ROMP polymeric architectures

Anionic polymerization was for a long time the only viable process to engineer sophisticated polymeric structures. Recently, the ROMP of cycloolefins has emerged of late as a powerful tool of macromolecular engineering. Its robust and "living" character that is associated with the development of catalysts tolerant of polar functions has been exploited to assemble original polymeric architectures. 

Figure 28 Synthesis of 7-oxanorbornene macromonomer derived from two (G = 1)dendronsfoiiowedby ROMP polymerization into a linear dendritic hybrid architecture. (Courtesy of Macromolecules, 39(19) 5786, 1997. Copyright 1997 American Chemical Society.)...

The catalysts mentioned above are the tools used to build various polymeric architectures via ROMP, ranging from the simplest linear homopol3mier to intricate double helices. This chapter aims to delve into these architectures from the most basic to the most complex. In many cases, only representative examples are discussed and should only be used as a guide as the inclusion of all work from the field into a single chapter is not possible. Ultimately, this chapter aims to give its readers an appreciation and understanding of the stmctural opportunities already obtained through ROMP and a vision for its future. 

Developments in the area of initiators for ROMP have resulted in the creation of a large armory of transition-metal compounds suited to these purposes. In addition, while the mechanistic details of Schrock-type initiators are well established, the mechanistic understanding of ROMP-particularly with ruthenium-based initiators-has experienced impressive progress such that today, highly sophisticated polymeric architectures can be produced that quite recently were barely achievable with other polymerization methods. Yet, further progress in both polymer and materials science may well be expected. 

A change of architecture is another route that enables diversification of the properties of aliphatic polyesters. This review will focus on star-shaped, graft, macrocyclic, and crosslinked aliphatic polyesters. It must be noted that the ROP of lactones has been combined with several other polymerization mechanisms such as ROP of other heterocyclic monomers, ionic polymerization, ROMP, and radical polymerization. Nevertheless, this review will not cover these examples and will focus on polymers exclusively made up of poly(lactone)s. 

The third class of olefin methathesis in Scheme 21.1 is addition metathesis polymerization (ADMET). This reaction is an alternative method to stitch together olefins into polymers, in this case by a combination of dienes with extrusion of ethylene. Control of molecular weight by the ADMET process is less precise than that by ROMP, but this reaction has been used to make polymers with precise architectures, such as polymers that would be perfectly alternating ethylene-propylene copolymers. ... 

Two kinds of well-defined copolymers were synthesized by sequential ROMP of PS and PEO or PB macromonomers [4,9]. The strategy used to obtain the expected architecture is illustrated in Scheme 3. The order of polymerization of macromonomers appeared to be essential for a complete crossover to occur indeed give rise to propagating species of highest reactivity. 

This contribution discusses the molecular characteristics of a series of original nonlinear and yet well-defined architectures based on PS, PB and PEO that were obtained by ROM polymerization or copolymerization of the corresponding macromonomers. Unlike other "living" chain addition polymerization, ROMP is robust and versatile and could successfully be applied to shape very common polymers into particular forms. 

This chapter discussed only a subset of the work conducted in the field of ROMP in the last 10 years, showing that. ROMP is a privileged polymerization method for the preparation of highly functionalized polymers and is used in almost every contemporary polymer research field. ROMP is fast, functional-group tolerant, reliable, flexible, and versatile, and allows the synthesis of a broad spectrum of different polymer architectures. However, precise control of the microstructure of the polymers is still a challenge, particularly in case of ruthenium-based initiators, which, in practice, are the most commonly employed. 

The group of Nomura has explored the end-functionalization of molybdenum carbene initiated ROMP polymers extensively. Terminal monool [81, 82] or diols [83] were prepared via ROMP and used to synthesize different copolymer architectures. Pyridine and bi-pyridine ligands were successfully introduced to the polymer chain end in order to complex to ruthenium carbenes. Polymeric recyclable hydrogen transfer reduction catalysts were prepared in this manner [84, 85]. Notestein et al. [86] used a polymeric mono-aldehyde to functionally terminate a living ROMP initiated with a molybdenum catalyst to prepare diblock copolymers during the end-capping step. 

To expand the variety of monomers available for polymerization and to increase the range of copolymer and nanoparticle architectures, attention has turned toward the development of methods whereby multiple polymerization techniques can be utilized to generate unique and functionally diverse materials. The grafting-from (Figure 6.29) approaches to ROMP-based micellar... 

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