Radical polymerization carbon-hydrogen bond, reaction

In 1947 Szwarc prepared a white polymeric material u by rapid flow pyrolysis of p-xylene under reduced pressure. On the basis of p-xylylene diiodide 2) detected in the reaction mixture of the pyrolysis products with iodine gas he proposed a formation 1,3) of p-xylylene(p-quinodimethane) (QM) in this pyrolysis. He claimed the polymeric material to be poly-p-xylylene(poly-QM)and proposed a mechanism 2) for the formation of poly-QM, involving thermal cleavage of carbon-hydrogen bonds of p-xylene to yield p-xylyl radicals which collide with each other to give p-xylene and QM through disproportionation. QM condenses and polymerizes to produce poly-QM. 

Figure 2.2 Mechanism of "backbiting" in formation of short chain branching initiated by attack of radical on a 5 carbon-hydrogen bond. In the reaction above, homolytic bond scission occurs resulting in a free radical on the 5 carbon atom and an n-butyl branch. R is a polymeric alkyl group.

CHS, and nitric oxide (Chap. 15). Radicals have a typical set of reactions in the gaseous state, many will remove thin films of metals from hot glass, and in solution they often can induce polymerization of compounds having carbon-carbon double bonds. Thus, a solution of ferrous sulfate and hydrogen peroxide (such a mixture is called Fenton s reagent) qatalyzes the conversion of acrylonitrile, CH2=CH—CN, to its polymer neither H202 alone nor Fe2+ alone is effective as a catalyst. 

The mechanism of the Ziegler polymerization has been the subject of many experiments and much speculation. It is certainly not of free radical nature, since hydrogen acts as a chain transfer agent. When tritiated alcohols are added as chain-terminating agents, tritium is found in the polymer. If ( C2H5)3A1 is used as a starter, the polymer is radioactive. Therefore the overall reaction can be formulated only in terms of metal-carbon bond participation ... 

The propagating center is neither an ion nor a radical, but a carbon to carbon double bond at the end of the chain. The monomer anion adds to this double bond. This process is a step-growth polymerization and the monomer anion is called an activated monomer. Not all acrylamide polymerizations, initiated by strong bases, however, proceed by a hydrogen transfer process. Depending upon reaction conditions, such as solvent, monomer concentration, and temperature some polymerizations can take place through the carbon to carbon double bonds [216]. 

Chain transfer reactions mostly proceed by abstraction of a monovalent atom such as hydrogen or a halogen. The scission of a bond carbon - oligovalent (e.g., H) atom is of interest for the introduction of endgroups into a polymer produced in a free radical reaction. Radically induced vinyl monomer polymerization with the possibility of chain transfer to a polymer of different chemical structure present in the reaction mixture leads to graft copolymers if bond scission occurs outside the main chain, no matter whether a single atom or a grouping is abstracted. Quite a different result is obtained if a radical attack involves a bond in the main chain of the polymer, if this bond scission occurs at a monovalent atom, which must be at the chain end, there is block copolymer formation. If bond scission occurs inside the polymer backbone, either block or random copolymers are produced [63]. 

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