Synthesis And Characterization Of Syndiotactic Polystyrene Graft Copolymers

Heterogeneous Friedel-Crafts acylation of highly stereoregular syndiotactic polystyrene (sPS) in carbon disulfide has been used to synthesize atom transfer radical polymerization (ATRP) macroinitiator (or initiator precursor) and photoinitiator. Then sPS based graft copolymers have been synthesized through ATRP technique or photochemical coupling process with as-pepared acylated sPS as macroinitiator or photoinitiator. Due to the"living"nature of the ATRP reaction, it is reasonable assumed that the initiator groups are quantitatively converted to"living"free radical centers. Therefore, the amount of graft chains that has been incorporated ionto sPS backbone can be controlled by the sPS acylation reaction, the length of the grafts can be controlled by the control/living graft copolymerization.(1) Friedel-Crafts acetylation of sPS has been accomplished in a heterogeneous process by using carbon disulfide as the dispersing medium, and acetyl chloride and anhydrous aluminium chloride as acetylating agent and catalyst, respectively. The acetylation reaction can be well controlled and carried out to a high degree of acetylation. The acetylated sPS, ATRP initiator precursor, can be converted to bromoacetylated sPS (BsPS) via the acid-catalyzed halogenation reaction. The proton NMR analysis results indicate that the transformation of acetyl group to bromoacetyl group can be carried out essentially to quantitative conversion (very near 100%) under the reaction conditions.Syndiotactic polystyrene graft copolymers, including sPS-graft- poly(methyl methacrylate) and sPS-graft-atactic-polystyrene, are synthesized by ATRP using BsPS as macroinitiator and copper bromide combined with N,N,N',N',N'-pentamethyldiethylenetriamine as catalyst. The graft copolymers were characterized by 1H- and 13C-NMR spectra. In order to gain more insight into whether the BsPS-initiated styrene polymerization process is a controlled polymerization. A systematic study was performed in which the monomer conversion and the polymer molecular weight were correlated with the polymerization time. The molecular weight and molecular weight distributionnof the graft chains were determined using gel permeation chromatography (GPC) after cleaving from the sPS backbone with Baeyer-Villiger oxidation followed by hydrazine-catalyzed hydrolysis. The results indicate that the semi-logarithmic plot of ln([M0]/[M]) versus reaction time is linear and passes through origin, the molecular weight of aPS side chain increases linearly with monomer conversion, which indicate that the polymerization is first order with respect to monomer concentration and the polymerization process is characteristic of a'living'nature. The thermal properties of modified sPS differ significantly compared to those of the neat sPS. The modified sPS samples were noncrystalline polymers. In addition, Tg of BsPS (13 mol% Br) is found to be much lower than that of the virgin sPS, while Tg of graft copolymers remains at around 100oC regardless of the different degree of grafting.(2) Syndiotactic polystyrene based ATRP macroinitiator was synthesized by directly reacting sPS with 2-bromo-2-methylpropanoyl bromide via Friedel-Crafts reaction in a heterogeneous process. It is found that acylation occurs predominantly in preference to alkylation reaction when the Friedel-Crafts reaction is conducted under optimum conditions. The outstanding virtue of 2-bromo-2-methylpropanoyl bromide modified sPS (BMPsPS) lies in that it is well suited for grafting of polyacrylates onto sPS backbone.Syndiotactic polystyrene-graft-poly(glycidyl methacrylate) copolymer was synthesized by a heterogenous ATRP using BMPsPS as macroinitiator and copper bromide combined with 2,2'-bipyridine as catalyst in anisole at room temperature. It was found that BMsPS macroinitiator was well swelled in the mixture of anisole and GMA, the equilibrium swelling degree could reach 370 wt%. The resultant polymer was characterized by FTIR and NMR spectroscopies. For the polymerization system without additional deactivator, CuBr2, the conversion of monomer increased rapidly in the early stages and approximately leveled off after 120 min, suggesting that the grafting process was not well controlled. This observation was not surprising because the polymerization was conducted in a heterogeneous process and the concentration of initiation sites was too low to establish the typical ATRP equilibrium. On the other hand, it was demonstrated that addition of the CuBr2 deactivator to the reaction mixture resulted in recovery of control over the polymerization, which was verified by the fact that the semilogarithmic plot of ln([M]0/[M]) versus reaction time was nearly linear.The thermal properties of the graft copolymers were also investigated with DSC. The results indicated that the incorporation of acyl groups into sPS significantly affected on the crystallization behavior of sPS. In addition, sPS-graft-PGMA samples are unable to crystallize regardless of the degrees of grafting.(3) Benzoylated sPS (BesPS), a functional polymer bearing benzophenone (BP) moiety, has been synthesized in a heterogeneous process by using carbon disulfide as the dispersing medium, and benzoyl chloride and anhydrous aluminum chloride as benzoylating agent and catalyst, respectively. The benzoylation reaction can be well controlled and the resultant polymer was characterized by FTIR and NMR spectroscopy.Well-defined poly(ethyl glycol) (PEG) was grafted onto sPS backbone by UV irradiated photochemical coupling reaction using the BesPS as photoinitiator. It was found that the photochemical coupling reaction was affected by the molecular weight, the concentration of PEG, as well as the irradiation time. The experimental results indicated that PEG with lower molecular weight was easily grafted to sPS than that PEG with higher molecular weight, and prolonging the irradiation time could improve the degree of graft. On the other hand, higher PEG concentration seemed to decrease the degree of graft.The incorporation of benzoyl groups into syndiotactic polystyrene was found to have a profound effect on the thermal properties of these new materials. The melting point and the degree of crystallinity decreased by the presence of benzoyl groups, while the glass transition temperature increased. In addition, the incorporation of PEG side chains onto sPS had a more profound effect on the thermal properties of the sPS.