Grafting Sites on the Cellulose and Mechanism for Okra Bast Fiber

GRAFTING SITES ON THE CELLULOSE AND MECHANISM

Okra bast basically is a cellulosic fibre. The structural unit of cellulose is C6H10O5 or anhydroglucose unit. In the presence of monomer, chain radicals such as (I), (II) and (III) load to graft copolymers, whereas (IV), (V) will give rise to block copolymers.


Actually both types of reactions are found to take place. In addition there will be radicals formed by radiolysis of the monomer and if there is a solvent present by radiolysis of the solvent, these monomer and solvent radicals lead to formation of homopolymer.

Cellulose is not simply possible to modified the properties owing to the fact that the cellulose chains become part of a three dimensions network, the material becomes harder and more rigid.

Grafting onto cellulose is therefore of necessity a heterogeneous reaction in which the physical structures and states of aggregation of the cellulose plays a significant role. The synthesis of cellulose graft copolymer differs from the fully synthetic graft copolymer by the fact that cellulose insoluble in all common organic solvents. Okra bast cellulose is a naturally occurring polymer which is not simply possible to modify in its properties. However, since the greatest number of synthesis involved heterogeneous grafting reactions these will form the first and most important part of the discussion of grafting methods. The polymerization of vinyl monomers may be initiated by free radicals or by certain ions. Free radicals can be generated on a cellulose chain by hydrogen abstraction oxidation, ceric-ion method, diazotisation and introduction of unsaturated groups or by irradiation. If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by growing chain radical or by a radical formed by the polymerization catalyst. This leads to an unshared electron on the cellulosic chain is capable of grafting.

It should be pointed out here that cellulose is a very poor transfer agent (1) and that very little graft copolymer results from the abstraction of hydrogen atoms by a growing chain radical. In most cases it is not the growing chain radical but a radical produced by the initiator which is responsible for formation of graft copolymer.

Thus in case of redox initiation with potassium persulphate (K2S2O8) and ferrous sulphate (FeSO4), •OH free radicals are generated which abstracts a hydrogen atom from the cellulose and thereby leads to grafting.

1 i) K2 S2O8 = 2K+ + S2 O8=
ii) FeSO4 = Fe++ + SO4=
2 i) S2 O8= + Fe (II) Adduct
ii) Adduct Fe (III) + 2SO4-
3 i) Fe (III) + H2O  Fe (II) + + H+ HO.
ii) SO4- + H2O  SO=4 + + H+