Propenes, polymerization

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

Collins et al. reported in 1995 that catalysts based on hafnium are desirable for the production of elastomeric polypropylene in that they polymerize propylene to a high molecular weight polymer and are indefinitely stable under typical polymerization conditions [8], Based on the theory that hafnium as a catalytic center leads to a significant increase of molecular weight in propene polymerization compared with the zirconium-based catalyst, Rieger et al. searched for hafnocene systems to obtain polymers with new properties. 

Resconi L, Cavallo L, Fait A, Pietmontesi F (2000) Selectivity in propene polymerization with metallocene catalysts. Chem Rev 100 1253-1345... 

Section 4 will deal with catalytic systems whose stereospecificity is controlled principally by the chirality of the closest tertiary carbon atom of the growing chain (chain-end stereocontrol). In Section 4.1 possible mechanisms for chain-end controlled isospecific and syndiospecific propene polymerizations will be reviewed. In Section 4.2 informations relative to the mechanism of chain-end controlled syndiospecific polymerization of styrene and substituted styrenes will be reviewed. In Section 4.3 chain-end controlled mechanisms for the isospecific and syndiospecific cis-1,4 and 1,2 polymerizations of dienes will be presented. 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

As for the Ti oxidation state after reduction with alkylaluminum compounds, literature reports are often contradictory, owing to the different catalysts and analytical methods used.108110,113 117 The only reasonable conclusion is that under polymerization conditions a considerable reduction of Ti(IV) takes place, not only to Ti(III) but to Ti(II) as well. However, Ti(II) is usually considered not to be active for propene polymerization.116... 

A syndiospecific chain-end controlled propene polymerization by Brookhart-type136 Ni(II) catalysts at low temperatures, also occurring through a primary... 

Recently, bis(imino)pyridyl Fe(II)-based catalysts have been reported to afford isospecific chain-end controlled propene polymerization occurring through secondary monomer insertion.138 139 Even more recently, catalytic systems based on the octahedral bis(salicylaldiminato)Ti complex have been reported to afford syndiospecific chain-end controlled propene polymerization,140 which possibly occurs through secondary monomer insertion.141... 

Propen-l-ol. See Allyl alcohol 2-Propenal. See Acrolein 2-Propenamide. See Acrylamide Propene, copolymerizations of, 16 111 Propene homopolymerization, 16 104-110 Propene polymerization, 16 94, 99 2-Propenenitrile. See Acrylonitrile (AN) Propenoic acid, physical properties, 5 31t Propenoic acid nitrile. See Acrylonitrile (AN)... 

Stereoselective alkylations, 12 165-166 Stereoselective hydrolysis, 16 400 Stereoselective propene polymerization, catalyst symmetries for, 16 104 Stereoselectivity, in a-olefin insertion, 16 99-102... 

Changes in the ligands of the transition metal component affect stereoselectivity [Natta et al., 1957a,b Rishina et al., 1976], For propene polymerization by titanium compounds in... 

Chain transfer to molecular hydrogen not only affects polymer molecular weight, but unlike other transfer agents, also affects polymerization rate. Hydrogen often decreases the rate of ethylene polymerization, but increases the rate of propene polymerization [Chadwick,... 

Some 3,1-placements (CH2CH2CH2) are observed in propene polymerization, a result of... 

Placement occurs through an isomerization process similar to that responsible for 3,1-placement in propene polymerization (Sec. 8-5c-l). 1,3-Placement is also observed with nickel and palladium a-diimine initiators [Sacchi et al., 2001] (Sec. 8-8b). 1,3-Placement has not been reported for other cycloalkene polymerizations. 

What are the mechanisms for syndiotactic and isotactic placements in propene polymerization Describe the reaction conditions that favor each type of stereoselective placement. 

Table 11 Propene polymerizations catalyzed by the phosphine-borane zirconocene 161e...

The second major topic in the field of olefin polymerization is that of the tacticity of the polymer [28]. If the olefin being polymerized is less symmetrical than ethylene, stereogenic centers will appear at the polymer, and the arrangement of these stereocenters can produce highly organized isotactic or syndiotactic polymers, as depicted in Fig. 5 for the case of propene polymerization. The alternative is an atactic polymer where the distribution of stereocenters is random. 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

Jiingling, S., Mulhaupt, R., Stehling, U., Brintzinger, H.-H., Fischer, D. and Langhau-ser, F., The Role of Dormant Sites in Propene Polymerization using Methylalumox-ane Activated Metallocene Catalysts , Macromol. Symp., 97, 205-216 (1995). 

In contrast to the oligomers formed by intermolecularly connected AT -N dative bonds, a series of monomeric metallacycles was synthesized and their XRD structures were determined (Figure 17).111-119 Several five-membered structures are comprised of one Al-G covalent and one AT -N dative bond provided by the bidentate ligand conformation. Their potential ability as candidates for co-catalysts in MgC /TiCLpmediated polymerization was also demonstrated (Figure 17).83 For the propene polymerization, all co-catalysts reach activities in the range of Et3Al... 

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