Cationic polymerization inductive influences

Figure 9-1 illustrates inductive influences in cationic polymerizations. The electron-releasing inductive influence of alkyl groups causes isobutene to polymerize very quickly at low temperatures where propylene reacts inefficiently and ethylene is practically inert. For similar reasons, a-methylstyrene (9-12) is more reactive than styrene, and substitution of an electron-withdrawing halogen for an ortho- or/ ara-hydrogen, decreases the monomer reactivity still further. As a corollary, ortho- and para-electron-releasing substituents (RO—, RS—, aryl) increase cation stability and monomer reactivity. 

The thermodynamics of TXN polymerization also influences the polymerization kinetics. Kern 92) has proposed that induction periods, frequently observed in the cationic polymerization of TXN, are due to the build up of the equilibrium formaldehyde concentration, which has to occur before polymer can be formed. According to other authors, it is not formaldehyde but TTXN that must be formed prior to polymerization 93). We conclude that formation of polymer may require that the equilibrium concentration of any monomeric species which is in equilibrium with polymer is reached first. 

On the other hand copolymer with a trioxane unit at the cationic chain end (Pi+) may be converted intp P2+ by cleavage of several formaldehyde units. These side reactions change the nature of the active chain ends without participation of the actual monomers trioxane and dioxo-lane. Such reactions are not provided for in the kinetic scheme of Mayo and Lewis. In their conventional scheme, conversion of Pi+ to P2+ is assumed to take place exclusively by addition of monomer M2. Polymerization of trioxane with dioxolane actually is a ternary copolymerization after the induction period one of the three monomers—formaldehyde— is present in its equilibrium concentration. Being the most reactive monomer it still exerts a strong influence on the course of copolymerization (9). This makes it impossible to apply the conventional copolymerization equation and complicates the process considerably. 

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