Polydienes free radical

In the free radical oxidation of polybutadienes, both the peroxides and carbonyl groups are formed typically. Consequently, the presence or absence of carbonyl groups in the reaction products of the photooxidation of polydiene may be an indication of the oxidation mechanism. 

Table 2 gives the structural composition of polybutadiene and polyisoprene obtained by free-radical polymerization [46-48]. The fact that the content of the vinyl structure units in polydienes is independent of the polymerization temperature over a wide range and that their content is much higher than the... 

Table 2. Effect of polymerization temperature on the microstructure of polydienes obtained by free-radical polymerization [46-48]...

This is still the most popular method for graft copolymerization of elastomers via free radicals. Free radicals (I) are generated from the same types of initiators which are used for free radical polymerization and copolymerization (see Section 2.4). In general, these radicals are formed in the presence of a polydiene elastomer and a monomer therefore, there are several possible reactions of these initiator-derived radicals which can occur as shown in Eqs. (2.93)-(2.96). The competition between initiation of monomer polymerization (Eq. 

Keoshkerian, B., Georges, M., Quinlan, M., et al., 1998. Polyacrylates and polydienes to high conversion by a stable free radical polymerization process use of reducing agents. Macromolecules 31 (21), 7559-7561. 

Besides the addition of halogens and hydrohalogens across the double bond just covered, there are many other reagents that will react similarly with unsaturated polymers by free radical, ionic, or radical-ion mechanisms. Of prime importance is the addition of ethylene derivatives to polydienes. One of the earliest reactions of natural rubber to be studied in detail was the combination with maleic anhydride (Cunneen and Porter, 1965). Depending on the reaction conditions and the presence or absence of free radical initiators, one or more of four basic reactions may take place, with the products shown (the arrows indicate where the addition has taken place and the new bonds formed). 

Polydienes can be made to undergo a cis-trans isomerization. Through addition of free radicals X to the double bond, this bond can rotate freely. On subsequent elimination of the free radical, the reformed double bond can take up a new position ... 

Table 1.2 Structure of free-radically initiated polydienes...

A new class of polymers with repeating cycloketonic units along their chain can be prepared by reaction of polydiene containing adjacent structural units derived from 1,4-d.s polymerization of conjugated dienes with carbon monoxide. This is best achieved in the presence of a free radical initiator and, preferably, a compound capable of acting as a hydrogen donor [36]. Infrared studies clearly indicate that the carbonyl moiety is incorporated in the forms of cyclopentanones and cyclohexanones. The content of carbonyl units as well as the ratio of cyclopentanones/cyclohexanones depends strongly on the experimental conditions. The simple mechanistic Scheme 5 for the chain modification of the polymer is not adequate to explain such a behavior. 

From these studies the following mechanism for free radical grafting of styrene or methyl methacrylate onto polydienes emerges as consistent with the data (Scheme 1). 

These studies are useful indicators of the reactivity of polydienes to free radicals. However, in the process for making HIPS or ABS the situation is more complex due to the fact that styrene acts as both a polymerizable monomer and an initiator. Stein et al. showed that the final 2% conversion of styrene is important for rubber crosslinking. There is competition between the monomer and the double bonds of the polydiene for the free radicals hence, only as the monomer approaches zero can addition of the free radicals to the elastomer double bonds occur, leading to crosslinking. 

---