IrCl

COCl AgClO4

,CO+ClO4 CO+ClO4

Silica-APC

h2o n

Figure 12. Grafting PMMA onto nanosilica. Reprinted with permission from [82], E. Schomaker et al., Polym. Commun. 29,158 (1988). © 1988, Elsevier Science.

Figure 13. Introduction of acylium perchlorate (APC) groups onto silica. Reprinted with permission from [84], N. Tsubokawa et al., Polym. Bull. 31, 456 (1993). © 1993, Springer-Verlag.

Silica-POE

Figure 14. Introduction of peroxyester (POE) groups onto silica. Reprinted with permission from [84], N. Tsubokawa et al., Polym. Bull. 31, 456 (1993). © 1993, Springer-Verlag.

an amide bond established between ACPA and an amine-bearing silane coupling agent on silica, an azo initiator can also be immobilized on silica substrates [89].

Various vinyl monomers, such as methyl methacrylate, styrene, acrylonitrile, and N-vinylcarbazole, were found to be effectively grafted to inorganic particles (silica (16 nm), titanium oxide (120 nm), and ferrite (15 nm)) initiated by the attached azo groups. Needless to say, these polymergrafted nanoparticles produced stable dispersion in organic solvents. Furthermore, the dispersion of polymer-grafted ferrite in organic solvent was found to behave like a magnetic fluid.

In fact, the azo groups introduced onto inorganic particles can also initiate graft polymerization by a photoirradiation technique [88]. Furthermore, the percentage of grafting by photografting polymerization is much higher than that initiated by thermal decomposition [86], indicating that the blocking effect of surface radicals by the grafted polymer chain is considerably reduced. In the process of pho-topolymerization at room temperature, the propagation rate of polymer radicals is smaller than that during the thermal polymerization at 70 °C. The surface radicals and the parted free radicals have the same chance to react with the monomers. However, in the thermal polymerization system, the formation of ungrafted polymer by free radicals preferentially proceeds with the progress of polymerization.

Tsubokawa et al. revealed that a redox system consisting of ceric ion and silica particles carrying reducing groups, such as alcoholic hydroxyl, amino, and mercapto, is also capable of initiating a radical polymerization (Fig. 15) [90]. The introduction of alcoholic hydroxyl groups onto

OCH3 h H

OCH3 O

och3 h H

OCH3 O

OCH3

(nSiO^O-SHCH^—<^CH2-C~C-H + HOOC-(CH2)2—C-N=N-C-(CH2)2—COOH O

OCH3 O CN CN

CH3 CH3

CN CN CH3

Scheme 5

the surface can be achieved by the treatment of the silica with 3-glycidoxypropyltrimethoxysilane (GPS) under acidic conditions in water. Amino and mercapto groups are introduced by the reaction of surface silanol groups with 3-aminopropyltriethoxysilane (APS) and 3-mercaptopropyl-trimethoxysilane (MPS), respectively (Fig. 16). The initiating ability of the redox system changes with the reducing activity of the reducing groups on the silica surface in the following order: alcoholic hydroxyl < amino < mercapto.

Based on the above-mentioned redox systems, polyacry-lamide (PAAM) was grafted to silica particles, and the grafting percentage reached about 25% [90]. It was found that both carbon and oxygen radicals can initiate the polymerization (Fig. 16). Since PAAM chains grafted to the silica surfaces interfere with the aggregation of the nanoparticles, the dispersibility of the modified silica in water was remarkably improved.

