Synthesis and characterization of model cyclic block copolymers of styrene and butadiene. Comparison of the aggregation phenomena in selective solvents with linear diblock and triblock analogues

The synthesis and characterization of model cyclic diblock copolymers of styrene (St) or perdeuterated styrene (St-d(8)) and butadiene (Bd) are presented. Since conventional methods of characterization cannot separate completely the cyclic copolymer from its linear precursor, differences in the micellar behavior were used as a method for investigation of their purity. For this purpose, in addition to the cyclic and linear triblock copolymers, two linear diblocks with similar compositions, and molecular weight equal or half of the cyclic diblocks, were also synthesized. The synthetic approach of the cyclics involved the reaction of (1,3-phenylene)bis(3-methyl-1-phenylpentylidene)dilithium initiator with butadiene in the presence of sec-BuOLi, followed by polymerization of St (or St-d8). The cyclization of the resulting a,co-difunctional triblock copolymer was performed by using bis(dimethylchlorosilyl)ethane, under high dilution conditions. The copolymers were characterized by size exclusion chromatography, membrane osmometry, NMR and UV spectrometry, and viscometry. The micelles formed in the selective solvents n-decane (for PBd) and dimethylformamide (for PS-d(8)) were characterized by small-angle neutron scattering and dynamic light scattering. It was found that the aggregation number of the cyclic copolymers was the smallest among the different macromolecular architectures. Moreover, the SANS data for the triblocks in n-decane indicated the presence of 37% dangling chains which did not appear in the data for the corresponding cyclic copolymers. Considering that 5% of dangling chains is possible to be detected, it proves that the cyclic copolymers are at least 87% pure. A scaling model was used in order to justify the difference in the aggregation numbers between the four different copolymers.