Synthesis and Molecular Characterization

Optimization studies were carried out with regard to (i) nature of the nonpolar co-solvent (i.e., hexanes vs methylcyclohexane), (ii) TiCl4 concentration, (iii) blocking time, and (iv) St concentration. Intermolecular alkylation occurred 

Both GPC traces showed the presence of two products. The major peaks correspond to the sought stars and the minor peaks correspond to by-product(s). The small peak at 34 ml in Fig. 8a is due to the linear PIB by-product formed by the side reaction discussed in Sect. 4.1. The small peak at 33 ml in Fig. 8b is 

The molecular weights of the PIB stars and PSt-b-PIB star blocks were studied by triple-detector GPC. The RI traces of the star blocks were monomodal, but the molecular weights determined by LLS were higher than expected. This discrepancy may be due to the presence of a small amount of higher molecular weight star blocks formed by intermolecular aromatic alkylation. Thus the molecular weight data of the star blocks and PSt segments calculated from LLS data should be viewed with caution. 

Since the presence of small amounts of alkylated (branched) products can distort molecular weights, NMR spectroscopy was used to analyze the PSt content and the molecular weight of the PSt blocks. Molecular characteristics of select samples obtained by GPC, NMR, and conversion are summarized in Table 3. The Mj of PIB stars determined by GPC (LLS) were slightly higher than theoretical. This M and the mole fraction of St obtained by NMR spectroscopy were used to calculate the M of PSt segments and the PSt content in star 

Calculated from homo PIB star samples assuming a theoretical number of arms of 8  

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Chen, G.W., Hayashi, T, and Nakajima, A. (1981) Synthesis and molecular characterization of A-B-A tri-block copolymers composed of poly(Y-ethyl L-glutamate) as the A component and polybutadiene as the B component. Polymer Journal, 13,433-442. 

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