Alkene polymerisation reaction

Figure 7.3 gives an overview of the reactions involved in the hydroformylation of internal alkenes to linear products. It has been suggested that cobalt, once attached to an alkene, runs along the chain until an irreversible insertion of CO occurs. Thus, the alkene does not dissociate from the cobalt hydride during the isomerisation process. There is no experimental support for a clear-cut proof for this mechanism. In alkene polymerisation reactions this type of chain running has been actually observed. 

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. 

Thermal cracking of wax. From thermal cracking a thermodynamic mixture might have been expected, but the wax-cracker product contains a high proportion of 1-alkenes, the kinetically controlled product. Still, the mixture contains some internal alkenes as well. For several applications this mixture is not suitable. In polymerisation reactions only the 1-alkenes react and in most cases the internal alkenes are inert and remain unreacted. For the cobalt catalysed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatisations the isomer composition is pivotal to the quality of the product. 

In the presenee of an organic peroxide initiator, the alkenes and their derivatives undergo addition polymerisation or chain growth polymerisation through a free radical mechanism. Pol3dhene, teflon, orlon, ete. are formed by addition polymerisation of an appropriate alkene or its derivative. Condensation polymerisation reactions are... 

The data discussed in Sections 8.5 and 8.6 make it clear that in the low-dielectric media typically employed for polymerisation reactions, the counteranions in metallocene ion pair catalysts are closely associated with the cationic complex as either inner-sphere or outer-sphere ligands. If anions are coordinated in the transition state, they must be expected to exert a significant influence on the stereochemistry of alkene polymerisation, even though the formation of syndiotactic and isotactic 1-alkenes have been readily explained by considering only the cationic metallo-cenium species and their ligand structure [21, 23, 122, 132, 133]. 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

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]. 

Later on, Calderon et al. [42,43] recognised that the ring-opening polymerisation of cyclic olefins is a special case of the more general alkene metathesis reaction, e.g. as for propylene ... 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

Both of these reactions have very important industrial uses (Section 14.3.9). In order to obtain alkene streams of sufficient purity for further use, the products of steam-cracking or catalytic cracking of naphtha fractions must be treated to lower the concentration of alkynes and alkadienes to very low levels (<5ppm). For example, residual alkynes and dienes can reduce the effectiveness of alkene polymerisation catalysts, but the desired levels of impurities can be achieved by their selective hydrogenation (Scheme 9.4) with palladium catalysts, typically Pd/A Os with a low palladium content. A great deal of literature exists,13,37 particularly on the problem of hydrogenating ethyne in the presence of a large excess of... 

The mechanism of this type of polymerisation reaction follows typical step-growth polymerisation rules, and the diene dithiol (AA BB) ratio is therefore a very important parameter [19]. This observation implies that, in contrast to some of the grafting reactions mentioned above, the dithiol compound cannot be used in excess to compensate for the lower reaction rates with internal alkenes. Hence, these polymerisations are limited to the use of structures bearing terminal double bonds [19]. If this rule is applied to vegetable oils, 10-undecenoic acid and its derivatives are the only species that fulfill this criterion. 

It is now apparent that seven-coordinate tungsten(II) and molybdenum(II) complexes of the type [(CO)4M( Li-Cl)3M(M Cl3)(CO)3] and [MCl(M Cl3)(CO)3(NCMe)2] (M = W, Mo M = Sn, Ge) can be synthesised and isolated in crystalline forms. These complexes readily react with alkenes and alkynes in mild conditions to give crystalline products. In the presence of an excess of terminal alkyne or cyclic olefins such compounds are catalysts of the polymerisation reaction. 

Schrock and Fischer type carbyne tungsten or molybdenum complexes are very interesting catalysts for alkene metathesis or alkyne polymerisation reactions. Within the first reaction steps they form carbene complexes and on these carbene complexes further metathesis or polymerisation occur. 

Dimolybdenum quadraple bonds can also act as catalysts for radical addition and polymerisation reactions. Their performance can be tuned by modifying the redox potential of the dimetal core. For example, the carbo)grlate bridged compound Mo2(TiPB)4 ( 4/2 = 0.140 V vs. Fc/Fc ) acts as a catalyst for radical addition reactions of polyhaloalkanes to 1-alkenes and cyclopentene, while the formamidinate and guanidi-nate bridged compounds ( 1/2 = —0.308 V and —0.581 V, respectively) are effective catalysts for the radical polymerisation of methyl methacrylate. 

Ross Xiao, 2002 Itoh, 2001 Stark et al., 1999], hydroformylation reactions [Magna et al., 2007 Sharma, 2009], Pd-mediated C-C bond formation [Park Alper, 2003], alkene polymerisation [Hardacre et al., 2002], and biotransformations [Bornscheuer Kazlauskas, 2006],... 

The catalytic activity of the phosphorus macrocycle complexes 23 (R = iBu, M = Re(CO)2Cl and Re(CO)2H) in cyclic alkenes polymerisation was studied. Addition of norbornene to the solution of activated catalyst results in the strongly exothermic reaction proceeding at a reasonable rate at 0 °C. The behaviour of the... 

Mixtures of C4 alkene isomers (largely isobutene) are polymerised commercially in contact with low levels of aluminium chloride (or other Lewis acid) catalysts. The highly exothermic runaway reactions occasionally experienced in practice are caused by events leading to the production of high local levels of catalyst. Rapid increases in temperature and pressure of 160°C and 18 bar, respectively, have been observed experimentally when alkenes are brought into contact with excess solid aluminium chloride. The runaway reaction appears to be more severe in the vapour phase, and a considerable amount of catalytic degradation contributes to the overall large exotherm. 

Polymers can be formed from compounds containing a c=c double bond. Alkenes, such as ethene, can undergo addition polymerisation to form a polymer. A polymer is a compound consisting of very long chain molecules built up from smaller molecular units, called monomers. The polymerisation of ethene, to form poly(ethene), is a free radical addition reaction. 

Since one of the substrates is a cyclic alkene there is now the possibility of ring-opening metathesis polymerisation (ROMP) occurring which would result in the formation of polymeric products 34 (n >1). Since polymer synthesis is outside the scope of this review, only alkene cross-metathesis reactions resulting in the formation of monomeric cross-coupled products (for example 30) will be discussed here. 

When analogous reactions were performed using symmetrical internal acyclic alkenes only polymerisation of the cyclobutene substrate was observed. 

Use of a symmetrical acyclic alkene limits the possible metathesis products to the desired diene (for example 45) and products formed from polymerisation of the cyclic substrate. Competing ROMP was suppressed in these reactions by using dilute conditions and a tenfold excess of hex-3-ene. By adding the cyclic substrate slowly to a solution of the catalyst and ris-hex-3-ene (which was significantly more reactive than the trans isomer), less than two equivalents of the acyclic alkene were used without causing a significant drop in the cross-metathesis yield. 

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