Current Organic Chemistry - Volume 6, Issue 2, 2002

This review provides a survey of Atom Transfer Radical Addition (ATRA) and Atom Transfer Radical Polymerization (ATRP) with the special emphasis on the copper mediated processes. Mechanistic features of these techniques are reviewed together with the major components of ATRP and examples of materials prepared by ATRP.

The essential reactions in radical polymerisation involve the addition of a radical to a molecule. In the propagation process, the radical is typically a polymer and the molecule is a monomer in transfer, the molecule is a transfer agent and, in initiation, the radical is a species of low molar mass. It is now possible to predict the reactivity of both the radicals and the molecules by means of a revised form of the Patterns Scheme, and a review is presented here of the historical development of the state of understanding of reactivity in the various component steps in a polymerisation process propagated by radicals.

Soluble coordination aggregates are usually multinuclear-multiligand entities (M x L y X z ) n hold together by μ-type of bonds, often said electron-deficient. A number of results scattered in the last three decades literature indicate that the size and shape of these aggregates, when used as catalysts, are factors controlling reactions selectivity with unexpected efficiency. This review aims at collecting and discussing a number of such examples in polymer-organic chemistry, including anionic polymerization of (meth)acrylates, ring-opening polymerization of lactones and oxiranes, coordination polymerization of diolefins and lithiation of alkyl aromatics. Under careful control of kinetic and thermodynamic conditions, those processes may exhibit remarkable chemo-, regio-, stereo- and even chrono-selectivity, provided a fine tailoring of the catalytic aggregate structural characteristics. Moreover, some of the examples strongly suggest the occurrence of a ”topochemical“ control in solution, a behaviour usually typical of solid surfaces. Due to the huge potential variety of such aggregates, extension of these ideas as a general and unifying concept is certainly possible, particularly if more sophisticated physical methods are used for the precise characterization of those entities 3D structure.

The ¹sup3;C-NMR chemical shifts of β-carbon (δCβ ) in the vinyl group of various monomers, which depend on the π-electron density on the carbon, were correlated with their reactivity parameters in polymerization reactions. The relations were studied between the δCβ and the e-value of the monomer and direct evidences were obtained for the validity of Q-e scheme. The meaning of Q-value was discussed in some detail from the careful examination of the correlations between the Q-values of homologous monomers and their ¹sup3;C- or ¹H-NMR chemical shifts and coupling constants for the vinyl groups. The method to estimate the monomer reactivity ratios for the copolymerizations of homologous monomers are proposed. The relative reactivities of monomers which were represented by the log(1 / r 1 ) were correlated with the δCβ values in cationic copolymerizations of styrene derivatives the β-carbon resonates at the higher field, the higher the reactivity is. The reverse was the case in anionic polymerizations. The correlations for the cationic polymerization of alkyl vinyl ethers and the coordinated anionic polymerizations of α-olefines were rather complex and the mechanisms of polymerizations were discussed from the results. The ¹H-NMR chemical shifts of vinyl groups of monomers could also be correlated with the reactivities of monomers, particularly among the homologous monomers. These results revealed that the NMR chemical shift of monomer can be used as a measure of reactivity of monomer and is an important tool for the examination of mechanism of polymerization.

This review summarizes the most popular linking agents (chlorosilanes, chloro(bromo) methylbenzenes, divinylbenzenes, diphenylethylene derivatives), which in combination with anionic polymerization lead to a plethora of macromolecular architectures i.e., symmetric and asymmetric star, α,ω-branched, exact comb and cyclic polymers, etc. Depending on the nature of their reaction product with living macroanions, linking agents can be characterized either as non-living (neutralization of active centers) or living (creation of a new anionic active center). The chlorosilanes and chloro(bromo)methylbenzenes belong to the first category, whereas divinylbenzenes and diphenylethylene derivatives in the second. The most important examples of both categories will be discussed in this review.

This article concerns the catalysis of polycondensations and polyadditions its scope is limited to the use of metal derivatives and enzymes. Palladium, nickel, ruthenium and copper derivatives are studied as catalysts of carbon-carbon forming polycondensations when applied to polymeric systems and models side reactions are analyzed. Palladium-derivatives mainly concern Heck and Suzuki polycondensations. The influence of the nature of the catalyst on reactivity is also an important part of this article it is not limited to carbon-carbon forming reactions but also to other polycondensations such as polyesterifications. Some examples concern systems activited by Ni(0), CsF and rhodium complexes as activators. A general bibliography of enzyme-catalyzed polycondensations is given however only polyesterifications are studied, particularly aliphatic, unsaturated and aromatic polyesters side reactions and linear chains / cycles equilibriums are particularly studied.

Recent topics on in vitro synthesis of polyesters by mainly lipase catalysis are reviewed. Lipase, an enzyme catalyzing an ester bond-cleavage reaction by water in living cells, induces the reverse reaction of hydrolysis, leading to polymer production by a bond-forming reaction. Polyester synthesis has been achieved from various monomer combinations, typically oxyacids or their esters, dicarboxylic acids or their derivatives / glycols, and lactones under mild reaction conditions. Lipase catalyzes ring-opening polymerization of lactones and their enzymatic polymerizability is quite specific in comparison with that by conventional chemical catalysts. Enzymatic synthesis of end-functional polyesters by facile procedures has been developed. By utilizing characteristic catalytic function of lipases, regio- and enantioselective polymerizations proceed to give functional polymers, many of which are difficult to be synthesized by conventional methodologies.