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CHEM 5384Polymer Characterization IILab 4: Titanocene dichloride controlled radicalpolymerization of isoprene Mengfang Li02/19/2013Introduction:Currently, living radical polymerization has become one the most efficient synthetic methodsin polymer industry. Apart from the common coordination polymerizations, there is increasedinterest in the radical polymerization using early transition metal complexes [1-3]. Theparamagnetic Cp2Ti(III)Cl complex, synthesized in situ by the Zn reduction ofCp2Ti(IV)Cl2, is a mild electron transfer agent and can be used to catalyzes variety of radical
polymerization. Cp2Ti(III)Cl is known to carry out radical halide abstraction from activated allylic orbenzylic halides which typically used as common initiators in ATRP polymerization. Radicalsgenerated by titanium abstraction can be readily add to a double bond, makes it possible toinitiate polymerization of vinyl monomers. The initiators used in ATRP are halogenatedalkanes, benzylic halides, a-haloester, a-haloketones, a-halonitriles[2,3]. However, theinactivated primary and secondary alkyl halides were not chosen as ATRP initiators becauseof the strong C-X bonds. Due to the ability of titanium catalyst to abstract radicals frominactivated halides, it is possible to use inactivated halides for polymerization[1-3]. Figure 1. Different conformation isomer of isopreneFigure 2. Mechanism of the Cp2TiCl-catalyzed styrene LRP initiated from epoxides,aldehydes, halides, and peroxides. Experimental:
Materials.a,a-dibromo-p-xylene (DBPX, 97%), Zn (nanosized activated powder> 99%)from Aldrich were used as received. Bis (cyclopendatienyl) titanium dichloride (CP2TiCl2,Acros, 97%) was recrystallized from methylene chloride. Isoprene (Aldrich, 99%) was driedover calcium hydride and passed through an Al2O3column. 1, 4-Dioxane (Fisher, 99%) wasdistilled. Chracterization. 1H-NMR (500MHz) spectra were recorded on a Bruker DRX-500 at 25 Cin CDCl3. GPC analyses were performed at 34 C on a Waters 150-C plus gel permeationchromatograph equipped with a Waters 410 differential refractometer, a Waters 2487 dualwavelength absorbance UV-Vis detector set at 254 nm. Tetrahydrofuran (THF) (Fisher;99.9%) was used as eluent at a flow rate of ml/min. Number-average(Mn) and weight-