OTHER LIVING COORDINATION POLYMERIZATIONS

The purpose of this review is to report on the recent developments in the macromolecular engineering of aliphatic polyesters. First, the possibilities offered by the living (co)polymerization of (di)lactones will be reviewed. The second part is devoted to the synthesis of block and graft copolymers, combining the living coordination ROP of (di)lactones with other living/controlled polymerization mechanisms of other cyclic and unsaturated comonomers. Finally, several examples of novel types of materials prepared by this macromolecular engineering will be presented. 

Molecular hydrogen has been known for a long time as an effective chain-transfer agent in the coordination polymerization of ethylene and a-oiefms with Ziegler-Natta catalysts 99-101,50). The mechanism for the reaction of a growing polymer chain with H2 has not been established, The living coordination polymerization system is well suited for an elucidation of the mechanism, since the reaction with H2 can be studied independently of any interference from other chain-terminating processes. 

The predominant approach toward the synthesis of olefin-based BCPs has focused on development of living coordination polymerization systems. Unfortunately, one feature that makes coordination polymerization catalysts so efficient for production of RCPs also limits their use for synthesis of conventional BCPs. These catalysts are susceptible to several chain termination and transfer mechanisms and typically produce many chains during polymerization. Therefore, a sequential monomer addition scheme produces a physical polymer blend with a conventional catalyst (Scheme 1). However, by designing systems that suppress these termination processes, advanced catalysts have been used to make BCPs via sequential monomer addition techniques (Scheme 1). These systems have produced many new BCPs with interesting structures. Unfortunately, the fundamental features that enable precision synthesis also make the processes very inefficient and thus of limited commercial appeal. Conventional catalysts produce hundreds to thousands of chains per metal center, but these living systems produce only one. For these materials to be competitive with other large-volume TPEs, more efficient protocols for BCP synthesis must be developed. 

First, new "living" initiators have been discovered (although not always as efficient), which respond to other mechanisms, i.e. cationic (5) or even radical ones (6), and can accordingly accomodate other types of monomers. A recent typical example is the coordination polymerization of butadiene by bis (n3-allyl-trifluoro-acetato-nickel) to yield a "living" pure 1.4 cis-poly-butadienyl-nickel, able to initiate in turn the polymerization of monomers like isoprene or styrene (7). 

Coordination Polymerization of 1,3-Dienes Single-Site (or Metallocene) Catalysts Living Radical Polymerizations Other Types of Polymerizations, Polymers Ring-Opening Polymerization... 

Coordination polymerization of dienes has progressed significantly within the last decade. Selective polymerization of 1,3-dienes is reinforced by conventional transition metal catalysts and by new organolanthanide catalysts. Nonconjugated dienes also polymerize selectively to produce polymers with cyclic units or vinyl pendant groups. Living polymerization of dienes has become common, which enabled preparation of block copolymers of dienes with alkenes and other monomers. Another new topic in this field is the polymerization of allenes and methylenecycloalkanes catalyzed by late transition metal complexes. These reactive dienes and derivatives provide polymers with novel structure as well as functionalized polymers. The precision polymerization of 1,2-, 1,3-, and l,n-dienes, achieved in recent years, will be developed to construct new polymer materials with olefin functionality. 

The book contains 22 chapters, logically organized into an overview chapter with three other subsections Part one The synthesis of functional polymers by direct methods, such as anionic, cationic, free radical and coordination polymerization. Part two The synthesis of functional polymers by postpolymerization functionalization. Part three Novel approaches and structures. Special emphasis is given to more modem techniques Aat allow for controlled and directed functionalization via living polymerization. 

A new strategy has been proposed for the one-step synthesis of block copolymers, based on living/controlled free-radical process. It involves the use of an asymmetric difimctional initiator that is able to start simultaneous polymerization of two comonomers by different polymerization chemistries in such a way that this initiator remains attached to each type of the growing chain (Mecerreyes et al., 1998). The implementation of one-step synthesis is not simple, however. The two catalysts must be tolerant to each other as also to the two comonomers and the reaction temperature must be closely controlled. Living radical polymerization and ROP by coordination and insertion can meet these requirements. 

In chain reactions the different types of monomers can be added subsequently to an active chain end. The most important techniques here are sequential living polymerization techniques, such as anionic or cationic polymerization. Certain metallocenes can be used in coordination polymerization of olefins leading to stereo block copolymers, like polypropylene where crystalline and amorphous blocks alternate with each other due to the change of tacticity along the chain [34]. In comparison to living polymerization techniques, free radical and coordination polymerization lead to rather polydisperse materials in terms of the number of blocks and their degree of polymerization. 

In addition, Cui-N-propyl-2-pyridylmethanimine mediated "living" radical polymerization of vinyl monomers by use of l-butyl-3-methylimidazolium hexafluorophosphate as solvent has been reported (Carmichael et al., 2000). It has been pointed out that the rate of polymerization was enhanced in comparison to other polar/coordinating solvents. Moreover, the polymerization product was made copper free by a simple solvent wash, which avoids the contamination of the polymer product by the catalyst. Other atom transfer radical polymerizations in ionic liquids have recently been reported (Sarbu Matyjaszewski, 2001 Biendron Kubisa, 2001). 

The quantitative data on fep in coordination polymerization are available almost exclusively for cyclic esters. A more detailed analysis of the reactivity-selectivity relationships has been presented in the already published papers. Among the first works when coordination initiators were used, the works of Teyssie et al. on Al(O Pr)3 ° and the dinuclear i-oxo-alkoxido compounds have to be mentioned.The actual step of propagation on the >Al-0-... bond has been described in terms close to our present understanding of this reaction. The living character of polymerization has also been demonstrated. Some time later, Kricheldorf et revealed a similar behavior for the other multivalent metal (Sn(IV), Ti, Zr) alkoxides. 

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