Synthesis and Characterization of Block Copolymers Using Polysiloxane Based Macroazoinitiator

Turk J Chem 28 (2004) , 645 – 657. c TÜBİTAK Synthesis and Characterization of Block Copolymers Using Polysiloxane Based Macroazoinitiator Sennur DENİZ, Nil BARAN, Mesut AKGÜN, Nalan A. AKGÜN, Salih DİNÇER∗ Yıldız Technical University, Chemical Engineering Department, Davutpaşa Cad.,No: 127, 34210, Esenler, İstanbul-TURKEY e-mail: Received 02.03.2004 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 hydroxyterminated polydimethylsiloxane (PDMS) and 4,40 -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 ◦ C. The decomposition temperature of MAI azo groups was determined to be 125 ◦ C by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). PDMMAI was characterized from the 1 H-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 (M̄n ) of PDM-MAI was determined to be 812 g/mol by vapor pressure osmometry (VPO) in chloroform. The number- and weight-average molecular weights (M̄n and M̄w ) 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. Key Words: Polydimethylsiloxane based macroazoinitiator, poly(dimethylsiloxane-b-styrene), poly (dimethylsiloxane-b-methyl methacrylate), macroazoinitiator, solution polymerization. ∗ Corresponding author 645 Synthesis and Characterization of Block Copolymers Using..., S. DENİZ, et al., Introduction The use of polymeric initiators such as a macroazoinitiator (MAI) having labile azo groups is a well-known method for preparing various block copolymers by conventional radical polymerization1−8. Synthesis using MAIs provides some advantages. Not only are block copolymers composed of addition polymer segments but also those of addition polymers and various oligomer segments can be synthesized by using MAIs in radical processes. In addition, MAIs enable researchers to combine different kinds of polymerization reactions, where MAIs can be prepared by the use of materials having both polycondensation9 or polyaddition functionality10,11and a thermally labile azo-linkage as radical initiation species12 . However, it is an important factor to synthesize a MAI containing one or more azo groups in the main chain for the molecular structure of block copolymers. Depending on the type of MAI and termination reaction, diblock (AB), triblock (ABA) or multiblock (AB)n copolymers can be obtained. Many studies have focused on the preparation of different block copolymers using different types of MAIs by heating in the presence of a vinyl monomer13,14. Incorporation of polysiloxane segments into common vinyl polymers would lead to an interesting group of thermoplastic elastomers with low glass transition temperature, high lubricity, high gas permeability, etc. In recent years, many studies were reported with MAIs containing polysiloxane segments for the synthesis of siloxane-vinyl block copolymers15−23 . The macroazoinitiator prepared by the condensation of 4,40 -azobis(4cyanopentanoyl chloride) (ACPC) and α,ω-bis(3-aminopropyldimethyl) polysiloxanes in equimolar amounts was found to be a useful candidate for the radical polymerization of vinyl monomers with a typical product of poly(dimethylsiloxane-b-methyl methacrylate) (PDMS-b-PMMA)15 . Inoue et al.15 showed that incorporation of the PDMS segment in block copolymers improved the thermal stability of PSt. Hamurcu et al. 17 reported the production of the block and graft copolymers as PDMS-b-Polystyrene (PDMS-b-PSt) and PDMS-gPolybutadiene (PDMS-g-PBd) by using NH2 -terminated PDMS based macroazoinitiators obtained by the reaction of ACPC with α,ω-amine terminated PDMS prepolymer. It was also reported that crosslinked block copolymers of styrene containing poly(dimethylsiloxane) units could be prepared with macroinimer having PDMS units and the thermal properties of crosslinked PDMS-b−PSt copolymers were compared with the copolymers of styrene prepared by PDMS-macroazoinitiators18. Chang et al.19studied the synthesis and thermodecomposition kinetics of hydroxybutyl-terminated macroazoinitiators (PDMS-ACP) of varying molecular weights, and prepared PDMS-b-PMMA copolymers using PDMS-ACP macroazoinitiators. It was found that the activation energy (Eo ), activation enthalpy (∆H∗ ) and activation entropy (∆S∗ ) of the thermodecomposition of PDMS-ACP in toluene increased with increases in poly(dimethylsiloxane) chain length (SCL) in PDMS moieties, while the activation free energy (∆G∗ ) was independent of the SCL. In another study, Chang et al.20prepared PDMS-b-PSt copolymers with PDMS-ACP macroazoinitiators, investigated the thermal and thermo-oxidative degradation of these copolymers, and found that the incorporation of PDMS segments in the block copolymers improved the thermo-oxidative stability of PSt. The aim of this study was the synthesis of block copolymers based on polydimethylsiloxane and polystyrene or poly(methyl methacrylate) sequences using different concentrations of PDM-MAI with a MEK/DCM solvent mixture (3:1 molar ratio) as the reaction medium. The indicated MEK/DCM solvent mixture was used to dissolve PDM-MAI completely so that a homogeneous reaction medium was obtained. First, the synthesis of PDM-MAI based on ACPC and hydroxy-terminated PDMS (M̄n = 500 g/mol) 646 Synthesis and Characterization of Block Copolymers Using..., S. DENİZ, et al., was performed in one stage. Then various initial amounts of PDM-MAI were used to initiate the radical polymerizations of methyl methacrylate (MMA) and styrene (St) in solution. The main contribution of this study is the use of OH-terminated PDMS as opposed to the NH2 -terminated (...truncated)