Isotactic polymers production

The synthesis of isotactic polymers of higher a-olefins was discovered in 1955, simultaneously with the synthesis of isotactic PP (1,2) syndiotactic polymers of higher a-olefins were first prepared in 1990 (3,4). The first commercial production of isotactic poly(l-butene) [9003-29-6] (PB) and poly(4-methyl-l-pentene) [9016-80-2] (PMP) started in 1965 (5). 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

Detailed modifications in the polymerisation procedure have led to continuing developments in the materials available. For example in the 1990s greater understanding of the crystalline nature of isotactic polymers gave rise to developments of enhanced flexural modulus (up to 2300 MPa). Greater control of molecular weight distribution has led to broad MWD polymers produced by use of twin-reactors, and very narrow MWD polymers by use of metallocenes (see below). There is current interest in the production of polymers with a bimodal MWD (for explanations see the Appendix to Chapter 4). 

We find an additional crystalline structure in rapidly quenched isotactic polypropylene products. In these materials, the polymer chains do not have the necessary time to orient,... 

In case of polypropylene some atactic polymer also gets formed in addition to the required isotactic polymer but much of this atactic material is soluble in the diluent so that the product isolated would be largely isotactic polymer. 

When the reaction is carried out with a racemic mixture of complexes, the product is a racemic mixture of the isotactic polymers. It was of interest to see what would happen if, after formation of a chiral block with one enantiomer of the bisoxazoline ligand, an equivalent of the other enantiomer was added. It was found that an excess of ligand changes the tacticity completely and the second block was syndiotactic In these diimine palladium complexes exchange of ligand is relatively fast and it can often be observed on the NMRtime scale as a broadening in the H NMR spectra. The process may well be associative. 

Note 4 Some stereospecific polymerizations produce tactic polymers [3] that contain a mixture of pairs of enantiomeric polymer molecules in equal amounts. For example, in the case of a polymerization leading to an isotactic polymer the product consists of... 

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

More sophisticated experimental and theoretical analysis of isoselective polymerizations have been performed by using a two-site model for propagation [Inoue et al., 1984 Wu et al., 1990]. The polymer product is fractionated into the highly isotactic, insoluble and atactic,... 

Looking ahead to Section 18.4, explain why isotacticity is not achievable by standard methods of organosiloxane polymer production. 

Hie study of effects of the catalyst components also help clarify the ionic factors in the steric control of isotactic polyvinylethers. Dall Asta and Bassi (15) studied the polymerization of butylvinylether with various alkylaluminum halides. They found that diethylaluminum chloride and ethylaluminum dichloride were the most effective catalysts for the production of isotactic polymer. Ethylaluminum dibromide and ethoxyaluminum dichloride were of questionable effectiveness, while diethylaluminum fluoride was completely ineffective. 

Nonpolar solvents favor the formation of ion pairs between the polymer cation and the counteranion and favor the production of isotactic polymers. 

All-trans-perhydrotriphenylene (PHTP) (cf. insert in Figure 14) is the product of exhaustive hydrogenation of triphenylene. It belongs to one of ten stereoisomers of PHTP. The chiral compound of high rotational symmetry (D3 — C3 -h 3 C2) forms inclusion complexes. The stereoselective polymerization via 7-radiation of the prochiral diolelin 1,3-pentadiene within the chiral nano channels of (.R)-(-)-all-trans-PHTP led to an optically active 1,4-trans-isotactic polymer (Nattaand Farina, 1976) (cf. Figure 13). 

Polyacetaldehyde is formed by cationic polymerization using boron trifluoride (BF3) in liquid ethylene (CH2=CH2). At temperatures below -75°C using anionic initiators, such as metal alkyls in a hydrocarbon solvent, a crystalline isotactic polymer is obtained. Molecular weights of the products fall into the range 800,000 to 3,000,000. 

Modifications of Cs-symmetric metallocenes may lead to Ci-symmetric metallocenes (Fig. 8). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so that every second insertion is random a hemiisotactic polymer is produced (276, 277). If steric hindrance is greater (e.g., if a /-butyl group replaces the methyl group), stereoselectivity is inverted, and the metallocene catalyzes the production of isotactic polymers (178-180). 

Polypropylene owes its current market success to the development of coordination polymerization. Before 1957 it was not produced commercially because radical polymerization gives an atactic polymer that is amorphous and has poor mechanical properties. Using a coordination catalyst, however, enables the production of an isotactic polymer that is semicrystalline. This material is stiff and hard and has a high tensile strength. Among its many useful products are rope, molded objects, and furniture. 

Synthesis. The early PP plants used a slurry process adopted from polyethylene technology. An inert liquid hydrocarbon diluent, such as hexane, was stirred in an autoclave at temperatures and pressures sufficient to keep 10-20 percent of the propylene monomer concentrated in the liquid phase. The traditional catalyst system was the crystalline, violet form ofTiCl3 and A1C1(C2H5)2. Isotactic polymer particles that were formed remained in suspension and were removed as a 20-40 percent solid slurry while the atactic portion remained as a solution in the liquid hydrocarbon. The catalyst was deactivated and solubilized by adding HC1 and alcohol. The iPP was removed by centrifuging, filtration, or aqueous extraction, and the atactic portion was recovered by evaporation of the solvent. The first plants were inefficient because of low catalyst productivity and low crystalline yields. With some modifications to the catalyst system, basically the same process is in use today. 

We now turn to the actual polymerization process and we will try to present a series of pictures that clarifies how chain-end control can be used to obtain either syndiotactic or isotactic polymers. Subsequently we will see how a chiral site can influence the production of syndiotactic or isotactic polymers. Finally, after the separate stories of chain-end control and site control, the reader will be confused by introducing the following elements (1) pure chain-end control can truly occur when the catalyst site does not contain chirality (2) but since we are making chiral chain ends in all instances, pure site control does not exist. In a polymerization governed by site control there will potentially always be the influence of chain-end control. This does not change our story fundamentally all we want to show is that stereoregular polymers can indeed be made, and which factors play a role but their relative importance remains hard to predict. 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

Substituents in the distal position of the cyclopentadienyl ring in these cyclopentadienyl-fluorenyl catalysts has a profound effect on the polymer product produced. A small substituent like methyl produces a novel material, hemiisotactic polypropylene, in which methine carbons of specific conformation alternate with methine carbons of random conformation. A larger substituent hke r-butyl makes isotactic polypropylene (Figure 6). 

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