Cyclic polymers ring opening metathesis

T. Sunaga, M. Okita, and T. Asanuma, Preparation of hydrogenated product of cyclic olefin ring-opening metathesis polymer, US Patent 6197 894, assigned to Mitsui Chemicals, Inc. (JP), March 6, 2001. 

Catalysts with an unsymmetrical NHC ligand featuring a vinylic side chain have the unique ability to metathesise their own ligand to form a metaUacycle as shown in Scheme 3.7 [119], Ring opening metathesis will then incorporate the monomers, e.g. cyclooctene, into that cycle until a cyclic polymer is cleaved by another intramolecular metathesis step. The catalyst is recovered and can restart this endless route to cyclic polymers [121]. 

Since one of the substrates is a cyclic alkene there is now the possibility of ring-opening metathesis polymerisation (ROMP) occurring which would result in the formation of polymeric products 34 (n >1). Since polymer synthesis is outside the scope of this review, only alkene cross-metathesis reactions resulting in the formation of monomeric cross-coupled products (for example 30) will be discussed here. 

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

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). 

Interest in cyclic olefin (co)polymers has increased dramatically over the past decade. This is because this class of polymers displays many attractive properties such as high thermal stability, high optical transparency, low dielectric constants, and low moisture absorption. Thus, these polymers can potentially be utilized in many electronic and optical applications. Although cyclic olefin (co)polymers typically include ring-opening metathesis polymers, vinyl-addition homopolymers, and vinyl-addition co-polymers with acyclic co-monomers such as ethylene, we will mainly deal with vinyl-addition homo- and co-polymers in this section. 

The ring-opening metathesis polymerization (ROMP) of cyclic alkenes yields polymers that still contain all double bonds [Eq. (13)]. 

An interesting instance of the ring-opening metathesis polymerisation of cyclic trienes is the polymerisation of the novel conjugated diene 3,4-diisopropylidene-cyclobutane in the presence of a bis(cyclopentadienyl)titanacyclobutane derivative as a catalyst, which affords a linear cross-conjugated polymer [156] ... 

Olefin metathesis has become one of the most important large-scale technical processes for the manufacture of olefins in the petrochemical industry [123]. When cyclic olefins are used as substrates, high-molecular polymers, which are formed by the so-called ring-opening metathesis (ROM), have found applications as elastomers and plastics. Gas-phase studies on the mechanism of olefin metathesis had been confined to simple metal carbenes, for example [Mn=CH2]+, [Fe=CH2]+,and [Co=CH2]+ [124-127]. Most of the metatheses have been observed with deuterated ethylene. 

The reaction is applied in industrial processes (Phillips triolefin process. Shell higher olefin process) and has importance in ring opening-metathesis polymerization (ROMP) in polymer chemistry [1]. In the past, olefin metathesis was not commonly applied in organic synthesis [2] because of the reversibility of the reaction, leading to olefin mixtures. In contrast, industrial processes often handle product mixtures easily. In ROMP, highly strained cyclic olefins allow the equilibrium of the reaction to be shifted towards the product side. 

Ring-opening metathesis polymerization, ROMP, using a cyclic and acyclic olefin monomer was used to prepare co-polymers with monomer alternation exceeding 95%. Monomer-to-ruthenium catalyst studies were also performed to minimize polydis-persities and maximize molecular weights. 

Unsaturated polymers can be produced by means of ring-opening metathesis polymerization (ROMP) of cyclic alkenes. These unique polymers can also be produced via intermolecular Acyclic Diene Metathesis (ADMET). Dienes can also react intramolecularly via Ring Closing Metathesis (RCM) to afford cyclic products. RCM is often applied to synthesis of compounds for fine chemical and pharmaceutical application. Generic examples of these reactions are shown in Figure 2. 

ADMET polymerization is performed on a,co-dienes to produce strictly linear polymers with unsaturated polyethylene backbones, as shown in Scheme 2. This step-growth polymerization is a thermally neutral process driven by the release of a small molecule condensate, ethylene [16-20]. Ring-opening metathesis polymerization (ROMP) is widely used to polymerize cyclic olefins and is performed with the same catalysts as in ADMET polymerizations. 

The developments of the metathesis reaction for polymer synthesis have paralleled the opportunities in acyclic chemistry. Acyclic and cyclic metathesis were discovered independently and only later were recognized as the same reaction. Most of the work in the area of ring-opening metathesis polymerization (ROMP) is outside the scope of this article, and only those areas where the results suggest possibilities in organic synthesis will be discussed in detail. 

Ring-opening metathesis polymerization (Problem 26.28) An olefin metathesis reaction that forms a high molecular weight polymer from certain cyclic alkenes. 

The cross-metathesis between an acyclic and a cyclic alkene, nowadays called ring-opening metathesis (ROM), provides a convenient route to certain polyunsaturated compounds. For example, ethenolysis of cycloalkenes yields a,w-dienes (eq. (6)), which may be useful, e. g., as crosslinking agents in polymers and as precursors for a, -difunctional compounds [13],... 

Amass, A. J. Ring-opening metathesis polymerization of cyclic alkenes. New Methods Polym. Synth. 1991, 76-106. 

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