Shirai et al. proved that haloalkyl groups introduced onto ultrafine inorganic particles can serve as a coinitiator for free radical grafting polymerization based on the interaction with transition metal carbonyls [91]. In their work trichloroacetyl groups were first introduced onto the surface of TiO2/Si-R-OH through the reaction of the surface alcoholic hydroxyl groups with trichloroacetyl isocyanate. The radical grafting polymerization of vinyl monomers was successively initiated by a system consisting of Mo(CO)6 and TiO2/Si-R-COCCl3. The percentage of grafting and grafting efficiency of the graft polymerization initiated by this coinitiation system are

R-CH-XH

OCH3 OH O CH3 CH3

- (nSlO2^O—Sl-(CH2)3—O-CH2C H-CH2-0—C-^H^—C-N=N-C-(CH2)2—COOH OCH3 CN CN

Scheme 4

Figure 15. Generation of radicals on the silica particles carrying reducing groups. Reprinted with permission from [89], N. Tsubokawa et al., Polym. J. 21, 475 (1989). © 1989, Society of Polymer Science, Japan.

or a

O-Si—(CH2)3OCH2CHCH2OH

O-Si-(CH2)3NH2

Lower temperature

Figure 16. Introduction of reducing groups onto silica particles. Reprinted with permission from [90], N. Tsubokawa et al., Polym. J. 21, 475 (1989). © 1989, Society of Polymer Science, Japan.

much higher than those initiated by azo groups, because of the absence of fragment radicals.

It is worth noting that a proper selection of polymer species, along with a control of molecular weight and density of the grafted chains, is needed in all cases to obtain desired properties of polymer-grafted particles. Therefore, factors including types of initiator, monomer concentration, polymerization temperature, etc. should be considered comprehensively.

For example, the molecular weight of PS grafted to silica obtained from radical graft polymerization was found to be much higher than that from cationic graft polymerization (Table 3). The number of grafted PS in the radical polymerization, however, is much less than that in the cationic polymerization. The difference can be attributed to the initiation efficiency. That is, the initiation efficiency was lower in the former case because of the blocking effect of the previously formed polymer chains. In the cationic polymerization system, in contrast, the initiating efficiency was quite high but the chain lengths of the grafted polymer were rather short, as a result of chain transfer reactions of the growing polymer cation. On the other hand, the molecular weight of PS grafted to silica obtained from radical grafting polymerization initiated by surface azo and peroxyester groups decreased with decreasing monomer concentration and polymerization temperature. The former was assumed to be related to the reduction of the polymerization rate, whereas the latter was assumed to be related to the decrease in the number of propagating chains from the surface with a drop in polymerization temperature (Fig. 17) [92]. By means of a chain transfer agent, the molecular weight of grafted PS can also be controlled to some extent without lowering the number of grafted chains (Fig. 18).

Among various polymerization methods, a controlled/ "living" polymerization technique appears to be optimal in

Table 3. Molecular weight and number of grafted PS on silica surface.

Ultrafine

Grafting number

particles

Mna (x104)

Mwb (x104)

Mw Mn

(^mol/g)

Silica-POE

55.9

120.0

2.1

0.74

Silica-APC

0.27

0.43

1.6

238.8

aMn, number-averaged molecular weight. bMw, weight-averaged molecular weight.

Source: Reprinted with permission from [84], N. Tsubokawa et al., Polym. Bull. 31, 456 (1993). © 1993, Springer-Verlag.

aMn, number-averaged molecular weight. bMw, weight-averaged molecular weight.

Source: Reprinted with permission from [84], N. Tsubokawa et al., Polym. Bull. 31, 456 (1993). © 1993, Springer-Verlag.

lrR"

Higher temperature

Figure 17. Effect of polymerization temperature on the number of propagating chains on particles' surfaces. Reprinted with permission from [92], N. Tsubokawa et al., Colloid Polym. Sci. 273, 1049 (1995). © 1995, Springer-Verlag.

