ROMP alkyne

Homogeneous, single-component catalysts such as, e.g., W(=CCMe3)(OCMe3)3 or W(=CMe)(OCMe2CF3)3, cannot only be used for exchange metathesis of alkynes but also for ROMP of cycloalkynes, ADMET of a,to-diynes, and RCM of a,co-diynes [751]. 

Introduction to Ring-Opening Metathesis Polymerization (ROMP) and 1-Alkyne Polymerization... 

Scheme 1 ROMP of a 2,3-disubstituted norbornadiene, a 2-substituted norborn-5-ene and polymerization of a 1-alkyne via a- and /3-addition, respectively. A and B are initiator-and termination-derived endgroups, respectively...

In 1975, it was discovered that WCk, which is a typical metathesis catalyst, is capable to catalyze the polymerization of phenylacetyl-ene. Subsequently, various substituted acetylenes have been polymerized by this type of catalyst. In 1983, poly(l-(trimethylsilyl)-l-propyne)) was synthesized in the presence of Tads and NbCls (35). The alkyne polymerization has many similarities with ROMP. 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

The intermolecular enyne cross metathesis, and consecutive RCM, between a terminal alkyne and 1,5-hexadiene produces cyclohexadienes, by cascade CM-RCM reaction, and trienes, formed during the sole CM step. Studies of various parameters of the reaction conditions did not show any improvement of the ratio of desired cyclohexadiene product [25] (Scheme 12). The reaction with cyclopentene instead of hexadiene as the alkene leads to 2-substituted-l,3-cycloheptadienes [26]. After the first cyclopentene ROM, the enyne metathesis is favored rather than ROMP by an appropriate balance between cycloalkene ring strain and reactivity of the alkyne. 

ROMP gel supported l-azo-2-oxopropylphosphonate (413) has been synthesized and the immobilized reagent successfully employed to convert a number of aldehydes possessing different substitution patterns into the corresponding terminal alkynes (414) (Figure 75). ... 

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. 

This chapter will cover both the synthesis and basic principles of modern, well-defined metal alkyhdenes related to their use in ROMP, ADMET-polymerization and alkyne metathesis polymerization. The term well-defined is limited to catalytic systems, usually metal alkyhdenes, that are characterized by a uniform and stoichiometric composition and for which the actual propagating species is well known and characterized. Consequently, the entire chemistry of standard binary or ternary systems such as WCl6/AlEt2Cl/ethanol has been neglected. Where ap-phcable, general aspects of the preparation of advanced materials via ROMP and alkyne-polymerization will be mentioned briefly. 

In principle, conjugated materials may either be directly synthesized via metathesis polymerization of acetylene or 1-alkynes, via ROMP of various cyclooctatetraenes (COTs) or via ROMP of polyene precursors as realized in the Durham route [107-111]. The first direct polymerization of acetylene to yield black untreatable unsubstituted polyacetylene was achieved with W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2 [112]. In order to obtain soluble polymers, polyenes were prepared via the ROMP of a polyene-precursor, 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02 5]deca-3,7,9-triene (TCDTF6), using initiators such as W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2) (Scheme 5.10) [113, 114]. 

Figure 10-17 Ring-opening metathesis polymerization (ROMP) [111, 112]. represents a cyclic olefin or an alkyne. Any of the steps may be reversible, depending in part on the relative stabilities of the metallacycle and metallacarbene, and on the ring-strain of the ring in the monomer which is opened.

Chromium oxide catalysts, supported on AI2O3 and activated by LiAlH4, have been claimed to cause ROMP of cycloalkenes but the yields are low (Eleuterio 1957). Cr(CO)3(mesitylene) initiates the polymerization of alkynes, possibly via a metallacyclobutene intermediate (Farona 1974 Woon 1974) see Ch. 10. 

The ROMP of cyclic alkynes like cyclooctyne may be used to make alkyne-containing polymers in a reaction similar to olefin ROMP (Fig. 4.24) [71]. Additionally, Bunz and co-workers have pioneered the application of acychc... 

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

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