by using the amphiphilic poly(acrylic acidn-b-styrene5) trithiocarbonate as both surfactant and mediator. The neutralization on acrylic acid (AA) units was found to play significant influence on the gelation. When half of the AA units were neutralized, the gelation occurred in the early stage of the polymerization so that the highest accessible molecular weight of polybutadiene was as low as 3K. In the non-neutralized conditions, the gelation was much retarded so that the highest accessible molecular weight was increased up to 23K. In the non-neutralized conditions, potassium persulfate could not initiate the polymerization. When azobisisobutyronitrile was used as initiator, the polymerization mediated by poly(acrylic acid27-b-styrene5) trithiocarbonate could proceed much faster than the solution polymerization did. The latex was stable. Before the gel point, molecular weight agreed well with the theoretical prediction while PDI was relatively high due to the crosslinking reaction. The polybutadiene-b-polystyrene core/shell particles could be obtained by extending polybutadiene chain. When the n value in poly (acrylic acidn-b-styrene5) trithiocarbonate was lower than 20, the coalescence would occur, leading to the formation of some coagulum. On the other hand, when n value was as high as 60, the molecular weight was out of control.Polystyrene-b-polybutadiene diblock copolymers (PS-b-PB) was synthesized by RAFT seeded emulsion polymerization of butadiene. Two types of the PS seed latex were made from RAFT miniemulsion polymerization, where PEPDTA was used as RAFT agent with SDS being surfactant and RAFT ab initio emulsion polymerization, where P(AA27-b-S5)-DSCTSPA was used as mediator and surfactant, respectively. In the case of the seed latex from the miniemulsion polymerization, the polymerization rate of butadiene was very low and the diblock copolymer was formed with controlled molecular weight. At the low conversion of butadiene, PDI was low and the crosslinking reaction of polybutadiene led to higher PDI and gelation occurred at the theoretical of polybutadiene Mn, theo=45K at 70℃. The crosslinking reaction and gelation were retarded at lower reaction temperature. The morphology of structured nanoparticles could be largely tuned simply by the copolymer composition. With the composition of the polybutadiene segment increased from 0.37 to 0.91, the morphology within the structured particles changed from the polybutadiene domains-in-polystyrene matrix, concentric-spherical multilayers, bicontinuous microphase, to broken layers of polystyrene in polybutadiene matrix. It was found that the morphology of the block copolymer within nanoparticles was also dependent on d/L values, which was in excellent agreement with the theoretical prediction. In the case of the seed latex from ab initio emulsion polymerization, the polymerization rate of butadiene was much faster than one in the case of miniemulsion polymerization. The morphology of diblock copolymer particle was observed to be core/shell microstructure. The molecular weight of diblock copolymer was poorly controlled and gelation occurred much earlier. Fortunately, two methods i.e. the addition of styrene being comonomer and post-addition of sodium dodecyl sulfate to decrease the interfacial tension could delay the gel point from Mn,theo 20K to 60K and 35K.Polystyrene-b-polybutadiene-b-polystyrene triblock copolymer (SBS) was synthesized. TEM observation showed clear microphase segregation in both bulky state and latex particle of SBS. The products showed good mechanical properties with the ultimate tensile stress higher than 15MPa and elongation at break above 600% when PS fraction was tuned from 34%-56%, indicating a promising application as thermoplastic elastomers.