Synthesis and characterization of block copolymers using polysiloxane based macroazoinitiator

Poly(dimethylsiloxane-b-styrene) (PDMS-b-PSt) and poly(dimethylsiloxane-b-methyl methacrylate) (PDMS-b-PMMA) block copolymers containing siloxane segments were studied by the radical polymerizations of vinyl monomers such as styrene (St) and methyl methacrylate (MMA) using polydimethylsiloxane based macroazoinitiator (PDM-MAI) in solution. PDM-MAI was synthesized by reacting hydroxy-terminated polydimethylsiloxane (PDMS) and 4,4'-azobis(4-cyanopentanoyl chloride) (ACPC) having a thermodegradable azo-linkage. The polycondensation reaction between PDMS and ACPC (2:1, molar ratio) was carried out at room temperature. The polymerizations of St and MMA initiated by PDM-MAI were investigated using different PDM-MAI concentrations (1, 5, 10, and 15 wt.%) for various reaction times with a methyl ethyl ketone (MEK) / dichloromethane (DCM) solvent mixture (3:1, molar ratio) as the reaction medium at 65 degreesC. The decomposition temperature of MAI azo groups was determined to be 125 degreesC by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). PDM-MAI was characterized from the H-1-NMR spectrum, where the signals of the -CH2 resonance (2.32-2.71 ppm) and -CH3 resonance (1.65-1.70 ppm) of ACPC and the dimethylsiloxane protons (0.072 ppm) in siloxane segments are observed. PDM-MAI was also identified by the peak at 1740 cm(-1) due to the C=O stretching, proving polyester formation, the peaks at 800 and 1260 cm(-1) as the Si-CH3 deformation bands, and the Si-O-Si asymmetric stretching vibration bands appearing at 1100 cm(-1) from the FTIR spectrum. The number-average molecular weight (M-n) of PDM-MAI was determined to be 812 g/mol by vapor pressure osmometry (VPO) in chloroform. The number- and weight-average molecular weights (M-n and M-w) of PDMS-b-PSt and PDMS-b-PMMA copolymers determined by gel permeation chromatography (GPC) show a decrease with respect to increasing PDM-MAI initial concentration in the polymerization mixture. In addition, the polymerization yields increase with increasing reaction time for constant PDM-MAI concentrations.