Polymerization of propene

Before the development of the Ziegler-Natta catalyst systems (Section 6.21), polymerization of propene was not a reaction of much value. The reason for this has a stereochemical basis. Consider a section of polypropylene ... 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Unusual annulated derivatives of pyrrole, 90 and 91, were found as efficient catalysts for the polymerization of propene (98JA10786). 

The addition of a large excess of bis(cj-alkenyl)zinc compounds to the TiC -catalyzed polymerization of propene resulted in an increased polymer yield, but a reduction in the molecular weights of the polymers.64 This suggests that the diorganozinc compounds are both co-catalysts and chain-transfer agents in this polymerization. The catalyst activity decreased in the order bis(3-butenyl)zinc < bis(7-octenyl)zinc < chlorodiethylaluminum. Bis(7-octenyl)zinc was co-polymerized with propene to afford hexylzinc side chains, whose zinc-carbon moieties were converted to vinyl groups by the addition of allyl bromide. 

Since the 1960s the syndiospecific chain-end controlled polymerization of propene in the presence of homogeneous vanadium-based catalytic systems has been known. For these systems, it has been well established by the work of Zambelli and co-workers that the polymerization is poorly regioselective and the stereoselective (and possibly syndiospecific) step is propene insertion into the metal secondary carbon bond with formation of a new secondary metal-carbon bond.133134... 

Many other compounds have been shown to act as co-catalysts in various systems, and their activity is interpreted by analogous reactions [30-33]. However, the confidence with which one previously generalised this simple picture has been shaken by some extremely important papers from Eastham s group [34], These authors have studied the isomerization of cis- and Zraws-but-2-ene and of but-l-ene and the polymerization of propene and of the butenes by boron fluoride with either methanol or acetic acid as cocatalyst. Their complicated kinetic results indicate that more than one complex may be involved in the reaction mechanism, and the authors have discussed the implications of their findings in some detail. 

By contrast with the polymerization of propene, there seems little scope here for the formation of branched structures. Detailed elucidation of the polymer structure would provide evidence on the question whether a non-classical ion can act as chain-carrier. 

The experimental evidence which has accumulated in recent years shows that in every system which has been rigorously investigated the polymerization of olefins by metal halides depends upon the presence of some third substance, the co-catalyst [2-8]. The function of the cocatalyst is to provide the ions which start the polymerization proper, by forming an ionogenic complex with the metal halide. In most systems the metal halide is not consumed in the course of the reaction, so that the term catalyst in its classical sense may be retained in this respect. Exceptions to this are some polymerizations involving aluminum halides in the polymerization of propene [9], and possibly of styrene and a-methyl styrene [10], these catalysts may be inactivated by the formation of stable complexes. In other cases, such as the... 

Aspects concerning the regio- and stereochemical behavior of these catalysts in the stereospecific polymerization of propene (or 1-olefins, in general) will be not discussed in details since these topics are at the center of several reviews recently published [11, 12, 14, 24, 25], Nevertheless, in the final sections we will briefly report about these points. 

On the other hand, a study of the kinetics of the polymerization of propene in the presence of dilute phosphoric acid (10-50% by weight) at 260-350° and 170-410 atmospheres has shown that the rate of polymerization is proportional to the square of the gas-phase propene concentration and the first power of the acid concentration, indicating that the polymerization involves addition of an ester to an olefin rather than interaction of two molecules of ester (Monroe and Gilliland, 58). It was pointed out, however, that both mechanisms may occur side by side and that under certain circumstances (as in dilute acid) one of these mechanisms may predominate over the other. 

Polymerization of propene at 330° in the presence of 90% ortho-phosphoric acid under about 100 atmospheres initial pressure yielded a product consisting of paraffinic, olefinic, cycloparaffinic and cycloolefinic, and aromatic hydrocarbons (Ipatieff and Pines, 70). About 8% of the product boiled in the dimer (C6) range and about 25% in the trimer (C9) range. Isobutane was formed to the extent of more than 2% by weight of the total polymer. 

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

Syndioselective polymerizations of propene are somewhat less regioselective than the isoselective reactions, with the typical highly syndiotactic polymer showing a few percent of the monomer units in head-to-head placement [Doi, 1979a,b Doi et al., 1984a,b, 1985 Zambelli et al., 1974, 1987]. The mode of insertion is secondary, contrary to what is expected for a carbanion propagating center. Apparently, steric requirements imposed by the counterion derived from the initiator force propagation to proceed by secondary insertion. 

