Hydrogen abstraction grafting

Hydrogen abstraction grafting to a simple polymer dissolved in the diluent for the dispersion [3.62]... 

In hydrogen abstraction grafting, the initiator abstracts a hydrogen atom from the main dissolved polymer. The graft copolymer is then formed by free-radical copolymerization of an appropriate monomer ... 

For optimization of the hydrogen abstraction grafting process, the soluble polymer must be carefully chosen and should be used with an initiator that generates very reactive free radicals (e.g., benzoyl peroxide). Adventitious hydrogen abstraction and grafting can also occur. Many dispersions, in which steric stabilization is claimed to arise from the adsorption of simple soluble polymers, may thus actually be stabilized by graft copolymers formed in situ during the preparation of the dispersions [3.64],... 

Although the hydrogen abstraction process is a useful technique for many practical purposes, it is not easy to control. Control is considerably improved when a diluent-soluble polymer carrying one or a few copolymerizable groups, often known as a stabilizer precursor, is used under conditions that minimize hydrogen abstraction grafting (e.g., use of initiators such as azobis-isobutyronitrile that produce moderately reactive free radicals). 

It was found that the sulfate radical anion S04 produced photochemically in Scheme (46) is responsible for generating the cellulose derivative macroradicals by hydrogen abstraction, which added the vinyl monomer to produce the grafted copolymer. The main disadvantage of this method is the production of large quantities of undesirable homopolymers in addition to the grafted copolymers. 

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

Irradiation of the polymer in air prior to grafting introduces hydroperoxide groups in the backbone polymer by the intermolecular, intramolecular, or hydrogen abstraction process. 

The hydrogen abstraction from —SH groups is faster than from —OH groups. Hebeish et al. [9] and Misra et al. [10,11] reported the chain-transfer method of initiation of graft copolymerization onto cellulosic substrates with azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) as initiators. 

Hydrogen abstraction from -SH is faster than from -OH groups. It is generally of interest to increase both the yield of polymer and the grafting efficiency and decrease the formation of homopolymer. This can be achieved by proper selection of the grafting conditions, e.g. monomer concentration, initiating system and its application, reaction temperature and time. 

Combination of BP with 2-propanol or amines induces homopolymerization alone. The rate constants of BP 3 - isopropylamine and triethylamine are 2.95 10 and 2.42 1()9m-1s-1, respectively(22) whereas that of BP 3 - isooctane as a model of OPP is 1.0 lO M s l (24). Also hydrogen abstraction from 2-propanol(k=1.0 106 M s"1) (25) is much more efficient than that from aliphatic hydrocarbons. Even methanol is more reactive (k=2.8 10% - s - -) (25) than OPP towards BP 3. The aforementioned results and the finding that surface grafting does not occur in methanol are well interpreted by the following elementary reactions. 

The reaction in THF requires comments. In THF, the reaction may not be true grafting. THF is known to be very susceptible to hydrogen abstraction and furthermore, benzoin isoproply ether... 

It is assumed by some authors that hydroxyl radicals formed from hydrogen peroxide and ferrous ions react with cellulose in a similar manner to ethanol in reaction (XI) and that radical formation caused by such hydrogen abstraction leads to active sites from which graft polymerization can start ... 

However, from the suppression of the hydrogen abstraction from the alcohol mentioned above one has to conclude, as Hermans (40) did, that cellulose will have to compete with the monomer, while at the same time, for grafting to occur, the cellulosic radical formed must be able to react with the monomer at a reasonable rate. It is clear that, as in the case of the persulphate initiation, the grafting due to such mechanism and the yield of copolymer will depend largely on the conditions of the experiment. 

This explanation for the acid effect on the radiation polymerisation of styrene solutions has a direct application in a grafting context in that higher G(H) yields or lowered effective concentrations of radical scavenging impurities will enhance existing hydrogen abstraction reactions from the substrate to create potential grafting sites as well as increasing the... 

These solvent radicals then can yield grafting sites in cellulose by hydrogen abstraction (Equation 3, CeH = cellulose). 

Although benzophenone derivatives have found application in a number of photopolymerization processes (10), these have usually involved cross-linking reactions induced by energy transfer from photo-exdted benzophenones to groups (e.g. cinnamate) attached to the polymer chains. In such cases, the benzophenones act as sensitisers and not as photoinitiators. An e>fample in which hydrogen abstraction by photoexdted benzophenone has been utilized for photoinitiation is the ultra-violet induced grafting of styrene on to polyethylene (8,45). Recently, extensive studies concerned with photoinitiation of vinyl polymerization by benzophenone derivatives in homogeneous media have been carried out, and are described in detail below. 

---