CHEM514 COPOLYMERS AND POLYMER BLENDS Graft Copolymerization

Grafting is one polymer modification technique in which a polymer is linked to the backbone of a parent polymer, the substrate, by chemical linkages.

Why grafting? 1)it allows surface modification 2) ) it allows modification such that bulk properties of the substrate remain unchanged while new modified properties are acquired from the polymer, which is grafted. Hence a polymer which fits for one application due its original bulk properties but does not fit due to some other unwanted qualities may be modified to enhance the required properties and minimize or overcome the unsuitable properties 3) Graft copolymers possess mechanical properties, solution properties different than linear polymers due to the branched nature of the polymer as a result of grafted chains.

How grafting is achieved? There are various methods of graft copolymerization. In general grafting is achieved by creating active sites on the backbone of the substrate polymer initiating polymerization of the monomer to be grafted on the polymer backbone. Creation of active sites and initiation of the polymerization can be achieved by using a free radical initiator, an ionic initiator, irradiation or enzymes.

Chemical Initiation 1.Free radical. 2.Ionic. 3. Living Radical Polymerization such as ATRP

Free radical grafting Most common method is free radical initiation Free radicals are produced from the initiator and transferred to the substrate to react with monomer to form the graft copolymer. General mechanism of free radical grafting:

1.Indirect oxidation Indirect method for free radical production 2+ / such as M 2+ / H 2 O 2, per sulphates. / Fe 2+ /H 2 O 2 : Fenton’s reagent / Fe 2+ /S 2 O 8 2- S 2 O 8 2- /HSO 3 - / Fe 2+ /hydroperoxides

Initiation Reactions for Various Systems / Fe 2+ /H 2 O 2 : Fenton’s reagent / Fe 2+ /S 2 O 8 2- S 2 O 8 2- /HSO 3 Note: Write down each one from (Bhattacharya and Misra, 2004 )

Direct Oxidation of the Substrate: Initiation by Ce 4+ Write from (Bhattacharya and Misra, 2004 )

Grafting onto Chitosan by per sulphate initiation 11

Grafting onto Chitosan by Ce 4+ Initiation (direct oxidation) 12

3. Free radical Initiators some compounds can produce free radicals on heating. Azo compounds, peroxides, hydroperoxides. Benzoyl peroxide; Azoisobutyronitrile AIBN.

4. Chemical pretreatment, such as ozonation, diazotation, xanthation. Study Scheme 3 Chemical pre-treatment methods ((Bhattacharya and Misra, 2004 )

Ionic grafting similar mechanism anionic or cationic active sites rather than radicals. Advantages of chemical grafting: simple and relatively inexpensive can be carried out in aqueous solution under mild conditions. Disadvantages simultaneous formation of homopolymer A heterogeneous distribution of grafting sites and grafted chain size Degradation of the chain backbone during grafting Impurities may be present 15

Living polymerization ATRP 16

Living Polymerization NMP and RAFT 17

Advantages: product qualities can be controlled. the graft size and the architecture of the graft copolymer produced can be controlled. homogeneous distribution of the grafting sites and graft size is possible. Homopolymerization possibility is lower though not zero. Disadvantages: More expensive chemicals are used. The reaction conditions can be harsh. 18

Study Scheme 4-9 in (Bhattacharya and Misra, 2004 )

Irradiation Free radicals or ions may be created by the irradiation of the polymer. The presence of an initiator is not required. Free from contamination, high purity. Molecular weight of the products can be better regulated in radiation.

There are three types: 1. pre-irradiation 2.Peroxidation 3.Mutual irradiation.

1. Pre-irradiation Polymer backbone is first irradiated in vacuum or in the presence of an inert gas to generate free radicals. The irradiated polymer substrate is treated with the monomer. The monomers are not exposed to radiation, so this technique is relatively free from homopolymer formation. Write eqns:

2. Peroxidation Polymer is subjected to high-energy radiation in the presence of air or oxygen to form hydroperoxides or diperoxides. Stable peroxy products are then treated with the monomer at high temperature. The peroxides undergo decomposition to radicals. Then they initiate grafting. The intermediate peroxy products can be stored for long period before performing the grafting step. Write eqns:

3. Mutual irradiation The polymer and the monomer are irradiated simultaneously to form free radicals. Disadvantage: homopolymer formation Write eqns:

Ionic grafting The ions formed through high-energy irradiation. Two different types, cationic and anionic. The polymer is irradiated to form the polymeric ion, then reacted with the monomer to form the grafted copolymer. Advantage of ionic grafting is high reaction rate. Small radiation doses are sufficient to achieve grafting. Write eqns:

Radiation Induced Grafting UV or photo initiated Microwave initiated Gamma initiated A similar mechanism to free radical mechanism given before free radicals generated by the action of radiation on the molecules. In UV and microwave initiation: small penetration depth only surface modification is possible. Gamma radiation higher penetration power but high energy radiation may lead degradation or to crosslinking. Homopolymerization is possible. Purification needed. 26

Photochemical grafting Chromophore on a macromolecule absorbs light. Then it goes to an excited state. After that they dissociate into reactive free radicals. Addition of photosensitizer is required If absorption of light does not lead to the formation of free radical sites Benzophenone is an example of photosensitizer. The sensitizer forms free radical, which can undergo diffusion so that they abstract H atom from the base polymer, and producing the radicals site required for grafting.