controlling the molecular weight, molecular weight distribution, and structure of the grafting polymer [93]. Recently von Werne and Patten reported a technique of atom transfer radical polymerization (ATRP) for conducting con-trolled/"living" radical polymerizations from the surface of silica nanoparticles [94]. For this research, two initiators were prepared: (3-(2-bromopropionyl)propyl) dimethyl-ethoxysilane (BPDS), in which the secondary a-bromoester could function as a styrene or acrylate ATRP initiator, and (3-(2-bromoisobutyryl)propyl) dimethylethoxysilane (BIDS), in which the tertiary a-bromoester could serve as a methacrylate ATRP initiator. Two sets of silica particles were used in this study, 75-nm and 300-nm diameter particles, with different surface area-to-volume ratios (0.08 vs 0.02, respectively). A monolayer of the ATRP initiator was deposited on the silica nanoparticle surface by heating the siloxane initiator and nanoparticles in tetrahydrofuran

HS: Chain transfer agent Sv Stable radical

Figure 18. Effect of chain transfer agent on the grafting polymerization on particles' surfaces. Reprinted with permission from [92], N. Tsubokawa et al., Colloid Polym. Sci. 273,1049 (1995). © 1995, Springer-Verlag.

(THF) at 80 ° C. The standard polymerizations of styrene and methyl methacrylate (MMA) were conducted by heating the initiator-functionalized silica with monomer, cop-per(I) bromide, ligand, and solvent at a temperature of 90-110 °C (typical conditions: temperature 110 °C, solvent p-xylene, [styrene]o = 8.73 M, [CuBr]o = 61.6 mM, [dNbipy]o = 116 mM, and 1.00 g of SiO2/initator). For the smaller (75 nm) silica nanoparticles, the grafting molecules of PS exhibited good molecular weight control (molecular weight polydispersity index <1.25), whereas PMMA grafting chains had higher molecular weight polydispersity index of about 2.0 under the same reaction conditions. When a small amount of free initiator was added to the polymerization solution, better molecular weight control was observed. Thus, the above difference should be induced by different termination modes during polymerization processes for PS and PMMA. In the case of larger (300 nm) silica nano-particles, neither PS nor PMMA grafting molecules exhibited molecular weight control. This lack of control was ascribed to the very high initial monomer-to-initiator ratio. It should be noted that only about 25% of the initiator present on the nanoparticle surface can initiate the growth of chains because of the sterically blocking effect of growing chains on the accessing of the catalyst to the neighboring initiation sites. As a result, the experimental molecular weights were a factor of ~4 higher than would be expected based upon the polymerization conversion factored with the ratio of monomer to nanoparticle initiator content.

Tsubokawa and Yoshikawa [95] prepared silica particles grafted to various polymers with well-defined molecular weight and structure by termination of living polymer cations with the amino groups already introduced on the particles. The introduction of amino or N-phenylamino groups onto the silica surface was made by the treatment of silica with y-aminopropyltriethoxysilane or N-phenyl-y-aminopropyl-triethoxysilane. It was found that these amino groups on silica can readily react with living poly(isobutyl vinyl ether) and poly(2-methyl-2-oxazoline). The percentages of amino and N-phenylamino groups used for the grafting reaction with living poly(isobutyl vinyl ether) were only 4.4% and 4.9%, respectively. This means that the surface amino groups were blocked by the neighboring grafted polymer chains.

Liu and co-workers [96] showed the feasibility of using diisocyanate as a coupling agent to introduce poly(ethylene glycol) (PEG) (weight averaged molecular weight = 1500) to the surface of nanoapatite particles. With the use of a monoisocyanate isocyanatoethyl methacrylate, the reactivity of the surface hydroxyl groups of nanoapatite was proved. The percentage of grafted polymer was about 20% by weight. The amount of hydroxyl groups of nanoapatite was decreased by 7.7% after a surface grafting reaction. Green et al. [97] found that the grafting of PEG to alumina powder (0.5 /m) can be obtained simply by heating the dispersion of alumina powder in a pure PEG melt. The amount of the grafted polymer is strongly dependent on temperature. Grafting from a solution of the polymer, instead of the polymer melt, was also found to be effective, and a poor dispersion medium is profitable to the grafting reaction.

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