Coordination-catalyzed polymerization

The coordination catalyzed polymerization is defined for the purpose of this paper as one which involves a concerted Insertion with concurrent cleavage of a covalent polymer-catalyst bond( ). It is illustrated in Equation (IV). 

The overall mechanism of an anionic-coordination catalyzed polymerization of f-caprolactone is dependent upon a number of factors. The most important of these are the type of catalyst and whether a coinitiator is used. A very large number of both have been reported( ). If R M represents the catalyst-initiator, R2OH an active hydrogen containing coinitiator, and CL the f-caprolactone monomer, the following steps have to be considered in order to arrive at a meaningful kinetic expression ... 

At present it is not possible to determine which of these mechanisms or their variations most accurately represents the behavior of Ziegler-Natta catalysts. In view of the number of variables in these catalyzed polymerizations, both mechanisms may be valid, each for different specific systems. In the following example the termination step of coordination polymerizations is considered. 

Polymers with much higher average molecular weights, from 90,000 to 4 x 10 , are formed by a process of coordinate anionic polymerization (43—45). The patent Hterature describes numerous organometaUic compounds, aLkaline-earth compounds, and mixtures as polymerization catalysts. Iron oxides that accumulate in ethylene oxide storage vessels also catalyze polymerization. This leads to the formation of nonvolatile residue (NVR) no inhibitor has been found (46). 

The basic assumptions common to most mechanism studies relative to transition metal catalyzed polymerizations are as follows (i) The mechanism is essentially monometallic and the active center is a transition metal-carbon bond.13-15,18,19 (ii) The mechanism is in two stages coordination of the olefin to the catalytic site followed by insertion into the metal-carbon bond through a cis opening of the olefin double bond.13,20,21... 

Anions, too, are not always innocent. The borate anion B(C6F5)4 is very popular in early-transition-metal catalyzed polymerization, where it acts as a rather inert and non-coordinating anion. Examples of decomposition of the anion by, e.g., CeFs transfer exist but are not very common. In aluminium chemistry, transfer of CeFs groups from B(C6Fs)3, MeB(C6F5)3 and B(C6F5)4 to the metal appears to be rather easy [14, 15], and it may be that other, even more innocent anions will be required here. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

Trivalent carbenium ions play a key role, not only in the acid-catalyzed polymerization of alkenes [Eq. (5.348)] but also in the polycondensation of arenes (7r-bonded monomers) as well as in the cationic polymerization of ethers, sulfides, and nitrogen compounds (nonbonded electron-pair donor monomers). On the other hand, penta-coordinated carbonium ions play the key role in the electrophilic reactions of cr-bonds (single bonds), including the oligocondensation of alkanes and alkenes (Section 5.1.5). 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

The structures of three of the compounds have been determined they show essentially octahedral coordination of the lanthanide with bond angles around 90°. Average lanthanum-carbon distances are 2.563(18) A (Ho) 2.57(2) A (Er), and 2.53(2)A (Lu). More complicated lanthanide methyl species have been synthesized by another route, involving reaction of main-group methyls, Lewis acids, with lanthanide alkoxides " and amides, " a process of the type implicated in the lanthanide-catalyzed polymerization of conjugated dienes... 

The reactive intermediates used in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes. Of the three common metal catalyzed polymerizations - coordination-insertion, ring-opening metathesis and diene polymerization - the last appears to possess the greatest tolerance toward protic solvents. The polymerization of butadiene in polar solvents was first reported in 1961 using Rh salts [18]. It was discovered that these polymerizations could be performed in aqueous solution with an added emulsifier (sodium dodecyl sulfate, for example). 

The coordination of the functionalities that may be present either in the monomer or in the growing polymer chain to the metal center is one key impediment in the development of new transition metal-based catalysts for the insertion polymerization of polar monomers. On the other hand, rapid -halide abstraction has prevented metal-catalyzed polymerization of vinyl halides. Several examples are discussed to illustrate these problems. 

Thus far, we have confined our discussions as to the effect of coordinating functionalities on metal-catalyzed polymerizations. Interestingly, vinyl halides, which... 

What is fhe implication of our work wifh respect to the metal-catalyzed polymerization of polar vinyl monomers FirsL for fhe late metal compounds, fhe polar vinyl monomers can clearly outcompete efhene and simple 1-alkenes wifh respect to insertion. However, fhe ground-state destabilization of the alkene complex that favors the migratory insertion of fhe polar vinyl monomers is a two-edged sword because it biases the alkene coordination towards ethene and l-alkenes. Indeed, we have observed fhe near quantitative displacement of vinyl bromide by propene to form 7 from 3 (Scheme 9.1). Thus, the extent of incorporation of fhe polar vinyl monomer in fhe polymer will depend on the opposing trends in alkene coordination and migratory insertion. The above discussion does not take into account the problem of functional group coordination for acrylates or halide abstraction for vinyl hahdes. 

Yold has determined by IR spectroscopy that the degree of silicon oxide network connectivity increases with increasing H O/TEOS mole ratio for add-catalyzed polymerizations in bulk solutions (21). These results, if applicable to the membrane in situ add-catalyzed polymerizations described herein, serve to reinforce our conclusion that a more highly-coordinated silicon oxide structure exists within microcomposites produced with short immersion times according to procedure A, or according to the slow stepwise TEOS addition in procedure B. In either case, more initial hydrolysis water molecules per alkoxide molecule are available to promote this situation. General Conclusions... 

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