Stereoselectivity monomer insertion

A necessary (but not sufficient) prerequisite for models of stereospecific catalytic systems is the stereoselectivity of each monomer insertion step. The possible origin of stereoselectivity for models of several kinds of catalytic systems has been investigated through molecular modeling. 

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

Molecular mechanics analyses of the kind described in previous sections are able to rationalize not only the stereoselectivity (and stereospecificity) of regioregular primary insertion steps but also the stereoselectivities relative to occasional secondary monomer insertions as well as relative to primary insertions following these secondary insertions.37d... 

In summary, there is substantial stereoselectivity of this isospecific C2 symmetric catalytic model for the lower energy (and experimentally observed) primary monomer insertion, and the stereoselectivity would also be higher for the higher energy (minor but experimentally detected) secondary monomer insertion. It is worth noting that the stereoselectivity of the isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions,37d... 

The mechanisms for chain-end stereoselectivity (isospecific and syndiospecific) for primary monomer insertion (Section 4.1.1) present relevant analogies with the well-established mechanism of chiral site controlled stereoselectivity (Section 3). In fact, for both mechanisms, the selection between the two... 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

It should be noted that Ci-symmetric metallocenes show a stereoselectivity increase with elevated polymerization temperature and lowered monomer concentration, a behavior opposite to that displayed by the C2-symmetric metallocenes. The regioregularity of PP samples prepared with most Ci-symmetric metallocene catalysts is fairly high. The predominant monomer insertion mode is 1,2. Isolated 2,1 and/or 3,1 units can be observed with <0.5 mol%o. 

Waymouth and Coates employed the homogeneously catalyzed cyclopolymerization of 1,5-hexadiene giving poly (methylene-1,3-cyclopentane) as previously developed in his group in order to utilize the stereoselectivity of the monomer insertion for the construction of a polymer with main-chain chirality. The cyclopolymerization is a remarkable chain growth reaction during which a conventional... 

In 1962. Natta and Zambelli reported a heterogeneous. vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures. " The regiochemistry of the insertion was determined to be a 2.1-insertion of propylene, and a chain-end control mechanism determined the s mdiospecificity of monomer insertion. This catalyst system suffered from both low activity and low stereoselectivity. Highly active single-site olefin polymerization catalysts have now been discovered that make syndiotactic polypropylene with nearly perfect stereochemistry. Catalysts of two different symmetry classes have been used to make the polymer, with Cs-symmetric catalysts typically outperforming their Q -symmetric counterparts due to different mechanisms of stereocontrol (Figure 10). 

The polymerization reaction is a sequence of different events, such as monomer insertions, site isomerizations, and chain release reactions. The polymer chain can be seen as a permanent picture of the sequence of these events, and it is possible to use a statistical approach to study their distribution along the chain to increase our knowledge on polymerization mechanisms. As a consequence, a mathematical model of the polymerization can be built by assigning a probability at each event in our system. In the case of propene homopolymerization, this approach is (largely) used to study the mechanisms governing the stereoselectivity of the catalyst from the NMR spectrum of the polymer. In fact, the type and the relative amount of the stereosequences present in the chain are obtained from the methyl region of the spectrum and are usually determined at the pentad level (see section II.G). This distribution can be studied using insertion probabilities for propene enantiofaces, which depend on the type of stereocontrol mechanism active for the catalytic... 

Enantiomorphic Site with Chain-End Control. In the case of less stereoselective Cz-symmetric metallocene catalysts, the magnitude of chain-end control can be comparable to that of site control. In this case, obviously, the former has to be added to the model using Markovian statistics. The probability parameters are the same found for pure chain-end control p si re), i.e., the probability of insertion of a si monomer enantioface after a monomer inserted with the re face, p re si), p si si), and p re re). In this case, the metallocene chirality prevents the equiprob-ability of the si olefin insertion after a re inserted monomer (see structure on the left in Scheme 36) and re olefin insertion after a si inserted monomer (see structure on the right in Scheme 36). 

A major consequence of this pathway, also known as migratory insertion, is that the growing chain sweeps from one side to another with every addition of monomer. This is a generalization, for some authors have explained the loss of stereoselection in metallocene polymerizations of propylene, for example, at low monomer concentrations to the action of a windshield wiper isomerization, in which the polymer chain and open coordination site switch places without the benefit of monomer insertion. At low insertion rates, this site inversion phenomenon may become competitive with insertion and thus render ineffective any substituent influences which differ between the two faces of the catalyst site. With appropriate ligand design, different or enantiomeric steric environments may be created for the two sides of the active site. This makes possible stereoselective polymerization of propylene and higher a-olefins, as will be seen below. 

Third, to the extent that the catalyst system preferentially utilizes the more stereoselective site for monomer insertion, enhancement of the stereoselectivity at that site will lead to higher isotacticity. Thus, efforts to increase the size of the fluorenyl ring have led to enhanced isoselectivity, presumably because a substituted benzo moiety is more repulsive in the transition state than the benzo group itself. Catalysts derived from fert-butylated fluorenes such as f-16 ([mmmm] = 95.7%, = 153 °C)... 

A prochiral monomer such as propylene offers two faces for coordination to a metal center. The steric environment at the active site, formed by the coordinated ligands and the growing polymer chain after activation with a cocatalyst, determines the orientation of the incoming monomer. In this case, the mechanism of stereoselection is referred to as enantiomorphic site control. The stereochemistry of the polymer is thus determined by the chirality relationship of the two coordination sites of the catalyst. However, every monomer insertion generates a new stereogenic center. As a consequence, chiral induction (enantioface preference) arises from the last-inserted monomer unit in the growing polymer chain. This mechanism is referred to as chain-end control (see Chapter 1 for an introduction to chain-end and enantiomorphic site control mechanisms in iPP synthesis). 

In the course of the polymerization, the monomer insertion may lec a either to a or to an n -allyl oonplex. In the first case, several mononner insertions may take place before going back to the stable dormant syn n -allyl isomer of the growing chain-end (17). In the latter case, only the formation of a very reactive anti n -adLlyl structure will ensure the formation of the cis-1,4-polybutadiene often observed es jerimentally such a stereoselectivity would inply the absence of any rapid anti-syn isomerization, in accordance with our WR studies on ANiUA (see above), and with the SurCollette diene-olefin dimerization mechanism (29). It is difficult to decide v ch one of these two possible pathways is actually followed, but v tever it can be, the genercLL schonne will not be changed. 

Isotactidty of poly(POx) chains corresponding to the crystalline fraction is explained by steric constraints and orientation of the complexed monomer, which induces stereoselectivity in the nudeophilic attack by the chain end. It was demonstrated that monomer insertion proceeds by attack at the carbon atom of the epoxide ring where it is deaved with inversion of the carbon configuration. This necessitates an attack of the complexed monomer by the nudeophile from the back, which requires the partidpation of two adjacent aluminum atoms and chain transfer from one aluminum atom to the other one at each monomer addition. The coordination polymerization mechanism proposed by Vandenberg for the trialkylaluminum/water system is shown in Scheme 23. 

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