Study Scheme 12 Mechanism for photochemical grafting method (Bhattacharya and Misra, 2004 )

Enzymatic grafting This is a new method. Enzyme work as an initiator for the chemical grafting reaction. There are several potential advantages for the use of enzymes in polymer synthesis and modification. respect to health and safety. enzymes offer the potential of eliminating the hazards associated with reactive reagents. Finally, enzymes specificity may offer the potential for precisely modifying macromolecular structure to better control polymer function.

Oxidative enzymes such as peroxidases have been used to generate active sites on the polymer backbone. The monomer is then grafted on the polymer backbone by a so called ‘domino type’ of reaction. The radical site created reacts with a monomer to graft one monomer. Then a radical is created on the monomer, which adds to another monomer. Hence homopolymerization does not occur and a clean product is obtained. It is an environmentally clean process.

Factors affecting grafting yield Monomer Substrate Initiator Temperature Time Additives Solvent pH Homogeneous or heterogeneous conditions 31

Controlling factors of grafting nature of the backbone the nature of the backbone (physical nature, chemical composition is important. Solubility is limited due to the big size of the polymeric chain. Swelling or dissolution of the backbone may take place in the presence of an appropriate solvent, which enhances the mobility of radicals. As the crystallinity decreases, grafting becomes easier. The presence of functional groups in the backbone also influences grafting.

Effect of monomer the reactivity of the monomer is also important in grafting. Factors affecting are polarity and steric nature, concentration of monomer. Grafting also depend upon the stability of the radicals. Monomer concentration has an influence on the grafting efficiency. The grafting efficiency increases with monomer concentration up to a certain limit and then decreases with further increase in the monomer concentration. Further increase in monomer concentration increases the homopolymerization reaction rather than grafting.

Effect of solvent solubility of monomer in the solvent is an important parameter. Swelling of polymer backbone in the solvent also enhance the grafting efficiency. Swelling of backbone facilitates the accessibility of the monomer to the active sites. Water is an excellent medium for grafting, because it has zero chain transfer constant. Chain transfer and H-abstractions decrease grafting.

Effect of initiator the nature of the initiator is important initiator concentration also effect the rate of grafting. Once a certain initiator concentration is reached, higher level of initiator does not increase the conversion of grafted monomer. above certain limit, extra radicals will participate in the termination of the growing polymer and lead to decrease in grafting. The solubility of the initiator in the grafting medium is another important factor.

Additives the presence of additives such as metal ions, acid and inorganic salts has an effect on the grafting yield. The addition of additives causes a competition between the reaction between the monomer and the backbone from one side, and the reaction between the monomer and additives from other side. Additives may enhance the reaction between monomer and backbone, and this leads to increase in grafting. sometimes the opposite will happen if the monomer/additives reaction is dominant. Sulfuric acid is an example of an effective additives. presence of an inorganic salt (e.g. LiClO 4 or LiNO 3 ) will improve the grafting due to monomer partitioning phenomena. Presence of metal ion (e.g. Fe 2+, Cu 2+ also enhance the grafting efficiency, because they reduce the homopolymer formation.

Effect of temperature.  The temperature is one of the prime factors that control kinetics of graft copolymerization.  Increasing temperature increases the grafting yield until a limit is attained.  increasing temperature leads to faster monomeric diffusion processes in the backbone.  Thermal decomposition rate of initiator increases as temperature becomes higher, so free radicals production enhance.  The maximum graft yield occurs for a temperature near the glass transition temperature.  At temperature below Tg the diffusion of monomer not enough so the polymeric radical cannot react with monomers.  At temperature. Above Tg, the number of radicals available for grafting will decrease with increasing temperature, due to combination of monomer radicals and result is lower grafting yield.

Figure 1 : (a) Effect of monomer concentration on the grafting yield conditions (b) Effect of reaction time on the grafting yield © Effect temperature on the grafting yield(d) Effect of initiator concentration on the grafting yield (Adali and Yilmaz, 2009)

References 1. A Bhattacharya, BN Misra, “Grafting: a versatile means to modify polymers: Techniques, factors and applications” Progress in Polymer Science, 2004, pp. 767– T Adali, E Yilmaz, “Synthesis, characterization and biocompatibility studies on chitosan-graft- poly(EGDMA)” Carbohydrate Polymers, 2009, pp