Grafting acid enhancement mechanism

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

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. 

Further work on the mechanism of the acid enhancement in both UV and gamma ray grafting is reported. The copolymerisation of styrene in methanol and dioxan to cellulose is used as model system. Acid enhances grafting and homopolymer formation in both radiation systems. Analysis of the homopolymer from the grafting runs indicates that acid reduces the chain length, but increases the numbers, of grafted chains. 

It is also known that chemical and physical abnormalities in the cellulose molecule can produce weak bonds (30,31) thus, structural abnormalities may set up strain at localized points. At such positions, the rate constants for hydrolysis may be increased by as much as 10,000 (31). In the presence of acid, these properties would be enhanced and would assist UV grafting. Such a mechanism may even be the predominant process by which acid accelerates UV grafting. 

More recent detailed work indicates that the acid enhancement in grafting is more complicated than previously considered. Chappas and Silverman have confirmed the presence of the acid effect and have suggested that hydrogen atoms need not be the only explanation although they proposed no alternative mechanism. 

Although these recent molecular weight distribution experiments indicate that H atom yields are the predominant mechanistic pathway for the acid enhancement in radiation grafting, recent work on the radiolysis of binary mixtures indicates that acid can also affect the yields and stability of the styrene-methanol grafting intermediates (MR ). An increase in yield of MR could also lead to an increase in grafting sites by H atom abstraction. Such a mechanism may also make a finite contribution to the overall grafting process. 

The above mechanism has proved to be an oversimplification of the grafting process since it fails to explain a number of further observations recently reported (7,. > > 1Z) including (i) the occurrence of enhancement only at certain acid and monomer concentrations,... 

Grafting and networking may modify the mechanical, chemical, and functional properties of polymers and enhance their utilization for some purposes, such as for water treatment (Kumar and Verma, 2007 Mishra et al., 2003). Psyllium derivatives were prepared by grafting acrylonitrile onto psyllium molecules using a ceric ammonium nitrate and nitric acid system (Mishra et al., 2003). The resulted grafted psyllium samples were not soluble in commonly used solvents or their combinations. In 2007, methacrylic acid derivatives of psyllium were prepared using ammonium persulfate as initiator and cross-linked using N,N-methylenebisacryla-mide as the crosslinker (Kumar and Verma, 2007). The modified psyllium... 

With radiation grafting, there is also an additional mechanism for enhancement unique to acid and not applicable to the polyfunctional monomer additives. This process is particularly relevant to irradiations performed in air and involves the acid induced decomposition of peroxy species formed radiolytically in the backbone polymer, thus generating further sites where copolymerisation may occur (Equation 3). Current evidence (17) indicates that the contribution... 

Both mechanical means and transfection reagents, among others, have been used to facilitate the cellular uptake of oligonucleotides. The application of intraluminal pressure enhances the uptake of particular oligonucleotides in vascular tissues such as carotid arteries or venous bypass grafts [14, 15]. Other approaches use chemical modifications in order to secondarily modify the nucleic acid backbone [16, 17]. In general, these modifications increase uptake through the cell membrane based on the classical receptor-mediated endocytosis pathway. However, once inside the cell, most nucleic acid compounds taken up by endocytosis are ultimately trapped in the lysosomal compartment... 

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