Copolymerization with coordination catalysts

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. 

The polymerization mechanisms for vinyl chloride and acrylonitrile (and also styrene) with coordination catalysts are also uncertain [222] and the copolymerization of butadiene/acrylonitrile (q.v.) also shows some features suggesting the formation of free radicals (or possibly radical-ions from charge transfer complexes). As these polar monomers can react, or form strong complexes, with the organo-metal compound it is likely that the kinetic schemes will be complex. As with styrene there is a good deal of scatter in the experimental kinetic data with these monomers which detracts from the certainty of the deductions, and much work will be required to put their polymerization by coordination catalysts on a sound mechanistic and kinetic basis. 

Anionic copolymerizations are very useful in forming block copolymers. (See Chapter 5 for discussion.) Ziegler-Natta catalysts also form block copolymers, similarly to anionic initiators. Much work on copolymerization with coordinated anionic initiators was done to develop ethylene propylene copolymers. Ethylene is considerably more reactive in these copolymerizations. To form random copolymers, soluble Ziegler-Natta catalysts are used. This is aided further by carefully controlling the monomer feed. ... 

The mechanism on long-chain branch (LCB) formation with coordination catalysts was discussed briefly in Section 2.2 and illustrated in Figure 2.18. LCB formation with coordination catalysts is nothing more than a copolymerization reaction with macromonomers made in the reactor through -hydride elimination and transfer to monomer reactions for polyethylene, and -methyl elimination for polypropylene (Scheme 2.2). At this point, the population balances could be re-derived to include LCB-formation reactions and solved by the method of moments. For brevity, however, only the final results of this derivation will be shown, leading to an analytical solution for the instantaneous distribution of MWD for chains containing LCB derivation details are available in the literature [45-47]. 

The LCB structure of polyolefins obtainable with a single metallocene catalyst can be altered in several ways. For instance, two or more metallocenes can be used to control, simultaneously, the MWD and LCB of polyolefins [48,49]. If an even more drastic micro-structural change is required, one that would make the LCB structure of polyolefins made with coordination catalysts resemble that of LDPE, dienes can be copolymerized with ethylene and a-olefins [50]. The cited references provide some additional information on this subject. 

Recently, Choo and Way mouth performed the copolymerization of ethylene with 1,5-HD using various metallocene catalysts (12, 13, 14, Figure 19.2). 1,5-HD cyclopolymerized exclusively to give MCP units in the copolymers, with only traces of uncyclized 1,2-inserted 1,5-HD. The diaste-reoselectivity of the cyclocopolymerization favored the formation of 1,3-cyclopentane rings for metallocenes (74% trans for 12, 81% trans for 13, and 66% trans for 14). For metallocenes 12 and 14, the ethylene/1,5-HD copolymerization yielded copolymers with similar comonomer compositions and sequence distributions to those observed for ethylene/1-hexene copolymerization with these catalysts. On the other hand, the copolymers derived from metallocene 13 showed very different compositions and sequence distributions. At comparable comonomer feed ratios, the poly(ethylene-c -l,5-HD)s were enriched in the 1,5-HD comonomer and deficient in ethylene as compared to the analogous polymers prepared from ethylene and 1-hexene. The copolymerization behavior of 13 provided support for a dual-site alternating mechanism for 1,5-HD incorporation, wherein one coordination site of the active catalyst center is highly selective for the initial 1,2-inserion of 1,5-HD and the other site is selective for cyclization. 

Polymerization with coordination catalysts proceeds via two main steps monomer coordination to the active site, and monomer insertion into the growing polymer chain, as illustrated in Figure 8.9. Before insertion, the double bond in the olefin monomer coordinates to the coordination vacancy of the transition metal. After the olefin is inserted into the growing polymer chain, another olefin monomer can coordinate to the vacant site thus the process of insertion is repeated to increase the size of the polymer chain by one monomer unit at a time until chain transfer takes place [17]. In the case of copolymerization, there is a competition between the comonomers to coordinate to the active sites and to be inserted into the growing polymer chains. Different rates of coordination and insertion of comonomers determine the final chemical composition of the copolymer chain. 

Polymerization Kinetics and Mechanism with Coordination Catalysts 389 Tab. 8.1. Terminal model for blna copolymerization of olefins.M... 

