Simultaneous Use of Free Radical and Ionic Chain-Growth Polymerizations
This technique allows formation of many different types of block copolymers [437]. Lithium metal can be used to initiate polymerizations in solvents of varying polarity. Monomers, like styrene, a methylstyrene, methyl methacrylate, butyl methacrylate, 2-vinylpyridine, 4-vinyl pyridine, acrylonitrile, and methyl acrylate, can be used. The mechanism of initiation depends upon formation of ion radicals through reactions of lithium with the double bonds:
Propagation reactions proceed from both active sites, the radical and the carbanion. When two different monomers are present, free-radical propagation favors formation of copolymers, while propagation at the other end favors formation of homopolymers. There is a tendency, therefore, to form AB—B type block copolymers. Several publications appeared recently that describe use of controlled living” radical polymerizations to form block copolymers. Thus, Jerome et al. [435] described formation of block copolymers by using an initiator capable of initiating simultaneously dual living polymerizations:
In a similar manner, Yoshida and Osagawa [436] synthesized poly(e-caprolactone) with 2,2,6,6 tetramethylpiperdine-1-oxyl (TEMPO) at one end by anionic polymerization of caprolactone using an aluminum tri(4-oxy-TEMPO) initiator. The TEMPO-supported polycaprolactone behaved as a polymeric counter radical for a controlled/ “living” radical polymerization of styrene to form block copolymers [436]. Also, Kotani et al. [437] reported using controlled/ “living” atom transfer radical polymerization (ATRP) to form block copolymers of ethyl and n-butyl methacrylates. A ternary initiating system that consists of carbon tetrachloride, tris(triphenyl-phosphine) ruthenium dichloride [RuCl2(PPh3)3], and aluminum compounds produced ABA triblock copolymers [437]. Huang and coworkers [437] reported preparation of a series of well-defined amphiphilic block copolymers containing conjugated poly(fluorene) (PF) block and coil like poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA). The block copolymers were synthesized through ATRP. The reactions were initiated by a 2-bromoisobutyrate end-capped macroinitiator using CuCl/ 1,1,4,7,10,10-hexamethyltriethylenetetramine as the catalyst. Matron and Grubbs formed block copolymers by combining ring opening metathesis polymerization with ATRP [437]. Use was made of fast initiating ruthenium metathesis catalyst to form three different monotelechelic poly(oxa)norbornenes. The ends were functionized and ATRP polymerizations of styrene and tert-butyl acrylate followed. Coca et al. [438] showed a general method of transforming living ring opening metathesis polymerization into controlled/“living” atom transfer polymerizations to form block copolymers. Ring opening polymerizations of norbornene or dicyclopentadiene were followed by Witting-like reactions with p-(bromomethyl) benzaldehyde to form efficient (ATP) macroninitiators for formation of block copolymers with styrene [478]:
Other cationic ring opening polymerizations can also be transformed to ATRP to yield block copolymers [439]. Thus, formation of block copolymers was initiated by poly(tetramethylene glycol) containing one bromopropionyl end group. These were used to form block copolymers by ATP polymerization of styrene, methyl methacrylate, and methyl acrylate.
