Olefin polymerization kinetics

Gas phase olefin polymerizations are becoming important as manufacturing processes for high density polyethylene (HOPE) and polypropylene (PP). An understanding of the kinetics of these gas-powder polymerization reactions using a highly active TiCi s catalyst is vital to the careful operation of these processes. Well-proven models for both the hexane slurry process and the bulk process have been published. This article describes an extension of these models to gas phase polymerization in semibatch and continuous backmix reactors. 

Scheme 4 Mechanism of chain growth for a all Pd(II) polymerizations and ethylene polymerizations with Ni(II), and b a-olefin polymerizations with Ni(II). Specific kinetic data shown for Ni catalyst 1.15b [63]...

In addition to the unique insertion kinetics, the polymerization of a-olefins by the Ni(II) catalysts affords polymers of narrow molecular weight distributions at low temperature. At -10 °C, chain transfer becomes reduced to the extent that polymerizations are living with polydispersities below 1.10 (Table 3) [70], This was... 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

The largest-volume polymers are polyolefins, and the kinetics of olefin polymerization are fairly similar to the ideal addition process just considered. All these olefins form condensation products to form a very long-chain alkane such as... 

The behavior of the different catalytic systems (containing transition metal crystalline compounds) in the a-olefin polymerization, except for the different degree of stereospecificity, may be connected with a definite kinetic scheme. This was shown by experimental work performed at the Institute of Industrial Chemistry of the Milan Polytechnic. 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then react with excess monomer to start propagation. The kinetics and mechanisms of cationic polymerization and polycondensation have been studied extensively.925-928 Kennedy and Marechal926 have pointed out that only cations of moderate reactivity are useful initiators, since stable ions such as arenium ions were found to be unreactive for olefin polymerization. On the other hand, energetic alkyl cations such as CH3CH2+ were too reactive and gave side products. 

Based on these kinetic and microscopic observations, olefin polymerization by supported catalysts can be described by a shell by shell fragmentation, which progresses concentrically from the outside to the centre of the support particles, each of which can thus be considered as a discrete microreactor. A comprehensive mathematical model for this complex polymerization process, which includes rate constants for all relevant activation, propagation, transfer and termination steps, serves as the basis for an adequate control of large-scale industrial polymerizations with Si02-supported metallocene catalysts [A. Alex-iadis, C. Andes, D. Ferrari, F. Korber, K. Hauschild, M. Bochmann, G. Fink, Macromol. Mater. Eng. 2004, 289, 457]. 

Propylene conversion over three SAPO molecular sieves (SAPO-5, SAPO-11, and SAPO-34) was conducted at a variety of operating conditions. Catalyst behavior was correlated with the physical and chemical properties of the SAPO molecular sieves. The objective of this work was to determine the relative importance of kinetic and thermodynamic factors on the conversion of propylene and the distribution of products. The rate of olefin cracldng compared to the rate of olefin polymerization will be addressed to account for the observed trends in the product yields. The processes responsible for deactivation will also be addressed. 

The kinetics and mechanism of olefin polymerization can be elucidated from the dependences of the number, reactivity and selectivity of the active centers (active sites, polymerization centers) on ... 

In this article the problems of kinetics and the mechanism of olefin polymerization proceeding on solid Ziegler-Natta catalysts are discussed, using the novel data accumulated in polymer research ... 

The kinetic parameters of the propagation of olefin polymerization on different active centers are compiled in Table 10. Apparently, for both the transition and non-transition metal compounds the insertion of the olefin into Mt—C bonds proceeds with the participation of coordinatively unsaturated metalalkyl compounds via intermediate n-complexes. The higher reactivity of transition metal compounds compared with organoaluminium compounds is primarily due to the lower activation energy of the propagation step when Mt is a transition metal. Many facts indicate that polarization of the Mt—C bond does not determine the reactivity of metalalkyl compounds in olefin addition, e.g. due to the decrease of reactivity in the order... 

Table 10. Kinetic parameters for the propagation reaction of olefin polymerization on various active centers...

The catalytic oligomerization of olefins in the presence of OAC and the olefin polymerization in the presence of transition metals are based on similar olefin insertions into the metal-carbon and metal-hydrogen bonds (see Section 3.2). However, in organoaluminium compounds, the structure of the active center is defined more simply and more reliably. Data on its coordination state, thermodynamic and kinetic parameters have been reported (e.g. Table 13). 

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