Free-radical copolymerizations have been performed ia bulb (comonomers without solvent), solution (comonomers with solvent), suspension (comonomer droplets suspended ia water), and emulsion (comonomer emulsified ia water). On the other hand, most ionic and coordination copolymerizations have been carried out either ia bulb or solution, because water acts as a poison for many ionic and coordination catalysts. Similarly, few condensation copolymerizations iavolve emulsion or suspension processes. The foUowiag reactions exemplify the various copolymerization mechanisms. 

Ethylene reacts by addition to many inexpensive reagents such as water, chlorine, hydrogen chloride, and oxygen to produce valuable chemicals. It can be initiated by free radicals or by coordination catalysts to produce polyethylene, the largest-volume thermoplastic polymer. It can also be copolymerized with other olefins producing polymers with improved properties. Eor example, when ethylene is polymerized with propylene, a thermoplastic elastomer is obtained. Eigure 7-1 illustrates the most important chemicals based on ethylene. 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

Currently, more SBR is produced by copolymerizing the two monomers with anionic or coordination catalysts. The formed copolymer has better mechanical properties and a narrower molecular weight distribution. A random copolymer with ordered sequence can also be made in solution using butyllithium, provided that the two monomers are charged slowly. Block copolymers of butadiene and styrene may be produced in solution using coordination or anionic catalysts. Butadiene polymerizes first until it is consumed, then styrene starts to polymerize. SBR produced by coordinaton catalysts has better tensile strength than that produced by free radical initiators. 

The trigonal planar zinc phenoxide complex [K(THF)6][Zn(0-2,6-tBu2C6H3)3] is formed by the reaction of a zinc amide complex, via a bis phenoxide, which is then further reacted with potassium phenoxide. TheoX-ray structure shows a nearly perfect planar arrangement of the three ligands with zinc only 0.04 A out of the least squares plane defined by the three oxygen atoms.15 Unlike the bisphenoxide complexes of zinc with coordinated THF molecules, these complexes are not cataly-tically active in the copolymerization of epoxides with C02. The bisphenoxide complex has also been structurally characterized and shown to be an effective polymerization catalyst. 43... 

The water-soluble palladium complex prepared from [Pd(MeCN)4](Bp4)2 and tetrasulfonated DPPP (34, n=3, m=0) catalyzed the copolymerization of CO and ethene in neutral aqueous solutions with much lower activity [21 g copolymer (g Pd) h ] [53] than the organosoluble analogue in methanol. Addition of strong Brpnsted acids with weakly coordinating anions substantially accelerated the reaction, and with a catalyst obtained from the same ligand and from [Pd(OTs)2(MeCN)2] but in the presence of p-toluenesulfonic acid (TsOH) 4 kg copolymer was produced per g Pd in one hour [54-56] (Scheme 7.16). Other tetrasulfonated diphosphines (34, n=2, 4 or 5, m=0) were also tried in place of the DPPP derivative, but only the sulfonated DPPB (n=4) gave a catalyst with considerably higher activity [56], Albeit with lower productivity, these Pd-complexes also catalyze the CO/ethene/propene terpolymerization. 

Attempts have been made to copolymerize conjugated dienes with olefins but there are no data on polymerization rates or reactivity ratios. They are ill suited for copolymerization in that polymerization rates are markedly reduced by the presence of the conjugated diene and the copolymers are heterogeneous in composition and may be crosslinked. The reasons for this behaviour have not been established but a possible explanation is that the conjugated diene coordinates preferentially with the catalyst and so excludes the olefin, but has a slow insertion rate compared with the more reactive olefin. 

Tertiary amine-functionalized olefins are not difficult to polymerize and copolymerize with group IV catalysts, provided that sufficient steric hindrance is present around the nitrogen atom. Amines of sufficient bulkiness, including diisopropyl and diphenyl derivatives, can be polymerized without the necessity of protection by Lewis-acid complexation. Smaller monomers (such as dimethyl and diethylamines) can be polymerized if 1 equiv of a proper alkylaluminum protecting group is used (vide infra). However, if the amine functionality is too near to the double bond, the additional steric bulk provided by the aluminum species may actually inhibit monomer coordination and polymerization. 

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). 

Ethylene and other olefins can also be copolymerized with carbon monoxide to form polymers of aliphatic ketones, using transition metal catalysts, like paUadium(ll) coupled with non-coordinating anions. There are numerous reports of such catalysts in the literature. One example is a compound composed of bidentate diarylphosphinopropane ligand and two acetonitrile molecules coordinating Pd coupled with BF3 counterions. This compound, bis(acetonitrile)palladium(II)-l,3-bis(diphenyl-phosphino)propane-(tetrafluoborate), can be illustrated as follows [97] ... 

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