Enchainment

In the case of a-olefin polymerisation, the monomer insertion into the Mt C bond in the propagation step can proceed by two pathways - via monomer 1,2-enchainment (primary insertion) or 2,1-enchainment (secondary insertion) ... 

For example, the rates of propylene and 1-butene polymerisation by the rac.-(IndCH2)2ZrCl2—[Al(Me)0]x catalyst increase in the presence of hydrogen roughly by a factor of 10-60 respectively [260]. These polymers were found to be free of misinserted units hydrogenolysis apparently eliminates the slowly inserted 2,1-enchained monomer units and allows the start of a new, fastgrowing polymer chain [scheme (44)] [30],... 

According to most models proposed for polymerisation with vanadium-based homogeneous Ziegier-Natta catalysts such as VCI4—A1R2C1, the active site may involve pentacoordinated three-valent vanadium species with three Cl atoms, the secondary carbon atom of the last monomer unit inserted (predominantly by 2,1 enchainment of the a-olefin) and the coordinating olefin [323-327]. However, a model of the active site that assumes a hexacoordinated V(III) species with four Cl atoms, the carbon atom of the last chain unit and the coordinating olefin has also been proposed [328],... 

In coordination polymerisation, a-olefins of the formula CH2=CH(CH2)aH may be inserted into the growing polymer chain in a 1,2 or 2,1 manner. Normally, these insertion steps lead to 1,2-enchainment or 2,1-enchainment of the monomer. Both of these steps form a -(CH2)aH branch. However, with some catalysts herein, some of the initial product of 1,2-insertion can be rearranged by migration of the coordinated metal atom to the end of the last inserted monomer before insertion of additional monomer, similarly to that presented in schemes (69) and (70). This results in m,2-enchainment and the formation of a methyl branch ... 

Halobutyls. Chloro- and bromobutyls are commercially the most important butyl mbber derivatives. The halogenation reaction is carried out in hydrocarbon solution using elemental chlorine or bromine (equimolar ratio with enchained isoprene). The halogenation is fast, and proceeds mainly by an ionic mechanism. The stmctures that may form include the following ... 

Halogenated Butyl Rubber. The halogenation is carried out in hydrocarbon solution using elemental chlorine or bromine in a 1 1 molar ratio with enchained isoprene. The reactions ate fast chlorination is faster. Both chlorinated and brominated butyl mbbers can be produced in the same plant in blocked operation. However, there are some differences in equipment and reaction conditions. A longer reaction time is requited for hromination. Separate faciUties are needed to store and meter individual halogens to the reactor. Additional faciUties are requited because of the complexity of stabilising brominated butyl mbber. 

Butyl Rubber. In butyl mbber, isoprene is enchained by 1,4-addition ia the trans configuration (74). 

The most interesting aminomethyl derivative of condensation polymers that we have prepared to date Is derived from direct reduction of poly(2-cyano-l,3-phenylene arylene ether), 20. Enchainment of benzonitrile repeat units Is accomplished by coupling 2,6-dichlorobenzonitrile with the potassium salt of bisphenol-A copolymers with lower nitrile contents can be produced by copolycondensation of bisphenol-A, 2,6-dichlorobenzonitrile and 4,4 -dichlorodiphenyl sulfone.21 The pendent nitrile function provides an active site for further elaboration. 

Scheme 9 Modes of catalyst enchainment in polymerizations of a-olefins as a result of 1,2- or 2,1-insertion...

Figure 1.25 Minimum-energy diastereoisomeric monomer free intermediates for butadiene polymerization catalyzed by titanium complexes presenting Cp group as ancillary ligand. Chiralities of coordination of allyl groups (assumed to be si) and back-biting double bonds (si or re) are indicated, in order to easily visualize possible stereoregularity (iso or syndio) of model chains. In fact, like and unlike chiralities would possibly lead to isotactic and syndiotactic enchainments, respectively.

Many discoveries changed human life in the last fifty years, and the polymerization of olefins catalyzed by transition metals certainly can be considered among them. At first glance, the job requested to polymerization catalysts seems rather trivial. It consists in the enchainment of monomeric units by insertion of olefins into Mt-P (P = Polymeric chain) bonds. Despite its simplicity and the amount of work by many research groups both in the... 

According to the scheme of Figure 12, in the framework of a regular chain-migratory mechanism the C2 and some Cs symmetric metallocenes lead to iso- and syndiotactic polymers, respectively. In fact, for the C2 symmetric systems the same propene enantioface is enchained at each... 

Figure 5.8 A single enchainment stereo isomer is obtained in the acetylcholine templated synthesis of a [2] catenane.

Following the above conclusion it is clear that the rather bizarre spectra of these polymers derive from the special features of 1,2-enchainment. Examination of molecular models reveals that runs of 1,2-enchained segments are considerably more restricted in their degrees of motional freedom than are runs of 1,4- enchained segments. The restriction arises partly from the absence of a "crankshaft" mode with 1,2-enchainment and partly from the steric interference of substituents on adjacent phenylene rings. 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

The anion dissociates, and the coordinatively unsaturated metal center then picks up a monomer molecule for subsequent enchainment. This dissociative model has been favored in the past [16, 21-23, 27-28] since it allows a convenient explanation of the observed polymer stereochemistry by considering only the roles of the ligand and the alkyl chain in the cationic metallocene complex. However, anion dissociation opposes the electrostatic attraction between cation and anion and is therefore energetically expensive. So does it operate at all ... 

Even if anion dissociation can be shown to be prevalent, there will be a strong tendency for anion binding. If anion re-association is slow, many monomer units may be consumed and polymerised before anion binding interrupts the process. On the other hand, if enchainment is slow, anion recombination can be expected to occur after each insertion step, to form a possibly long-lived resting state. Which mechanism applies ... 

NMR analysis of the polymer also detected low levels of stereo-errors due to enchained 2,1-misinsertions (cf Section 8.9), about 1 in 500. The data suggested that 2,1-insertion is slow but is responsible for the accumulation of dormant states carrying Zr-sec-alkyl chains. 

In principle, each enchainment of a prochiral monomer such as propene generates a chiral carbon center. The relative orientation of two such centers can therefore give rise to meso-diads or racemo-diads. Three monomers consecutively enchained in meso-fashion give rise to a mm triad, and so forth (Scheme 8.37). 

Kinetic measurements of the ring-opening polymerization of trimethylene carbonate (TMC) versus the enchainment of oxetane and CO2 to provide poly (TMC) reveal that these processes in the presence of (salen)CrCl and an ammonium salt have similar free energies of activation (AG ) at 110°C. This similarity in reactivity coupled with the observation that in situ infrared studies of the copolymerization of oxetane and CO2 showed the presence of TMC during the early stages of the reaction has led to the overall mechanism for copolymer production shown in... 

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