Polymerization reaction ROMP

It has been shown that [(r]6-arene)RuCl2]2 6 and [(r 6-arene)RuCl2] PR3 7 complexes can be activated in situ to afford active metathesis catalysts, either on treatment with diazoalkanes [15] or by UV irradiation [16]. The structure of the active species thus formed is unknown, but it initiates the ring opening metathesis polymerization reactions (ROMP) of various cycloalkenes very efficiently. Therefore these in situ recipes may also be useful in the context of preparative organic chemistry. 

At this point it is appropriate to discuss the mechanism for ADMET, because ADMET polymerization is more involved than its chain polymerization counterpart— ROMP. Figure 8.6 illustrates the accepted mechanistic pathway which leads to productive metathesis polymerization, as first described by Wagener et al.14a A general model reaction between an a,o>-diene with a metal alkylidene... 

For the synthesis of carbohydrate-substituted block copolymers, it might be expected that the addition of acid to the polymerization reactions would result in a rate increase. Indeed, the ROMP of saccharide-modified monomers, when conducted in the presence of para-toluene sulfonic acid under emulsion conditions, successfully yielded block copolymers [52]. A key to the success of these reactions was the isolation of the initiated species, which resulted in its separation from the dissociated phosphine. The initiated ruthenium complex was isolated by starting the polymerization in acidic organic solution, from which the reactive species precipitated. The solvent was removed, and the reactive species was washed with additional degassed solvent. The polymerization was completed under emulsion conditions (in water and DTAB), and additional blocks were generated by the sequential addition of the different monomers. This method of polymerization was successful for both the mannose/galactose polymer and for the mannose polymer with the intervening diol sequence (Fig. 16A,B). 

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

The polymerization of cyclic, strained olefins by transition metal alkylidenes of general formula L M = CRR (L = ligand, R, R = H, alkyl, aryl) yields polymers formed via ring-opening that contain unsaturated double bonds within each repetitive unit. Since the mechanism is based on repetitive metathesis steps, this polymerization reaction is known as ring-opening metathesis polymerization (ROMP) (Scheme 1). 

Initially, the polymerization of macromonomers was achieved by free radical polymerization reactions, which allowed only a limited control of the final properties. With the advent of ROMP and new free radical polymerization techniques, such as atom transfer radical polymerization (ATRP) the control of final properties became more facile (16). ATRP and ROMP techniques can be combined for the synthesis of macroinitiators (17). 

Living ring opening metathesis polymerization is a special kind of ROMP. In order to approach the conditions of a living polymerization reaction, the following requirements must be fulfilled (12) ... 

Although the alkylidene complexes initiate the ROMP of functionalized norbomenes and 7-oxanorbomenes in aqueous solution quickly and completely in the absence of acid, the propagating species in these reactions often decompose before the polymerization reaction is complete. For example, in the ROMP of the water-soluble... 

In absence of excess of a second reaction partner, polymerization occurs (ROMP) ... 

In the context of ROMP chemistry, living polymerization reaction conditions have only been observed when well-defined carbene complexes are used as the catalysts. The first catalyst to behave in this fashion was the titanocene complex (4), while more recently, complexes containing Ta, W, and Mo have been shown to be catalysts for the living ROMP of a variety of cyclic alkenes. The Mo complex (5b) is an especially promising catalyst since it is compatible with a number of functional groups and thus can be used to synthesize a variety of functionalized polymers. 

D-Limonene and ot-pinene have been used as renewable solvents and chain transfer agents in metallocene-methylaluminoxane (MAO) catalysed polymerization of ot-olefins. Chain transfer from the catalyst to the solvent reduces the achieved in limonene compared with toluene and also reduces the overall catalyst activity. This was confirmed, as in the ROMP studies, by performing identical reactions in hydrogenated limonene. However, an increase in stereospecificity was seen when D-limonene was used as the solvent. This is measured as the mole fraction of [mmmm] pentads seen in NMR spectra of the polymer. 100% isotactic polypropylene would give a value of 1.0. On performing the same propylene polymerization reactions in toluene and then in limonene, the mole fraction of [mmmm] pentads increased from 0.86 to 0.94, indicating that using a chiral solvent influences the outcome of stereospecific polymerizations. Unfortunately, when a-pinene was used, some poly(a-pinene) was found to form and this contaminates the main polymer product. 

Although alkyne metathesis has been used in polymerization reactions, these applications are not nearly as well advanced as those involving ROMP or ADMET polymerization of alkenes. Further discussion of alkyne metathesis polymerizations is beyond the scope of this text. 

ADMET has been shown to be a step-growth polycondensation reaction [31[. The kinetics of step-growth polymerization and consequences thereof are completely different than those of chain polymerizations. Since ROMP and many other single-site transition metal-catalyzed polymerizations discussed in this book proceed... 

Fig. 4.24. Another variation of the alkyne metathesis reaction alkyne ring-opening metathesis polymerization (alkyne ROMP).

The metathesis reaction may be used to produce a range of polymers by two processes (Scheme 10). Cyclic olefins with a threshold level of ring strain can be polymerized via ROMP chain process. Under appropriate conditions, certain acyclic dienes can be polymerized in a condensation process known as acyclic-diene metathesis polymerization, or ADMET. The elimination of a volatile olefin byproduct (usually ethylene) provides a driving force for the latter process, while alleviation of ring strain is the primary driver in ROMP. Because ADMET is a condensation... 

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