Ring-opening metathesis polymerization cyclopentene

Whereas only limited stereoselectivity is characteristic of the metathesis of acyclic olefins, ring-opening metathesis polymerization of cycloalkenes may be highly stereoselective provided the proper catalysts and reaction conditions are selected. Cyclopentene, for instance, is transformed to either all-cis [Eq. (12.25)] or all-frans polypentenamers [Eq. (12.26)] in the presence of tungsten catalysts 21 92... 

Alkene metathesis, a remarkable reaction catalyzed by transition metal catalysts, can be traced back to Ziegler-Natta chemistry as its origin [11], In 1964, Natta et al. reported a new type polymerization of cyclopentene using Mo- or W-based catalyst, without knowing the mechanism. This was the first example of ring-opening metathesis polymerization (ROMP eq. 1.9) [12],... 

R and R refer to organic groups that contain some number of carbon atoms and may or may not be different from each other. With cyclic alkenes, metathesis leads to polymerization (specifically, ring-opening metathesis polymerization [ROMP]), shown here for cyclopentene ... 

When certain cycloalkanes are used in metathesis reactions, ring-opening metathesis polymerization (ROMP) occurs to form a high molecular weight polymer, as shown with cyclopentene as the starting material. The reaction is driven to completion by relief of strain in the cycloalkene. 

Weiss and co-workers reported a series of papers on the use of alkylidyne complexes as precursors for olefin metathesis catalysts. W(CCMe3)-Cl3(dme) was found to initiate ring-opening metathesis polymerization of cyclopentene [Eq. (209)] and metathesis of -hexene as well as other 1-alkenes 203.204). The rate of metathesis is slowed down by branching... 

This was utilized to form several novel homopolymers with l,2-bis(3-thienyl)cyclopentene derivatives using ring-opening metathesis polymerization techniques. The color forming cyclization reaction in the polymers can be illustrated as follow ... 

During ring-opening metathesis polymerization (ROMP) of norbornadiene (NBD) with the Grubbs catalyst a free radical is also observed as a 1 2 1 triplet, which is also formed, but to a much weaker extent, with norbornene (NBE), cyclopentene and 1,7-octadiene. The identity of this triplet, and that of a transient doublet observed together with the triplet in the case of benzonorbornadiene, are discussed as well as the possible role of radicals in initiation of ROMP, crosslinking of ROMP products, and polymerization of a-methylstyrene. 

Ring-opening metathesis polymerization of dicyclopentadiene (DCPD) can take place by two different pathways in the first, the reaction occurs with opening of the norbornene unit leading to linear polydicyclopentadiene while in the second both the norbornene and cyclopentene rings are successively opened forming a cross-linked polymer (Scheme 1) [1-3]. Due to substantial differences in reactivity of the double... 

Table 7.1 Gas-phase, Gibbs free energies at 298 K of the intermediates and transition states in the ring-opening metathesis polymerization (ROMP) of norbornene, cyclopentene, cycloheptene, and Z-cyclooctene. Adapted from Ref [39] . ...

Metallocene/methylaluminoxane (MAO) and other single site catalysts for olefin polymerization have opened a new field of synlhesis in polymer chemistry. Strained cyclic olefins such as cyclobutene, cyclopentene, norbornene (NB), and their substituted compounds can be used as monomers and comonomers in a wide variety of polymers." Much interest is focused on norbornene homo- and copolymers because of the easy availability of norbornene and the special properties of their polymers. Norbornene can be polymerized by ring opening metathesis polymerization (ROMP), giving elastomeric materials, or by double bond opening (addition polymerization). Homopolymerization of norbornene by double bond opening can be achieved by early and late transition metal catalysts, namely Ti, Zr, Hf, Ni, - ° and Pd (Scheme 16.1). 

On the other hand, a hi -trans polymer is formed with TiCU/AlEts [370,380] or WCle in presence of suitable cocatalysts (e.g., AlEts) [380,381], Not only the catalyst and cocatalyst choosen are crucially for the microstructure of the resulting polypentenamers, but also the particular reaction conditions, especially the temperature or concentration [379], Selected examples for the ring-opening metathesis polymerization of cyclopentene are listed in Table 10. 

Cyclopentene has been polymerized in the presence of numerous catalytic systems leading either to poly(ey-clopentenylene) by addition polymerization or to poly-pentenylene or polypentenamer by ring-opening metathesis polymerization. Thus, in an early report, Hoffman [54] observed that the oUgomerization of cyclopentene in the presence of BF3/HF proceeded to dimers, trimers, and tetramers as well as to solid resins. The structure probably corresponded to a 1,2-addition polymer, poly(l,2-cyclopentenylene) [Eq. (10)]. 

P is a polymer chain, W stands for wolfram) Scheme 2 Ring-opening metathesis polymerization of cyclopentene. 

The use of ROMP has been widely applied. Thus, cyclopentene can be polymerized by olefin metathesis to a linear pol5mier. In this reaction the cyclic hydrocarbon is opened up by the catalyst and joined together in a linear fashion (see Eq. 2.34). The catalysts that perform this operation are called ring-opening metathesis catalysts. 

As can be seen in the two examples (cyclopentene and norbomene) polymerization of cyclic olefins leads to polymers containing double bonds at periodic intervals in their backbone. An interesting application of the metathesis polymerization is the preparation of tra 5-polyacetylene [1]. Metathesis of the tricylic monomer leads to the ring-opening of the cyclobutene ring to afford a polymeric derivative. Heating this polymer eliminates bis-trifluoromethylbenzene to leave behind traiis-polyacetylene (see Eq. 2.36). 

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