Carbene-catalyzed ring-opening polymerization

Remarkable development over the last 10-15 years in the synthesis of well-defined functional-group-tolerant ruthenium carbenes (Grubbs-related catalysts) also caused real development of the metathesis-based reactions in organosilicon polymers. For recent reviews on metathesis of organosilicon compounds see Refs. [6,7]. Unsaturated organosilicon polymers can be synthesized via ruthenium carbene catalyzed ring-opening metathesis polymerization (ROMP) of silylsubstituted cycloalkenes (Eq. 113). 

Vishwa Prasad, A., Yinghuai, Z. Syntheses of cyclic poly(lactones) by zwitterionic ring opening polymerization catalyzed by N-heterocyclic carbene. I. Annl Pnivm.. 2013,128, 3411-3416. 

Fig. 3.47. Mechanism of ring-opening metathesis polymerization catalyzed by carbene complexes.

The observation that the metal carbene complex, (CO)5W = C(Ph)2 [22], catalyzed the polymerization of cyclic olefins to ring opened polymers containing the diphenylmethylene unit of the catalyst provided additional evidence that carbenes were involved in the catalytic cycle. The formation of the initiating metal carbenes in the classic systems that consist of transition metal halides and alkylating agents was proposed to involve metal alkylation followed by oc-hydrogen loss, Eq. (6). Methane and propene were detected in the early stages of these reactions [23]. 

The complex is also active in ring-opening metathesis polymerization of 1,5-cyclooctadiene (COD), where the ruthenium—carbene bond is now the initiating point. Therefore, a mixture of MMA and COD undergoes a dual or tandem living polymerization of both monomers to generate block copolymers of COD and MMA, which can be converted into ethylene-block-MMA copolymers on subsequent hydrogenation, also catalyzed by the complex. 

Scheme 1.41. Carbene complexes and their behavior in catalyzing the olefin metathesis (a), ring-opening metathesis polymerization (b), and ring-closing olefin metathesis (c).

With the work by Grubbs et al. [27] and Herrmann et al. [28], the use of ruthenium carbene complexes as homogeneous catalysts for the ROMP (Ring-Opening Metathesis Polymerization) of olefins was estabhshed (see Section 2.4.4.3). The development of catalysts that can catalyze hving polymerization in water was an important goal to achieve, especially for applications in biomedicine. In this context, two water-soluble ruthenium carbene complexes (3 and 4) have been reported that act as initiators for the living polymerization of water-soluble monomers in a quick and quantitative manner [29]. 

Fig. 12 Left. One-Pot enzymatic ring opening and iiving free radical cascade polymerization [44], Right. Block copolymers by combination of enzymatic ROP and carbene-catalyzed ROP [120]...

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