Homopolymerization, coordinate

Coordinate Homopolymerization. When Ziegler-Natta catalysts of the type TiCl4/alkylaluminum compounds are used, no polymerization occurred because the cyanoprene (like acrylonitrile for instance) reacts with the catalyst and destroys it. Polymerization occurs, however, when metal acetyl acetonates and organoaluminum compounds are used. For example, coordinate polymerization with a mixture of cobalt acetyl acetonate and ethylaluminum dichloride results in a polymer that corresponds mainly to the radical-produced polymer. 

Ethylene is conveniently polymerized in the laboratory at atmospheric pressure using a titanium-based coordination catalyst [34]. It may also be polymerized less conveniently in the laboratory under high pressures using free radical catalysts at high and low temperatures [35-37]. Other olefins such as propylene, 1-butene, or 1-pentene homopolymerize free radically only to low molecular weight polymers and require ionic or coordination catalysts to afford high molecu-... 

POLYALLOMER RESINS. These are block copolymers prepared by polymerizing monomers in the presence of anionic coordination catalysts. The polymer chains in polyallomers are composed of homopolymerized segments of each of the monomers employed. The structure of a typical polyallomer can be represented as ... 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

The radical model cannot be applied for ionic and coordination polymerizations. With a few exceptions, termination by mutual combination of active centres does not occur. The only possibility is to measure the rate of each copolymerization independently. The situation can be greatly simplified for copolymerizations in living systems. The constants ku and k22 can usually be measured easily in homopolymerizations. Also, the coaddition constants fc12 or k2] are often directly accessible when the M] and M2 active centres can be differentiated spectroscopically or when the rate of monomer M2 (M[) consumption at M] M 2 centres can be measured. Ionic equibria, association, polarity of medium and solvation must be respected, even when their quantitative effect is not known exactly. The unusual situations confronting macromolecular chemistry will be demonstrated by the example of the anionic copolymerization of styrene with butadiene initiated by lithium alkyls in hydrocarbon medium. 

T-Hg +-T interstrand cross-linking can occur in the alternating copolymer poly(dA-T) poly(dA-T), but not within or between homopolymeric strands in poly(dA) poly(T). The related complex [Hg(l-methyl-T)2] in which Hg + ion coordinates the deprotonated N-3 atoms of two pyrimidine residues, has been described as well. ... 

In 2013, Schafer s group [22b] reported titanium bis(amidate) and bis(pyridonate) complexes for the homopolymerization of rac-lactide and e-caprolactone, and also the formation of a random copolymer of the two. These complexes form pseudo-octahedral six-coordinate species, which were characterized in the solid state. Complexes were synthesized by first installing 2 equiv. of the ligand on homoleptic TifNMe ) followed by protonolysis of dimethylamido ligands with 2 equiv. of alcohol (Figure 19). 

Oxetanes, 4-membered cyclic ethers, polymerize exclusively by cationic mechanism 1 3), although coordinative anionic homopolymerization and copolymerization with C02 was claimed 4 5) for the unsubstituted oxetane. 

TABLE 5.2 Basic Mechanism for Olefin Homopolymerization with Coordination Catalysts... 

The contribution of complex formation to MCM copolymerization is more substantial than it is to homopolymerization. The coordinational unsaturation of central metal atoms, for example the pentacoordination state of Sn(IV), plays a definite part. The transfer of an electron from an MCM (electron-donor monomer) to a multiple bond of the comonomer is comparatively easily carried out in the transition state [112] as shown in Eq. 4-36, for example for maleic aldehyde the complex formation constant. 1= 0.17 0.002 L mol". ... 

This effect, as in the case of homopolymerization, appears to be due to the electron-acceptor nature of M", the coordination with which involves an electronic density distribution at the ligand bonds including those of the vinyl group. In this case, a remarkable role is played by the nature of the anion, which decreases the electron affinity of the metal ion in a different way in that the e values are more positive as 1 is replaced by Cl" (Fig. 4-11). 

Earlier we pointed out possible changes in MCM composition during homopolymerizations. The products of such a process can be regarded as offbeat MCM copolymers with a smaller number of coordinated ligands and with a free ligand. This can be exemplified by polymerization of methylvinyltetrazole complexes in ethanol (Eq. 4-40) [4]. 

Ethylene homopolymerization using Phillips catalyst PC600 calcined at 600°C followed by activation with DEAE cocatalyst during the slurry polymerization process was carried out with Al/Cr molar ratios of 7.5, 15.0, and 22.5 [84]. As shown in Fig. 14, a typical single-type polymerization kinetics corresponding to type b in Fig. 10b was observed, which was completely different from the kinetics with the same catalyst activated by TEA at the same conditions (as shown in Fig. 13). This t3 pe of polymerization kinetics could be ascribed to one type of active site (Site-B) formed in two ways. One was similar with the PC600 activated by TEA some chromate Cr(VI) species were reduced to Cr(II) species by ethylene monomer and coordinated with formaldehyde, then formaldehyde-coordinated Cr(ll) sites were transformed to DEAE-coordinated Cr(II) sites by substitution, as shown in Scheme 8. On the other hand, some chromate Cr(VI) species were reduced by DEAE, and then the Al-alkoxy product coordinated with the Cr(Il) sites. Site-B had relatively low activity and high stability. Based on the microstructure analysis, the relative amount of SCBs of polymers obtained from the DEAE systems was even more than that from TEA catalyst systems. This can be explained as follows. Firstly, the reduction ability of DEAE was weaker than that of TEA. More Cr(VI) species... 

With respect to titanium catalysts for polymerization of ethylene or propylene, Ziegler synthesized the first high-density polyethylene and Natta prepared isotactic polypropylene by means of coordination catalysts about 50 years ago. The preparation methods of catalysts have been studied extensively. These TiCL3 catalysts have very high activity for homopolymerization of ethylene and propylene, whereas, they exhibit low activity for random copolymerization of ethylene with propylene when compared to vanadium catalysts. Refer to Ziegler-Nata Catalyst, Vanadium Catalysts, and EP Terpolymer, (Source Elastomer Technology Handbook, N. P. Cheremisinoff - editor, CRC Press, Boca Raton, Florida, 1993). 

Figure 18 Homopolymerization of PS macromonomers via coordination poiymerization. Reproduced from Catari, E. Peruch, F. Isel, F. etal. Macromol.

Even though the discussion has been mainly on homopolymerization, the same polymerization mechanism steps are valid for copolymerization with coordination catalysts. In this case, for a given catalyst/cocatalyst system, propagation and transfer rates depend not only on the type of coordinating monomer, but also on the type of the last monomer attached to the living polymer chain. It is easy to understand why the last monomer in the chain will affect the behavior of the incoming monomer as the reacting monomer coordinates with the active site, it has to be inserted into the carbon-metal bond and will interact with the last (and, less likely, next-to-last or penultimate) monomer unit inserted into the chain. This is called the terminal model for copolymerization and is also commonly used to describe free-radical copolymerization. In the next section it will be seen that, with a proper transformation, not only the same mechanism, but also the same polymerization kinetic equations for homopolymerization can be used directly to describe copolymerization. 

We will start with a very simple homopolymerization model that includes only initiation, propagation, transfer to hydrogen, -hydride elimination and imimolecular catalyst deactivation, as depicted in Table 2.4. From our previous discussion of the standard model for polymerization with coordination catalysts, it is known that several steps are not included in Table 2.4. It will be shown, however, that general expressions for population balances and the methods of moments starting with this simplified mechanism can be developed and later they can be extended, rather easily, to include more polymerization steps. 

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