Coordination of monomers

Addition of two moles of a donor substance, 5,6-benzoquinoline, to one mole of R2A OAlR2 deactivated completely the activity of the latter. This fact provides positive experimental evidence for the currently accepted assumption that the coordination of monomer to catalyst is an essential factor for occurrence of stereospecific polymerization of propylene oxide. 

According to Cabassi et al. the reduction of polymerization rates by aromatic compounds is caused by competitive coordination of monomer and arenes to vacant Nd-sites. The following coordination equilibrium was put forward in order to account for the observed effects (Scheme 12) [165,166]. 

The most outstanding feature of alkyl metal catalysts is their stereospecificity. Viewing the entire field in this respect, it appears that a single, common feature is beginning to emerge. This is the coordination of monomer with one part of the catalyst prior to the addition of a partially stabilized polymer chain end. The coordination takes place between jr-electrons or lone-pair electrons of the monomer with vacant orbitals of a metal component. The polymer chain end is fixed in position and partially stabilized by either simple or complex gegen-ions. Such polymerizations are referred to as coordination or coordinated polymerizations to emphasize coordination of monomer. It should be noted that prior usage of these terms frequently implied either coordination of catalyst components or a concerted polymerization mechanism. 

Although a strong case can be made for coordination of monomer with a cation prior to addition, the question as to whether complexing occurs on the growing alkyl metal or on a second metal site in a dimer or higher aggregate cannot be answered at this time. Polyisoprenyl... 

If a coordination of monomer is considered as a pre-insertion step, one should modify Eq. [1] to ... 

In the copolymerization of phenylacetylene with various acetylenes by W- and Mo-based catalysts, sterically less hindered acetylenes always show higher reactivity 87). This suggests that the propagation reaction consists of two stages, i.e., monomer coordination and reaction of the coordinated monomer with the metal carbene, and that the relative reactivity of monomers in copolymerization is governed by the competitive coordination of monomers. 

The counterion is very stable in the polymerization of 3,3-bis-(chIoromethyl)-oxetane no termination, due to its decomposition, was observed up to 90 °C. Thus, it was proposed that the anion has a stabilized cyclic structure, formed after preliminary coordination of monomer with initiator ... 

The sites for coordination of monomer (indicated by the open dotted lines in (I) to (III)) have been the subject of much speculation in regard to the mechanism of stereoregulation [23, 24]. It has also been suggested that the active site may contain two vacancies [25]. In the above, catalyst complexes have been represented as bridged structures but they have also been considered to be ionic complexes [26], e.g. 

Molecular weights of polymers from styrenes deuterated in the side chain are the same as those of polystyrene prepared under the same conditions [78], and this is true also of polymers from ethylene and deuteroethylene [79]. If hydride ion transfer were rate determining an isotope effect would be expected with higher molecular weights in the deuterated polymers. The rate determining step would therefore appear to be coordination of monomer followed either by rapid transfer or insertion into the polymer chain. 

The occurrence of this reaction has been deduced [81] from the effects of the concentration of transition metal compound on polymer molecular weight, and the mechanism is not understood. It is conceivable that the chains are transferred to transition metal atoms without vacant sites for coordination of monomer and which do not then propagate or that the chain is eliminated with deactivation but exposes a fresh transition metal atom for alkylation and subsequent propagation. 

For the rate to pass through a maximum with increase in [M] it follows that Kfji [M] > 1, in accord with published values of for propene, since strong coordination of monomer to the sites would give rise to independence of rate on [M]. A fall in rate, however, implies that monomer is competing with active site formation it is difficult to see the mechzmism by which this could occur as exclusion of organometal from the catalyst surface by monomer would appear to be unlikely. 

The operator fix represents here the electrostatic potential generated by the unperturbed monomer X and depends on electron coordinates of monomer Y... 

In the preceding section the behavior of the catalyst at Al/ Ti 1.0 was examined. Next, lower ratios will be discussed, but first it is instructive to include some description of a-TiCla, another crystalline modification of the trichloride (Natta et al., 1961a). In combination with trialkylaluminum or dialkylalumi-num chloride the a form produces trans-l,4-poly dienes with butadiene or isoprene (Natta et al., 1959b). The reason for the difference in behavior between the /3 and a modifications has not definitely been established, but it is thought to be related to the different Ti-Ti ionic distances (Saltman, 1963). In /3-Ti-CI3 this is 2.9 A, about the same as the 1-4 carbon-carbon distance for isoprene in the cis conformation. The a-TiCls has a Ti-Ti distance of 3.54 A, more in line with the 1-4 carbon-carbon distance for isoprene in the tram conformation (3.7 A). Perhaps these atomic distances are fortuitously similar, but if one assumes two-point coordination of monomer on the surface the difference between the allotropic forms can be explained. 

At low propylene monomer concentrations, the back-skip of the polymer chain (B C) is faster than monomer coordination. This leads to high isotacticities at low propylene concentrations and elevated temperatures. At higher propylene monomer concentrations, however, coordination of monomer at the less hindered site D is favored over back-skip of the polymer chain from site B to site C. Subsequent coordination of the monomer at site D followed by migratory insertion leads to the formation of a stereoerror (D E). Insertion from E F proceeds in a stereoselective way, similar to the process A B, but instead leads to the formation of an rr triad owing to the previous nonselective insertion (D E). At low propylene monomer concentrations, the back-skip of the polymer chain to the less encumbered site (F C) is favored over monomer coordination (F D). At site C, the catalyst follows the isotactic cycle A B C. 

The TROP mechanisms were examined in detail, and the results of kinetic experiments were presented in [3], With consideration of these data, the mechanism of TROP of silacyclobutanes was advanced. The TROP of silacyclobutanes proceeds through ring opening via the heterolytic scission of the endocyclic Si-C bond and occurs as a zwitterion process, in which the coordination of monomer molecules to active centers plays an important role. In this case, the process is limited by the activity of a positively charged part of the zwitterion. 

Figure 4.10 Proposed structure of the active site (eiiminate hydrogen) and the dormant site for the styrene polymerization, (a) Active site, (b) dormant site by error coordination of monomer, and (c) dormant site by polymer chain rotation.

Polymerization occurs by cleavage of the oxygen bond attached to the less-substituted carbon atom of the oxirane ring with inversion of the configuration of the secondary carbon atom and formation of a secondary alkoxide polymer molecule while the approaching monomer oxirane complexes with the now vacant aluminum atom site. Stereospecificity of the resulting polymer depends on whether the coordination of monomer is enantiomorphic selectic (97). 

The polymerization of 2,3-epoxybutane with the same initiator as used with propylene oxide shows that the oxygen/substituted-carbon atom bond can be cleaved and, hence, a mechanism can be logically proposed to account for the head-to-head, tail-to-tail structures identified by Price and Vandenberg. With 2,3-epoxybutane, it was found that amorphous polymer could also be as pure disyndiotactic as the crystalline forms. Amorphous polymer could arise from short sequences of stereoregularity that were too short to form crystallizable segments. These could arise when coordination of monomer temporarily displaced alkoxide, interrupting chain growth, which, when resumed, could be selective for the antipode monomer. 

Bailey and France (104) proposed that with a range of coordination initiators, copolymerization propagation consisted of two steps coordination of monomer to an active site, metal or hydrogen, followed by rearrangement during addition with alkoxide and regeneration of an alkoxide ion. 

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