Atom transfer radical polymerization controlled chain lengths

ESR spectroscopy was successfully applied to quantify radical concentration in the polymerizations [4, 8-11], However, the direct detection method of ESR did not reveal information on many additional points that ate very significant in radical polymerization chemistry so far. For example, the length of propagating chain is not known, direct observation of the penultimate unit effect is almost impossible, and detailed mechanisms of radical reactions remain extremely difficult to examine. These problems have not yet been fully resolved but the development of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP), enables us to resolve some of these problems. 

We have seen previously that polymerization initiated by free-radicals suffers from some disadvantages. Mainly, the chain-length cannot be controlled and branching occurs. Some of these disadvantages are overcome in newer methods of radical polymerization. An important new development in this regard is the atom transfer radical polymerization (ATRP) [29-30]. In this process all the chains are initiated essentially at the same time (at the point of catalyst injection) and all the chains grow at the same rate until the monomer is consumed. The important principles of the atom transfer polymerization process are illustrated by the following sequence of reac-... 

This type of chemistry towards well-defined star-shaped (polymer)j,Qo is closely linked to progress made during this last decade in radical polymerization [3]. Indeed, the various controlled radical polymerizations such as nitroxide-mediated polymerization (NMP) or atom transfer radical polymerization (ATRP) allow us to produce polymer chains with tailored chain length and low polydisper-sities. Furthermore, the as-prepared chains bear at their end a function (stable radical or C—halogen bond) that can be converted under the right conditions into a macro-radical. If this latter is produced in the presence of Cgo, several chains should add to the fullerene. 

TEMPO (2,2,6,6-tetramethylpiperidinyloxy) [82], has been shown to afford hyperbranched structures. Copol)maerization with styrene was also shown to afford branched polymers with a controlled branch and chain length. Atom transfer radical polymerization (ATRP) [83] of /7-(chloromethyl)styrene similarly provided hyperbranched pol)miers [79]. 

One of the problems of radical polymerization is high-termination-rate constants by combination ( ) or by disproportionation ( ). In view of this, polymer chains of controlled chain length cannot be formed and this technique is ill-suited for precise control of molecular structure (e.g., in star, comb, dendri-mers, etc.) required for newer apphcations like microelectronics. The major breakthrough occurred when nonterminatmg initiators (which are also stable radicals) were used. Because of its nonterminating nature, this is sometimes called living radical polymerization and the first initiator that was utilized for this purpose was TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxo) [36,37]. A variation of this is atom-transfer radical polymerization (ATRP) in which, say for styrene, a mixture of 1 mol% of 1-phenyl ether chloride (R—X) and 1 mol% CuCl with two equivalents of bipyridine (bpy) is used for initiation of polymerization. Upon heating at 130°C in a sealed tube, bpy forms a complex with CuCl (bpy/CuCl),... 

In Scheme 3.7(a), the transfer of a radical center from a polymeric radical to another polymer chain occurs via H-atom abstraction. Addition of monomer to the resulting midchain radical produces a polymer with a branch point, with the final length of the newly formed branch controlled by the kinetic chain length of the system. An additional subscript is added to track the number of LCBs formed. 

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