The initiator formed from VCLt and A1(C2H5)2C1 is one of the most efficient means for syndioselective polymerization of propene, especially in the presence of a Lewis base such as anisole (methoxybenzene) [Doi, 1979a,b Natta et al., 1962 Zambelli et al., 1978, 1980], Other vanadium compounds such as vanadium acetylacetonate and various vanadates [VO(OR)xClp x), where x — 1,2,3] can be used in place of VCI4 but are more limited in their stereoselectivity [Doi et al., 1979]. Trialkylaluminum can also be used as a coinitiator, but only for VCI4. Syndiotacticity increases with decreasing temperature most of these syndioselective polymerizations are carried out below —40°C and usually at —78°C. The initiators must be prepared and used at low temperatures since most of them undergo decomposition at ambient and higher temperatures. There is considerable reduction of V(III) to V(II) with precipitation of ill-defined products that are low in activity and do not produce syndiotactic polymer, when the initiators are prepared at or warmed to temperatures above ambient. 

The open nature of the metal site limits catalyst site control by CpA initiators. Polymerization of propene proceeds with weak chain end control at low temperatures. The highest stereoselectivity reported is (mmmm) — 0.77 using Me2Si(Flu)(N-t-Bu)ZrCl2. 

Discuss the use of homogeneous versus heterogeneous reaction conditions for the coordination and traditional Ziegler-Natta polymerizations of propene, isoprene, styrene, methyl methacrylate, and n-butyl vinyl ether. 

The importance of stabilizers for SCF polymerization was briefly outlined in Section 9.1.4. The drawback with existing stabilizers, however, is that most of them are based on fluorocarbons or siloxanes, which are high-cost chemicals. Cheaper polymeric stabilizers are usually only soluble in SCCO2 at pressures too high to make viable their widespread use. Very recently, Beckman and co-workers reported [68] a totally new approach to the problem polymers were prepared by co-polymerization of propene oxide and SCCO2. These polymers are not only much cheaper than fluorinated polymers but are more soluble than these materials in SCCO2. The polyether polymers are likely to have widespread applicability, not only as building blocks for stabilizers for SCF polymerization, but also as the basis of... 

One of the most important technical reactions of alkenes is their conversion to higher-molecular-weight compounds or polymers (Table 10-4). A polymer is defined as a long-chain molecule with recurring structural units. Thus polymerization of propene gives a long-chain hydrocarbon with recurring... 

Polymerization of propene by the Ziegler process gives a very useful plastic material. It can be made into durable fibers or molded into a variety of shapes. Copolymers (polymers with more than one kind of monomer unit in the polymer chains) of ethene and propene made by the Ziegler process have highly desirable rubberlike properties and are potentially the cheapest useful elastomers... 

Kaminsky W, Kulper K, Brintzinger HH, FRWP Wild, Polymerization of Propene and Butene with a Chiral Zirconocene and Methyl Aluminoxane as Cocatalyst, Angew Chem Int Ed Engl, 24,507 (1985)... 

Michalak A, Ziegler T, Pd-catalyzed Polymerization of Propene - DFT Model Studies, Organometallics, 18, 3998-4004 (1999)... 

Arlman EJ, Coose P, Ziegler-Natta Catalysis III Stereospecific Polymerization of Propene with the Catalyst System TiCl3-AlEt3, J Catal, 3, 99 (1964)... 

Mitsui Petrochemical Industries has developed a process for the gas-phase fluidizcd-bcd polymerization of propene, a plant using the process came on stream in 1984 [94] The Unipol-Shell process was jointly developed by Union Carbide and Shell and commissioned in 1986... 

Fig. 6.19. Isotactic polymerization of propene with rac-l,2-ethanediyl-bis(indenyl)zirconium complexes. The plane of the drawing coincides with the plane bisecting the two planes of the indenyl ligands. Propene coordination takes place with the alkene Tt-orbitals in the plane of the drawing. The carbon atoms of the growing chain are depicted in the plane of the figure in between insertions no rearrangements to more stable conformations have been drawn. Indenyl ligand above the plane drawn in full.

Fig. 6.20. Syndiotactic polymerization of propene with isopropyl(l-fluorenyl-cyclopentadienyl) zirconium complexes. See also Fig. 6.19. The cyclopentadienyl ligand above the plane is drawn in full. The fluorenyl ligand below the plane is drawn with dotted lines.

Scheme 5.7 Three examples of ansa-zirconocene dichlorides directing the polymerization of propene, in the presence of MAO, to isotactic, syndiotactic and atactic polypropylene (PP)...

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