Lab 4 - Isoprene - CHEM 5384 Polymer Characterization II...

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CHEM 5384 Polymer Characterization II Lab 4: Titanocene dichloride controlled radical polymerization of isoprene Mengfang Li 02/19/2013 Introduction: Currently, living radical polymerization has become one the most efficient synthetic methods in polymer industry. Apart from the common coordination polymerizations, there is increased interest in the radical polymerization using early transition metal complexes [1-3] . The paramagnetic Cp2Ti(III)Cl complex, synthesized in situ by the Zn reduction of Cp2Ti(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 or benzylic halides which typically used as common initiators in ATRP polymerization. Radicals generated by titanium abstraction can be readily add to a double bond, makes it possible to initiate polymerization of vinyl monomers. The initiators used in ATRP are halogenated alkanes, benzylic halides, a-haloester, a-haloketones, a-halonitriles [2,3] . However, the inactivated primary and secondary alkyl halides were not chosen as ATRP initiators because of the strong C-X bonds. Due to the ability of titanium catalyst to abstract radicals from inactivated halides, it is possible to use inactivated halides for polymerization [1-3] . Figure 1. Different conformation isomer of isoprene Figure 2. Mechanism of the Cp2TiCl-catalyzed styrene LRP initiated from epoxides, aldehydes, halides, and peroxides. [1] Experimental:
Materials. a,a-dibromo-p-xylene (DBPX, 97%), Zn (nanosized activated powder> 99%) from Aldrich were used as received. Bis (cyclopendatienyl) titanium dichloride (CP 2 TiCl 2 , Acros, 97%) was recrystallized from methylene chloride. Isoprene (Aldrich, 99%) was dried over calcium hydride and passed through an Al 2 O 3 column. 1, 4-Dioxane (Fisher, 99%) was distilled. Chracterization. 1H-NMR (500MHz) spectra were recorded on a Bruker DRX-500 at 25 C in CDCl 3 . GPC analyses were performed at 34 C on a Waters 150-C plus gel permeation chromatograph equipped with a Waters 410 differential refractometer, a Waters 2487 dual wavelength 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-

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