Propylene coordination polymerisation

Coordination polymerisation was first proposed in 1956 for the unusual, at that time, low-pressure polymerisation of ethylene and polymerisation of propylene with the transition metal catalysts discovered by Ziegler in 1953 [1], and for the ferric chloride catalysed ring-opening polymerisation of propylene oxide to crystalline polymer reported by Pruitt et al. in a Dow patent [2]. 

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

The type of solvent or diluent should be specified in reporting a Ziegler-Natta catalyst system. Alkene polymerisations are usually carried out in inert solvents, such as aliphatic or aromatic hydrocarbons (e.g. some gasoline fractions or toluene). The use of protic or aprotic polar solvents or diluents instead of the hydrocarbon polymerisation medium can drastically alter the reaction mechanism. This usually results in catalyst deactivation for alkene coordination polymerisation. Modern alkene polymerisation processes are carried out in a gas phase, using fluidised-bed catalysts, and in a liquid monomer as in the case of propylene polymerisation [28,37]. 

In the model proposed for the active centre of the propylene syndiospecific polymerisation, the V(III) atom is pentacoordinated [327]. Its ligands include three chlorine atoms (two of which are bridge-bonded to the aluminium atom), the chiral carbon atom of the last monomer unit of the growing chain and the coordinated propylene molecule. Prior to its coordination and after its insertion, the vanadium atom is tetracoordinated. In the alternative similar model, two chlorine atoms are substituted by a bidentate dionate, and the chlorine atom is bridge-bonded to the aluminium atom in dimeric A1R2C1 [2]. 

Figure 3.59 Life cycles of catalysts for olefin coordination polymerisation (a) early-generation Ziegler-Natta catalysts for ethylene and propylene polymerisation (b) Phillips catalysts for ethylene polymerisation (c) fourth-generation Ziegler-Natta catalysts for ethylene polymerisation (d) fourth-generation Ziegler-Natta catalysts for propylene polymerisation (e) metallocene-based catalysts for olefin polymerisation leading to polymers of various stereoregularity...

The coordination polymerisation and copolymerisation of heterocyclic monomers have been restricted in industry to a much smaller volume than the polymerisation and copolymerisation of hydrocarbon monomers polyether elastomers from epichlorohydrin and ethylene oxide or propylene oxide, and allyl glycidyl ether as the vulcanisable monomeric unit, are produced on a larger scale [4-7],... 

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

The first isolated and characterised species that could be envisioned as intermediates in the initiation step for the coordination polymerisation of epoxides when using metal carboxylate catalysts were complexes formed between cadmium carboxylates, solubilised in organic solvents by the tris-3-phenylpyrazole hydroborate ligand, and epoxides such as propylene oxide and cyclohexene oxide [68]. Other epoxide complexes with various metal derivatives have also been reported in the literature [69-72],... 

Semiempirical molecular orbital calculations on this model [309] suggest that, in the case of propylene polymerisation, equatorial 2,4-substitution of the metallacyclopentane ring is the most stable form this would lead to regiose-lective head-to-tail propagation during the polymerisation of propylene and, moreover, to the formation of isotactic polypropylene [51]. Such calculations concern a case, however, that has not been confirmed by experiments a coordination of propylene at Ti(II) species and subsequent reaction according to the above scheme is not as obvious as that of ethylene. 

This model would explain the inability of metallocene-alkylaluminium halide systems to promote the polymerisation of propylene and higher a-olefins [94] it is obvious that there is insufficient capability of the more weakly coordinating a-olefins to form reactive, olefin-separated ion pairs by displacement of an aluminate anion from the metal centre. At any rate, the limitation of homogeneous catalysts to the polymerisation of only ethylene was a crucial obstacle to progress in this field for many years. This impediment was fortunately overcome, however, by a series of serendipitous observations [90-95, 100,101,103] that led, around the 1980s, to the discovery by Kaminsky, Sinn et al. [90, 91,94,95,100,101] that metallocenes are activated for catalysing the polymerisation of propylene and other a-olefins (without a, a-disubstituted olefins) by methylaluminoxane [30],... 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

Figure 3.38 Possible model site for isospecific propylene polymerisation with the (R, A>)-/Y/r.-(ThindCH2)2 ligand-containing catalyst. The broken lines indicate the forbidden conformation of the growing chain and the favoured coordination of propylene corresponding to this forbidden chain conformation. For the sake of clarity, hydrogen atoms are omitted. O Zr O C, CH, CFF or CH3. Reproduced (by permission from Elsevier Science) from Ref. 1. Copyright 1989 Pergamon Press...

If 1-butene or 1-hexene is chosen instead of propylene as the monomer polymerising with the Me2C(MeCp)(Flu)ZrCl2-based catalyst, the polymers obtained become enriched in m diads. This has been suggested to testify to the preference of site isomerisation prior to the coordination of the next monomer molecule with increasing size of the polymerising a-olefin [121]. 

The polymerisation of racemic propylene oxide with coordination catalysts leads to a polymer that can be fractionated into crystalline and amorphous polypropylene oxide)s ... 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

Copolymerisation of propylene oxide as well as other oxiranes with carbon dioxide in the presence of zinc-based coordination catalysts is generally accompanied with the formation of a cyclic five-membered carbonate, propylene carbonate or another alkylene carbonate [147,206,207,210,212,230]. The alky-lene carbonate, however, is not the precursor for poly(alkylene carbonate), since it hardly undergoes a polymerisation under the given conditions [142-146],... 

The study of external and internal donors has been made in [73], in terms of their influence on the composition of polymer fractions and their stereoregularity in the process of propylene polymerisation on titanium-manganese catalysts. Three types of AC are assumed aspecific centres with two coordination vacancies isospecific centres with one coordination vacancy, and isospecific centres with a mixture of donor and coordination vacancies from an aspecific active centre. 

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