Molar mass branching polycondensation

The chemical constraint reduces the number of possible reactions considerably, and consequently it leads to a much narrower molar mass distribution. Furthermore, the extent of reaction a of the A-group can cover all values from zero to unity, but the extent of reaction P of the equally reactive 5-groups cannot become larger than P=a/(f-l). One important consequence of this strict constraint is that gelation can never occur [1,13]. A much higher branching density than by random polycondensation can be achieved. For this reason one nowadays speaks of hyperbranching. 

Very early reports on these systems described them as polycondensates, consisting of broad molar-mass distributions with randomly branched topologies. The methods of synthesis included Friedel-Crafts coupling of benzyl alcohols [108] and the polymerization of 2,5,6-tribromophenol involving aryl ether formation [109], In addition, hyperbranched natural carbohydrate polymers, such as amylopectin, dextrin, and glycogen have been extensively studied [73-75]. 

Control of Mn was feasible below 10 kDa and DPs <1.5 were found. Higher molar masses with little control via the M/I ratio were obtained from other initiators [94—97]. Numerous polymerizations of metal salts of various halophenols were reported by several research groups beginning with the work Torey and Hunter in 1911 [98]. In many cases, more than one haloatom was attached to the phenol and branching or other side reactions occurred. The hypothesis of CCP versus polycondensation was not discussed. A detailed review of aU these polymerizations has been published [99]. 

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