Silica anion polymerization

The precipitated silica (J. Crosfield Sons) was heated in vacuo at 120° for 24h. before use. Two grades of surface areas 186 and 227 m g l (BET,N2), were used during this project. Random copolymers, poly(methyl methacrylates) and polystyrene PS I were prepared by radical polymerization block polymers and the other polystyrenes were made by anionic polymerization with either sodium naphthalene or sodium a methylstyrene tetramer as initiator. The polymer compositions and molecular weights are given in Table I. 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

Among the two ionic polymerization techniques mentioned above, a living anionic polymerization should show the best possible control of polymer architecture and composition. Mono dispersed homopolymers, complex-block, graft, star, and miktoarm architectures have been accessible primarily by anionic polymerization methods [22]. They have been used to grow polymer brushes from various small particles such as silica gels graphite,carbon black, and flat surfaces [23-26]. Recent results have been reported on living anionic polymerizations on clay [27] and silica nanoparticles [28,29]. 

Anionic Polymerization. Anionic grafting polymerization initiated on the inorganic particles is also possible to give composite particles. The amino group incorporated into the surface of silica caused anionic polymerization of N-carboxy-a-amino acid anhydride to give poly(amino acid) grafting [Reaction (8)1 (21). 

Specific properties of polysilanes have been linked to the method of synthesis.35 For example, in the case of anionic polymerization of poly[l-(6-methoxy-hexyl)-l,2,3-trimethyldisilanylene] a new type of chromism was induced in the polysilane film by the difference in the surface properties of substrates and was termed a surface-mediated chromism. The polysilane exhibited thermochromism with an absorption maximum at 306 nm at 23°C, but <15°C a band at 328 nm began to appear. A monolayer of the polysilane was transferred onto both a clean hydrophilic quartz plate and a hydrophobic one treated with hexamethyldisilazane by the vertical dipping method. With the hydrophobic plate, a broad UV absorption at 306 nm is obtained, whereas the absorption on a hydrophilic plate shifts to 322 nm. The conformation of the polysilane is preserved by hydrogen bonding between the silica surface and the ether section of the substituent on the hydrophilic plate. The polysilane is attached to the hydrophobic surface only by van der Waals forces, and this weaker interaction would not sustain the thermodynamically unstable conformational state that is attained on the water surface. 

The synthesis and purification of polystyrene methacryloyl macromonomers (PS-MA) in the molecular weight range Mn= 1000-2000 g mol 1 by living anionic polymerization of styrene (S), termination with ethylene oxide (EO), and subsequent reaction with methacrylic chloride has already been described in detail elsewhere [180] (see also Scheme 16). In this context it has to be emphasized that the hydroxyethyl-terminated PS-MA macromonomer precursor (PS-OH) as obtained after purification of the crude PS-OH by silica column chromatography (cyclohexane/dichloromethane 1/1 v/v) and as charged in the PS-MA synthesis still contains up to about 15 wt-% of non-functionalized polystyrene (PS-H). This PS-H impurity of the PS-MA macromonomer does not interfere with the PS-MA synthesis and the subsequent TBA/PS-MA copolymerization and is easily and conveniently removed from the resulting PTBA-g-PS graft copolymer (see below). 

The systems studied by Clarke and Vincent (1981a) were closely analogous to those of de Hek and Vrij mentioned previously. Polystyrene was grafted on to silica particles by an anionic polymerization technique. Its molecular weight lay in the range S 000 to 24 000. The diameter of the monodisperse silica particles was varied from ca 100 to 300 nm. Ethyl benzene was chosen as the dispersion medium. 

Two other classes of silicones deserve mention. These are the water-based silicones that are used in sealant and coating applications and the silicone pressure-sensitive adhesives. Water-based silicones can be prepared by anionic polymerization of siloxanes in water using a surface-active catalyst such as dodecylbenzenesulfonic acid [4]. The resulting emulsion can then be cross-linked in several ways, including the use of alkoxysilane copolymerization or tin catalysts in conjunction with colloidal silica. The result is essentially an emulsion of cured PDMS in water. Various fillers and other components are added, resulting in a sealant composition. Upon evaporation of water. 

Polymer properties are dependent on many factors, including chain end interactions with substrates such as carbon black or silica fillers, as well as clay and calcium carbonate. At this point, there is a large volume of work that has been done on chain end modification, particularly those made by anionic polymerization with group 1 or group II metals (Bielinsk et al., 1995 Yamato and Oahu, 1996 Schulz et al., 1974), as seen later. 

After these initial considerations, the complete analysis of a number of diblock copolymers of styrene and methyl methacrylate shall be discussed in detail. The poly(styrene-ftlodc-methyl methacrylate)s under investigation were prepared via anionic polymerization of styrene and subsequent polymerization of methyl methacrylate, varying molar mass and composition (B1-B3). The polystyrene precursors (P1-P3) were isolated and characterized separately. As the PMMA block is the more polar block in the block copolymer, a polar (silica gel) column was chosen for establishing the critical point of PMMA. According to case (1) in Fig. 14, the PS block is then eluted in the SEC mode. The behavior of PMMA of different molar masses on silica gel Si-100 in eluents comprising methylethylketone and cyclohexane is shown in Fig. 15A [37]. 

The catalysts for these polymerizations can be separated into two groups. To the first belong the so-called Ziegler-Natta catalysts, and to the second, transition metal oxides on special supports, like carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also includes transition metal-TT-allylic compounds and transition metal hydrides. 

Living Anionic Surface-Initiated Polymerization. Anionic polymerization (qv) is a most versatile method to make well-defined architectures of polymers. It has been employed to grow polymer brushes from various silica gels (53), graphite and carbon black (54), and also flat surfaces (27,55). Results on living anionic pol5unerization on clay nanoparticles have been reported (56). Special conditions are required since difficulties arise owing to the effects of moisture and other impurities on anionic polymerization. A major limitation of the prior work... 

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