Ring-opening metathesis polymerization molecular weight control

We have reported the first example of a ring-opening metathesis polymerization in C02 [144,145]. In this work, bicyclo[2.2.1]hept-2-ene (norbornene) was polymerized in C02 and C02/methanol mixtures using a Ru(H20)6(tos)2 initiator (see Scheme 6). These reactions were carried out at 65 °C and pressure was varied from 60 to 345 bar they resulted in poly(norbornene) with similar conversions and molecular weights as those obtained in other solvent systems. JH NMR spectroscopy of the poly(norbornene) showed that the product from a polymerization in pure methanol had the same structure as the product from the polymerization in pure C02. More interestingly, it was shown that the cis/trans ratio of the polymer microstructure can be controlled by the addition of a methanol cosolvent to the polymerization medium (see Fig. 12). The poly(norbornene) prepared in pure methanol or in methanol/C02 mixtures had a very high trans-vinylene content, while the polymer prepared in pure C02 had very high ds-vinylene content. These results can be explained by the solvent effects on relative populations of the two different possible metal... 

The lack of polyethylene samples with tailored architectures, controlled molecular weight, low PDI, and absence of branching defects has hmited a systematic study of the relationships between molecular structure and physical properties of polyethylene. Several notable exceptions include cyclic polyethylene that has been synthesized by ring-opening metathesis polymerization (ROMP) (PDI of 2) (Bielawski et al., 2002 Bielawski et al., 2003) and star, comb, H-shape and pom-pom polyethylenes, containing 17—25 ethyl branching defects per 1000 carbons, which were produced by anionic polymerization (Hadjichristidis et al., 2000). 

A new synthetic method to prepare soluble phenyleneviny-lene polymers by the ring-opening metathesis polymerization (ROMP) of 4,7,12,15-tetraoctyloxy-[2.2]paracyclophane-1,9-diene was also presented under microwave irradiation (Figure 23). The polymerization showed a living character and gave polymers of controlled molecular weight, with a narrow polydispersity, fewer chain defects, and an alternating cis, trans-microstructure. 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

The developments over the past few years in the understanding of organometallic mechanisms and structure has led to a new generation of catalysts for the polymerization of cyclic olefins by the ring opening metathesis mechanism. These catalysts provide the means to control molecular weight and molecular weight distribution and to introduce functionality into polyolefins. 

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