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Homopolymer formation

Metal chelates afford a better initiating system as compared to other redox systems since the reactions can be carried out at low temperatures, thus avoiding wastage reactions due to chain transfer. Homopolymer formation is also minimum in these systems. It was observed by Misra et al. [66,67] that the maximum percentage of grafting occurs at a temperature much below the decomposition temperature of the various metal chelates indicating that the chelate instead of undergoing spontaneous decomposition receives some assistance either from the solvent or monomer or from both for the facile decomposition at lower temperature. The solvent or monomer assisted decomposition can be described as ... [Pg.488]

The formed radicals may abstract hydrogen from cellulose or a cellulose derivative leading to the formation of grafted cellulose or may initiate homopolymer formation. [Pg.507]

It was found that acid enhances grafting and homopolymer formation. Analysis of homopolymers shows that acid reduces the chain length but increases the number of grafted chains. [Pg.507]

To avoid homopolymer formation, it is necessary to ensure true molecular contact between the monomer and the polymer. Even if this is initially established, it needs to be maintained during the radiation treatment while the monomer is undergoing conversion. Several methods are used for minimizing the homopolymer formation. These include the addition of metal cations, such as Cu(II) and Fe(II). However, by this metal ion technique, both grafting and homopolymerization are suppressed to a great extent, thus permitting reasonable yield of graft with little homopolymer contamination by the proper selection of the optimum concentration of the inhibitor [83,90,91]. [Pg.510]

TABLE V. Effect of Acid on Homopolymer Formation in the Radiation Grafting of Styrene in Methanol to Polyethylene.0... [Pg.249]

In the pre-radiation procedure, the substrate is first irradiated, usually in vacuo or inert gas, to produce relatively stable free radicals, which are fhen reacted with a monomer, usually at elevated temperatures. A major advantage is minimization of homopolymer formation. ... [Pg.120]

As stated above, the formation of at least some homopolymer is normally inevitable using radical techniques even if the transfer reactions can be reduced to a negligible level. The extent of homopolymer formation then depends mainly on the position of the azo group in the prepolymer ... [Pg.177]

Metallic ions such as Fe+3, Fe+2 and Cu+2 are known to have an inhibitory effect on free-radical polymerisation and have been used for several years in the radiation grafting systems to interdict homopolymer formation. The mechanism of their action is meticulously discussed in the literature [195,... [Pg.270]

This technique is based on the use of a linear polymer with pendant functional groups that can be activated to initiate the polymerization of a second monomer. Based on this definition, the linear precursor polymer can be considered as a multifunctional macromolecular initiator. The importance of the grafting from technique by cationic polymerization of the second monomer increased considerably with the advent of living cationic polymerization. The advantage is the virtual absence of homopolymer formation via chain transfer to monomer. [Pg.41]

The elimination, or at least minimization, of concurrent homopolymer formation. [Pg.5]

A nonagueous medium was chosen in this procedure, as in the work of Gangneux et al (11), to inhibit diffusion of CAS out of the fiber, thus lessening homopolymer formation in the solution, and to increase the availability of the acrylic acid. Selective experiments were performed to examine the potential of homopolymer formation. A mixture of CAS solution, water and acrylic acid was found to form homopolymer throughout whereas a mixture of CAS solution, toluene and acrylic acid formed homopolymer only in the aqueous CAS phase. This offers evidence of the advantages of using a two phase system. Toluene was used instead of benzene, as employed in the French work (11), for safety considerations. [Pg.47]

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. [Pg.141]

Dependence of grafting efficiency and monomeric conversion on reaction time are shown in Figures 2 and 3. It is shown that a rapid polymerization takes place during the first 2 hrs of the reaction the rate then decreases, and after about 8 hrs the reaction almost ceases. It is interest to note that grafting efficiency is directly proportional to time during the first 2 hife of the reaction and after that remains constant at about the 85% level. It seems that the homopolymer formation (see diagrams II-III) is remakably suppressed at pH 5 at which there is not very much ferrous ion in solution to form free OH radical and polymerization takes place mostly on partially xanthated cellulose. [Pg.275]


See other pages where Homopolymer formation is mentioned: [Pg.257]    [Pg.491]    [Pg.492]    [Pg.493]    [Pg.504]    [Pg.504]    [Pg.504]    [Pg.508]    [Pg.535]    [Pg.551]    [Pg.869]    [Pg.256]    [Pg.257]    [Pg.257]    [Pg.261]    [Pg.175]    [Pg.402]    [Pg.142]    [Pg.257]    [Pg.258]    [Pg.37]    [Pg.6]    [Pg.6]    [Pg.6]    [Pg.45]    [Pg.115]    [Pg.115]    [Pg.146]    [Pg.166]    [Pg.269]   
See also in sourсe #XX -- [ Pg.226 , Pg.227 ]




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