Grafting mechanism

Paine et al. [99] tried different stabilizers [i.e., hydroxy propylcellulose, poly(N-vinylpyrollidone), and poly(acrylic acid)] in the dispersion polymerization of styrene initiated with AIBN in the ethanol medium. The direct observation of the stained thin sections of the particles by transmission electron microscopy showed the existence of stabilizer layer in 10-20 nm thickness on the surface of the polystyrene particles. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results supported the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents. 

Figure 32 Grafting mechanisms in styrenic graft copolymers.

The fact that acid enhances grafting also indicates the possibility that ionic processes may also contribute to the present grafting mechanism. In this context, acid may be considered to be a catalyst for the cationic process especially since ionising radiation is the initiator for the reaction and both free radicals and ions are known to be species formed from interaction between molecules and such radiation. However, the ionic mechanism for grafting is favoured by anhydrous conditions, thus, in the present system, acid enhancement via the ionic pathway would not appear to be a predominant process. 

Scheme 14.15 Possible grafting mechanism of[W(=Ar)(=CH Bu)(CH5Bu)j] on silica dehydroxylated at 700°C vs silica dehydroxylated at 200°C, proposed through analogy with silsesquioxane chemistry. Intermediates that have not been observed...

Nagai, N., Nakayama, Y., Zhou, Y. M., Takamizawa, K., Mori, K., and Munekata, M. (2008). Development of salmon collagen vascular graft Mechanical and biological properties and preliminary implantation study. J. Biomed. Mater. Res. 87B, 432-439. 

Fig. i Schematic illustration of H-bond grafting mechanism. Illustration a demonstrates hydrogen-bonded incorporation between MPEO-derived SMA and matrix PEU hard blocks, which mimics, as indicated in Illustration b, the similarly hydrogen-bonded selfcrosslinking of PEU hard blocks in PEU bulk [76-78, 82]. Reproduced from [173-176]... 

Fig. 2 Identification of H-bond grafting mechanism. A ATR-FTIR spectrum highlighting stretch bands of hydrogen-bonded carbonyl groups, - CO - (b), at 1700 cm 1, and free carbonyl groups, - CO - (a), at 1730 cm, respectively. B ATR-FTIR data curves indicating ratios of - CO - (b) vs - CO - (a) of each sample SMA-MSPEO, un-modified PEU surfaces, SPEO-modified PEU surfaces, and MSPEO-modified PEU surfaces [77], Reproduced from [173]...

The existence of a relationship between anionic catalyst concentration and incorporation of the acrylate again rules out the possibility of a grafting mechanism, but it is consistent with an active chain mechanism. If acrylate entered the polypropylene molecule at random along the backbone of the molecule, which would result in a grafting process, the level of anionic catalyst would be immaterial as long as the total weight of the polypropylene were kept constant. 

Obviously, the biocatalytic grafting mechanism differs significantly from the chemical mechanism [47] and further studies will be necessary to clarify the mechanism. 

Scheme 10.1 Grafting mechanism by hydrogen abstraction iUnstrated on a 1-2 bntadiene unit (1) of a PB chain. R can be either a primary radical from the initiator or a polystyryl radical. A side chain (PSt graft) grows from (3) by the addition of styrene to the radical center.

Zahedmanesh, H., Mackle, J. N., Sellborn, A., Drotz, R, Bodin, A., Gatenholm, R, and Lally, C. (2011). Bacterial cellulose as a potential vascular graft Mechanical characterization and constitutive model development. 1. Binmed. Mater. Res. BAddI. Biomater... 97,105-113. 

Fig. 8.4 Grafting mechanisms on polypropylene substrate and subsequent monomer grafting [36]...

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