Propylene insertion

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

A site-inversion mechanism (the key feature of which is that isomerization between diastereomeric and A configurations is rapid on the propylene-insertion time scale) based on theoretical calculations was proposed by Cavallo and coworkers in order to explain the ligand-directed chain-end controlled polymerizations (Fig. 35) [42]. The site-inversion mechanism allows chain-end control to work in concert with the site control effects. Our experimental results and the expected catalytic behavior resulting from the site-inversion mechanism concur with each other very well. 

Figure 4.80 Optimized structures of (a) the transition state (IIpri ) and (b) the product (IIpri) of the model propylene-insertion reaction (4.107b).

Cp Th(CH3)2] MgCl2.3oo (73) than for surface species 67. This proves the importance of cahonic surface species in polymerizahon reachons, since the number of active sites is >35% for the former. The reactivity of [Cp Th(CH3)2] MgCl2.3oo (73) was further examined toward propylene and/or 3,3 -dimethylbutene. This study rather suggests an aUyhc C-H bond activation/methane ehmination (Equation 12.1) followed by olefin inserhon than direct propylene insertion into the Th-R bond (Equahon 12.2). This observed reactivity is in agreement with that one described previously for organolanthanide complexes [CpJLnR] [142, 180, 181]. 

These reactions can be explained by the following mechanism. At first, isopropyltitanocene (521) is formed by transmetallation and its -elimination generates Cp2Ti-H and propylene. Insertion of the alkene to Ti-H affords the alkyltitanium 522. Then the alkyl Grignard reagent 523 is formed by transmetallation... 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

In agreement with this finding, it has been shown that, in ethylene/propylene copolymerisation with vanadium-based catalysts, propylene insertion after an ethylene insertion is substantially non-stereospecific (both cases (a) and (b) in Figure 3.46 are possible) [1,390]. 

Ans. 4.24 undergoes /3-elimination to give 4.27 and propylene. Insertion of propylene into the Rh-H bond in the Markovnikov and anti-Markovnikov manner followed by CO insertion gives 4.26 and 4.25. In other words, reversible /3-elimination and insertion initiates the hydrocarboxylation cycle. 

Figure 6.8 Stereospecific propylene insertion in a metallocene catalyst of the type 6.22. For clarity the coordination of Zr to the second indene ring (broken line) is not shown, (a) Preferred orientation of the growing polymer chain. Note the trans orientation of the methyl group (above the plane of paper) and (below the plane of paper), (b) Rotation along the Zr—C bond may make and CH3 cis to each other, (c) Agostic interaction that prevents rotation around the Zr—C bond and keeps and CH3 away from each other.

Since the desired product from propylene hydroformylation is -butyralde-hyde, considerable attention has been devoted to increasing the selectivity this focussed attention on the mechanism, especially the step where the propylene inserts into the Co-H bond, since this can be either Markovnikov or anti-Markovnikov. 

Figure 14 Alternative propylene insertion modes in diimine-nickel catalysts (RLS = rate-limiting step).

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. 

Scheme 14 /3-H transfer to a coordinated propylene monomer after a secondary propylene insertion.

The main drawback of propylene/ethylene co-polymerization with metallocenes is the frequent strong decrease of molecular mass at increasing ethylene incorporation. The available mechanistic explanation is a fast chain transfer to ethylene after a propylene insertion (Scheme 30), and has been reported for metallocenes of different symmetries C2 v-symmetric, (/ -symmetric, and fluxional bis(2-arylindenyl). This phenomenon has strongly limited the... 

Further NMR analysis of chain-end groups of PPs produced with similar catalysts provided additional evidence for the prevailingly secondary propylene propagation with this class of catalyst. In fact, it was shown that the main chain-release reaction is /3-H elimination, and that propylene insertion into the Ti-H bond in the initiation step is almost exclusively primary. 1 Moreover, NMR analysis of a co-polymer of propylene with a small amount (< 2 mol%) of l-13C-ethylene, obtained with the perfluorinated catalyst (137), showed that the large majority of ethylene units in the co-polymer was present as two methylene units (see Scheme 40). This clearly indicated that ethylene units bridge blocks of propylene units with opposite regiochemistry, which is consistent with and further supports the whole mechanistic scenario.161... 

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