Isoprene, coordination

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

As we did in the case of relaxation, we now compare the behavior predicted by the Voigt model—and, for that matter, the Maxwell model—with the behavior of actual polymer samples in a creep experiment. Figure 3.12 shows plots of such experiments for two polymers. The graph is on log-log coordinates and should therefore be compared with Fig. 3.11b. The polymers are polystyrene of molecular weight 6.0 X 10 at a reduced temperature of 100°C and cis-poly-isoprene of molecular weight 6.2 X 10 at a reduced temperature of -30°C. 

Table 3.4 Temperature Coordinate and Relative Height (in Parenthesis) for the Two Loss Tangent Maxima Observed in Mixtures of Isoprene-Butadiene Block Copolymers with Homopolymers of These Two Repeat Units in the Same Proportion ...

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

When polymerizing dienes for synthetic rubber production, coordination catalysts are used to direct the reaction to yield predominantly 1,4-addition polymers. Chapter 11 discusses addition polymerization. The following reviews some of the physical and chemical properties of butadiene and isoprene. 

In anionic and coordination polymerizations, reaction conditions can be chosen to yield polymers of specific microstructurc. However, in radical polymerization while some sensitivity to reaction conditions has been reported, the product is typically a mixture of microstructures in which 1,4-addition is favored. Substitution at the 2-position (e.g. isoprene or chloroprene - Section 4.3.2.2) favors 1,4-addition and is attributed to the influence of steric factors. The reaction temperature does not affect the ratio of 1,2 1,4-addition but does influence the configuration of the double bond formed in 1,4-addition. Lower reaction temperatures favor tram-I,4-addition (Sections 4.3.2.1 and 4.3.2.2). 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

A study of the reactions of butadiene, isoprene, or allene coordinated to nickel in a metallacycle, with carbonylic compounds, has been reported by Baker (example 11, Table IV). In the presence of phosphines, these metallacycles adopt a cr-allyl structure on one end and a ir-allyl structure on the other, as mentioned in Section II,A,1. The former is mainly attacked by aldehydes or electrophilic reagents in general, the latter by nucleophiles (C—H acids, see Table I, or amines, see Table IX). 

First, new "living" initiators have been discovered (although not always as efficient), which respond to other mechanisms, i.e. cationic (5) or even radical ones (6), and can accordingly accomodate other types of monomers. A recent typical example is the coordination polymerization of butadiene by bis (n3-allyl-trifluoro-acetato-nickel) to yield a "living" pure 1.4 cis-poly-butadienyl-nickel, able to initiate in turn the polymerization of monomers like isoprene or styrene (7). 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

The anionic polymerization of 1,3-dienes yields different polymer structures depending on whether the propagating center is free or coordinated to a counterion [Morton, 1983 Quirk, 2002 Senyek, 1987 Tate and Bethea, 1985 Van Beylen et al., 1988 Young et al., 1984] Table 8-9 shows typical data for 1,3-butadiene and isoprene polymerizations. Polymerization of 1,3-butadiene in polar solvents, proceeding via the free anion and/or solvent-separated ion pair, favors 1,2-polymerization over 1,4-polymerization. The anionic center at carbon 2 is not extensively delocalized onto carbon 4 since the double bond is not a strong electron acceptor. The same trend is seen for isoprene, except that 3,4-polymerization occurs instead of 1,2-polymerization. The 3,4-double bond is sterically more accessible and has a lower electron density relative to the 1,2-double bond. Polymerization in nonpolar solvents takes place with an increased tendency toward 1,4-polymerization. The effect is most pronounced with... 

Discuss the use of homogeneous versus heterogeneous reaction conditions for the coordination and traditional Ziegler-Natta polymerizations of propene, isoprene, styrene, methyl methacrylate, and n-butyl vinyl ether. 

Conjugated dienes (1,3-butadiene, isoprene) have suitable nucleophilicity to undergo cationic polymerization. There is, however, not much practical interest in these processes since the polymers formed are inferior to those produced by other (free-radical, coordination) polymerizations. A significant characteristic of these polymers is the considerably less than theoretical unsaturation due to cyclization processes.132 A fully cyclized product of isoprene has been synthesized163 by constant potential electrolysis in CH2C12. 

A large number of metal-coordinated chalcogenacycles were prepared from the heteroaldehyde and -ketone complexes [M(CO)5 E=C(Ph)R ] (M = Cr, Mo, W E = S, Se, Te R = H, Aryl) and 1,3-dienes. The cycloadditions proceeded rapidly, even at low temperatures.179180,228-233 As dienes 2,3-dimethyl-l,3-butadiene [for an example see Eq. (27)], isoprene, transA, 3-pentadiene, cyclopentadiene, pentamethylcyclopentadiene, and 1,3-cyclohexadiene were used. Decomplexation with pyridine/THF yielded the free heterocycles. Although the tellurobenzaldehyde complex [W(CO)5 Te = C(Ph)H ] (99c) proved too unstable for isolation, it could be generated and employed in subsequent cycloadditions with dienes.180... 

The mechanism for the polymerization of dienes to the ds 1,4 structure is also parallel. Sterns and Foreman (112) presented a cyclic 6-membered transition state to produce the cis structure. Orr (113) concluded that the configuration of the monomer itself has no role to play in determining the amount of cis or trans isomer produced from isoprene and butadiene. There is no requirement for the prealignment of the diene monomer by coordination between a diene molecule and the gegen ion. The diene must only assume the ds transition state during reaction. 

Slcreospecific solution polymerization has been emphasized since the discovery of the complex coordination catalyses that yield polymers or butadiene and isoprene having highly ordered microstructures. The catalysts used are usually mixtures of organometallic and transition metal compounds. An example of one of these polymers is cis- 1.4-polybutadiene. 

Alkylation of cyclohexane with isoprene can be carried out with alkyl radicals formed at 450°C and 20.3 MPa (200 atm) (73). 40% Pentenylcyclohexanes, 20% dipentenes (ie, substances having the general formula C1QH16), and 40% higher hoiling compounds are obtained using a 6.8 molar ratio of cyclohexane to isoprene and a space velocity of 2.5 h-1. Of the pentenylcyclohexanes, the head and tail products are in equal amounts. Even stable radicals, eg, triphenylmethyl, add readily to isoprene (74). Olefins, eg, ethylene, propylene, and styrene, add to isoprene in the presence of coordination catalysts that are based on cobalt, nickel, or iron (8). 

A Rh complex coordinated by the water-soluble phosphine TMSPP (XLVII) catalyses the 1 1 coupling of isoprene with the malonamide derivative 179 to give 180, and feprazone (181) was prepared after isomerization [78]. Under similar conditions, reaction of myrcene (182) with methyl acetoacetate gives 183 and 184 in 97% yield,... 

The hydrides readily lose their hydrogen upon reaction with other materials. In some cases, the hydrogen is transferred to the reactant, which then occupies the coordination position originally held by the hydride ligand (127), e.g., HV(CO)4PP + EtsN -> HNEt3V(CO)4PP and HV(CO)4(dppm) + isoprene -> (T)3-dimethylallyl)V(CO)3(dppm). 

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