Double-Bond Opening

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

Unsaturated hydrocarbons are quite reactive —in contrast to the relatively inert saturated hydrocarbons. This reactivity is associated with the double bond. In the most characteristic reaction, called addition, one of the bonds of the double bond opens and a new atom becomes bonded to each of the carbon atoms. Some of the reagents that will add to the double bond are... 

Given the high regioselectivity, the type of addition in the insertion step must always be the same. It was found that in the polymerization of cis- and trans-[l-2H ]-propene a yyn-type double-bond opening takes place during insertion, eventually resulting in em/iro-diisotactic and f/ireo-diisotactic polymers, respectively 332-334... 

We consider styrene homopolymerization by a free-radical mechanism. Styrene, like any other vinyl monomer, is bifunctional, because the double bond opens two arms in the polymerization processs (one of the carbons is attacked by a free radical, activating the other carbon atom, which may continue to propagate the chain). 

The polymerisation of monocyclic non-conjugated diolefins, which takes place by the double bond opening with simultaneous transannular migration, leads to cyclopolymers having bicyclic repeating units present in the main chain. A typical example of such cyclopolymerisation is the polymerisation of 1,5-cyclooctadiene [31,32] ... 

A tetrahedral carbon atom in the active centre usually preserves its configuration even during monomer addition. With monomer addition to a chain end, the configuration of the new end depends on the mode of double bond opening, cis [(a) in eqn. (34)] or trans [(b) in eqn. (34)]. If the approaching... 

Zambelli et al. reported on the mechanism of styrene polymerization [36]. They showed that the main chain of the syndiotactic polymer has a statistically trans-trans conformation. It was established then the double-bond opening mechanism in the syndiospecific polymerization of styrene involves a cis opening. The details in the control of the monomer coordination for this polymerization mechanism were examined by Newman and Malanga using detailed, 3C NMR. It was shown through the analysis of tacticity error (rmrr) that the tacticity in the polymer is chain-end controlled and that the last monomer added directs the orientation and coordination of the incoming monomer unit prior to insertion [37]. 

The mechanism of the polymerization reaction is presumed to be essentially that of a homogeneous bulk or solution free-radical polymerization. The concern is exclusively with the polymerization by double-bond opening of carbon compounds that contain at least one caibon-carbon double bond. The reactive species that propagates to produce the polymer chain is a free radical formed by opening of the rc-bond of the carbon-carbon double bond. The basic steps of the polymerization reaction are initiation, propagation, termination (by various means), and various transra reactions. Tbe structure of the polymer produced is determined by the balance of the propagation, termination, and transfer reactions. 

It has been established from the polymerization of l-dj-deutero-propene that the double bond opens in the cis direction [71], in conformity with the proposed mechanism. A repetition of this process with the monomer molecules approaching the active centre in the same direction would give the isotactic polymer while alternate approach to mirror im e positions would give the syndiotactic form. 

While it is very difficult to polymerize cyclic olefins such as cyclopentene or norbomene using heterogeneous catalysts without ring opening, a metallo-cene/alumoxane catalyst polymerizes them exclusively by double-bond opening [60]. 

In chain polymerization a simple, low-molecular-weight molecule possessing a double bond, referred to in this context as a monomer, is treated so that the double bond opens up and the resulting free valences join with those of other molecules to form a polymer chain. For example, vinyl chloride polymerizes to poly(vinyl chloride) ... 

There exist two main ways for the synthesis of polymers polycondensation and polymerization. A polycondensation is a series of chemical reactions between difunctional compounds thus the esterification of a diacid by a dialcohol produces a polyester. A polymerization is a chain reaction the polymers have at one extremity an active site, and they grow step by step by adjunction of monomers containing a double bond when a monomer clings to a chain, the double bond opens and the activity is transferred to the new extremity of the chain. Therefore, such a transformation requires an initiator able to create active sites. Industrially, the polymerization processes correspond to the largest tonnage and scientifically, for the preparation of good samples, they are also the most interesting. 

Most V-based catalysts of the Ziegler-Natta type cause double-bond opening (addition polymerization) rather than ROMP of cycloalkenes. For example, V(acac)3/Et2AlCl causes only Ziegler-Natta-type addition polymerization of cyclobutene (Natta 1963) and its copolymerization with ethene (Natta 1962). 

Oxyberberine (XVIII) is the first product of the oxidation of berberine. It may be reduced to tetrahydroberberiiie at a lead cathode in alcoholic sulfuric acid (215). Its synthesis is therefore a synthesis of berberine. The general procedure for the synthesis of this type of compound is described in connection with the synthesis of cryptopine, and oxyberberine itself was prepared by this procedure. Another synthesis of oxyberberine was achieved by Perkin, Ray, and Robinson (216). The 8-piperonyl-ethylimide of meconinecarboxylic acid (XX) was heated with phosphoryl chloride to yield an uncharacterized isoquinoline derivative, presumably XXI, which when reduced with zinc in acetic acid suffered reduction of the doubl bond, opening of the lactone ring, lactam formation, and dehydration to yield oxyberberine. This ready elimination of water was later observed in ophiocarpine (6). 

Let us consider the hydrogenation of ethylene on Ni catalysts in more detail. The adsorption of ethylene on niekel is assoeiative, especially in the presence of hydrogen. Speetroseopie investigations have shown that the ethylene double bond opens, forming two a bonds to neighboring Ni atoms and giving the ethane structure. 

In the hydrogenation of benzene and the dehydrogenation of eyelohexane, sixpoint adsorption of the moleeule on the catalyst has been found. The double bonds open, and chemisorption occurs by formation of a bonds. Here, too, the (111) surface is clearly favored. 

This blocks one mode of double-bond opening and assists stereoregulation. This conclusion is supported by the lack of any crystalline polymer in the product when the ethyl and isopropyl groups are used where no blocking is possible. 

Studies of model reactions for cationic polymerization of alkyl propenyl ethers showed that the mode of double bond opening is independent of the geometric structure of the ether. Mainly a threo opening takes place, but the mode of monomer addition is dependent on the geometric structure of the monomer or on the balkiness of the substituent. ... 

Stereospecific placement appears to result from steric interactions between the substituents on the ultimate units of the growing chains and the incoming monomers. NMR spectral evidence shows that the double bond opening is also cis, as with the heterogenous catalysts. 

The buUt-in distinguishing feature of a thermoset polyester, however, is that it is unsatorated. That is, prior to cross-linking, Aere are some R, groups present in which there is a carbon-carbon double bond (C—C bond) contained in the molecular backbone. The mixture for polymerization contains (XXVID plus monomers such as styrene (or other unsaturated non-pofymeric molecules) in liquid form. When catalyst is added and heat is applied the double bonds open and an addition reaction leads to the formation of a highfy cross-linked network, with pofyester chains... 

STRATEGY AND ANSWER Since there is only a single product containing the same number of carbon atoms as the reactant, the only reasonable explanation is that the reactant has a double bond contained in a ring. Ozonolysis of the double bond opens the ring ... 

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