Secondary insertion, propylene polymerization

The (co)polymerization of dienes can be a good method for the preparation of polymers with reactive vinyl groups, a method that enables the preparation of polymers possessing plural vinyl groups per polymer chain. A fluorinated bis(phenoxy-imine) Ti complex was shown by Coates and co-workers to convert 1,5-hexadiene to poly(methylene-l,3-cyclopentane-fti-3-vinyl tetramethylene), which contained multiple vinyl groups. As already discussed, Saito et al. and others revealed that bis(phenoxy-imine) Ti complexes favored secondary insertion. " This is probably responsible for the formation of 3-vinyl tetramethylene units. Likewise, the same catalyst system can form sPP-/ -poly(methylene-l,3-cyclopentane-z -3-vinyl tetramethylene) from propylene and 1,5-hexadiene. Very recently. 

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

Due to this chain-migration process ethylene is polymerized to macromolecules containing multiple branches - rather than to the linearly enchained polymer obtained with classical solid-state catalysts. In propylene polymerization with these catalysts 1,2-insertions give the normal methyl-substituted polymer chains, but after each 2,1-insertion the metal centre is blocked by the bulky secondary alkyl unit and can apparently not insert a further propylene. Instead the metal must then first migrate to the terminal, primary C atom before chain growth can continue by further propylene insertions. By this process, also called 1,CO-enchainment or polymer straightening, some of the methyl or (in the case of higher olefins) alkyl substituents are incorporated into the chain. 

Monocyclopentadienyl complexes of titaninm (Cp TtXs) perform poorly as catalysts for ethylene or propylene polymerization, bnt in the presence of MAO, they polymerize styrene to stereo- and regioregnlar syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a Ti complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

At —78°C, the soluble catalyst system VCLi/AlEt2Cl polymerizes propylene to a predominantly syndiotactic polymer. A secondary insertion of propylene into the bond was invoked to explain this phenomenon, a hypothesis subsequently confirmed experimentally. ... 

When the catalyst is not fully regioselective, chain release by a /3-H transfer after a secondary insertion with formation of internal double bonds is often observed. This has been reported for ethylene/a-olefin co-poly-mers, PP, and other polyolefins, as well as for 1-hexene polymerization with dialkoxide catalysts. The reaction is shown in Scheme 14 for the case of propylene, where kinetic studies have shown it to be a bimolecular process, following the rate law s/J/ -h=s / -h[sZr][m].217,257 [sZr] refers to the concentration of active Zr centers bearing a growing chain having a secondary propylene unit linked to the metal. 

We would like to emphasize here that the branching of polypropylene is controlled by different factors to that of polyethylene [29]. In the case of ethylene the primary/secondary insertion ratio is crucial, whereas in the propylene polymerization catalyzed by diimine catalysts, the ratio between the two alternative insertion pathways (1,2- and 2,1-) is more important [27]. As a result, an opposite temperature effect has been observed for ethylene (increase in branching number with T) and propylene (decrease in branching number with T). 

Ho, S. C. H. Wu, M. M. Xiong, Y. Novel cyclopolymerization polymers from nonconjugated dienes and 1-alkenes. PCT International Patent Application WO 95/06669 (Mobil Oil Corp.), March 9,1995. Hustad, P. D. Coates, G W. Insertion/isomerization polymerization of 1,5-hexadiene synthesis of functional propylene copolymers and block copolymers. J. Am. Chem. Soc. 2062,124, 11578-11579. Hustad, P. D. Tian, J. Coates, G. W. Mechanism of propylene insertion using bis(phenoxyimine)-based titanium catalysts an unusual secondary insertion of propylene in a group IV catalyst system. J. Am. Chem. Soc. 2002,124,3614-3621. 

In 1962, Natta and Zambelli reported a heterogeneous, vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures.45 jjie regiochemistry of the insertion was determined to be a secondary insertion of propylene, and a chain-end control mechanism determined the syndiospecificity of monomer insertion. This system suffered from both low activity and low stereospecificity. 

A comparative study was made for the stereoregularities of the polypropylene and the polystyrene formed by various metallocene catalysts is studied (Table 7) [101]. When the chiral metallocene was used, stereoregular polymers, IPP and SPS, were produced. In the syndiotactic polymerization of styrene, the secondary insertion occurred. On the other hand, in the case of isospecific polymerization of propylene. 

The polymerization of propylene and/or higher a-olefins with heterogeneous Ziegler-Natta catalysts proceeds by a primary insertion with occasional errors [9]. Vanadium complexes produce syndiotactic polypropylene by secondary insertion [10]. 

In metallocene-catalyzed propylene polymerization, propagation proceeds via 1,2-insertion of the monomer. 2,1-Insertion gives rise to a secondary alkyl species. This species is known to be much less active for the next insertion and tends to be involved in chain transfer or isomerization into 1,3-inserted species. As shown in Scheme 4, 0-hydrogen elimination followed by rotation and re-in-... 

Other approaches include the use of difunctional olefins such as 1,7-octadiene, 1,9-decadiene, or para-(3-butenyl)styrene.875 While the former method also generates chain cross-linking (thus unprocessable polymer gels), the latter leads only to LCB formation through hydrogenolysis after a secondary styrene insertion. Tandem Zr/Fe catalysis has been used as well.876 The preparation of iPP with PS branches has been achieved by co-polymerization of propylene with allyl-terminated PS macromonomers.877... 

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