Polydienes

Polydienes. The most important diene homopolymers are polybutadiene and polyisoprene produced by anionic or coordination polymerization.184,186,187,487 189 Highly purified starting materials free from acetylenes, oxygen, and sulfur compounds are required. 

The commerical polybutadiene (a highly 1,4 polymer with about equal amounts of cis and trans content) produced by anionic polymerization of 1,3-butadiene (lithium or organolithium initiation in a hydrocarbon solvent) offers some advantages compared to those manufactured by other polymerization methods (e.g., it is free from metal impurities). In addition, molecular weight distributions and microstructure can easily be modifed by applying appropriate experimental conditions. In contrast with polyisoprene, where high cis content is necessary for suitable mechanical properties, these nonstereoselective but dominantly 1,4-polybutadienes are suitable for practical applications.184,482 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

Dimerization of other simple alkenes such as propylene and butenes is of practical significance because of the problems associated with the use of MTBE the presently widely used high-octane oxygen blending component. The transformation of low-value streams, such as butenes, into valuable gasoline blending components is particularly attractive. 

Traditionally, solid acidic catalysts are applied in industry for the oligomerization of butenes and are still studied. MTS-type aluminosilicates,522 a NiCsNaY zeolite,523 and a silica-alumina containing 13% alumina524 proved to be active and selective catalysts. Moreover, deactivation rates of these catalysts are also favorable. Sulfated zirconia promoted with Fe and Mn was active and selective to yield primarily dimethylbutene isomers under supercritical conditions.525 A small amount of water improved productivity and decreased deactivation. A study showed that the blending octane number of Cg hydrocarbons is directly linked to the number of allylic hydrogens in the molecules.526 

Polydienes are produced by the polymerization of dienes such as butadiene, isoprene, chloroprene, etc. They differ in constitution and 

The photo-induced microstructural changes in polydienes have been extensively studied by Golub who is also the author of a recent review of the photochemistry of unsaturated polymers [47]. 

The photodegradation and photo-oxidative degradation of different polydienes have been the subjects of many publications (Table 3.19). 

Chlorinated rubber Polymeric blends containing polybutadiene segments Polybutadiene blends with  

Poly(styrene-butadiene) rubber (SBR) (high impact polystyrene) 

Blends of poly(butadiene-co-acrylonitrile) with poly(vinyl chloride) Poly(acrylonitrile-co-butadiene-co-styrene) (ABS) 

A diene contains two double bonds and during polymerisation only one of them opens, which leads to the formation of polymers of three distinctly different kinds. The three forms for polybutadiene, obtained from butadiene, H2C=CH—CH=CH2, are shown in fig. 4.6. The vinyl 1,2 form, for which the double bond is in the side group X, can have any of the types of tacticity discussed above. The two 1,4 types, for which the double bond is in the chain backbone, illustrate a form of configurational isomerism. Rotation around a double bond is not possible, so these two forms are distinct. In the cis form the two bonds that join the unit shown to the rest of the chain are on the same side of a line passing through the doubly bonded carbon atoms of the unit, whereas for the trans form they are on the opposite side of that line. 

5 J/(K mol), very close to that of cis-l,4-polybutadiene [ASj = 32.3 J/(K mol)] This second geometrical isomer of 1,4-poly butadiene does not show any condis phase For both isomers one can easily identify three flexible bonds, i.e. according to the fusion rules of Sect. 1.1, one expects 3 x 9.5 J/(K mol) for the overall entropy of fusion of either isomer, as is observed. The low-temperature transition of the 

The crystallization kinetics from the melt to condis crystals and then from condis crystals to form I crystals shows two-stage crystallizations and excqjtionally low Avrami coefficients (0.02-1.74). Little is know about the detailed mechanisms, except that the kinetics indicates significant differences from normal aystalliza-tions. 

The closely related trans-I,4-poly(2 ethylbutadiene) (gutta percha) has also two crystalline polymorphs. The stable, monoclinic a-crystal form (P2jC) grows from the melt above 318 K (T = 353 K) and has an entropy of fusion of 36.4 J/(K mol), indicative of full conformational order. The s nd polymorph, the orthorhombic P-crystal form (Pnam) grows at lower crystallization temperatures, but has only a somewhat lower entropy of fusion [29.7 J/(K mol)]. Conversion from p to a is not possiUe, or at least extremely slow. Both polymorphs have been asigned distinct chain conformations , i.e. neither seems to show large stale dynamic conformational disorder. 

It was shown already above that cis-1,4-poly butadiene melts in one step with the expected entropy of fusion. In contrast, cis-l,4-poly(2-methylbutadiene) (natural rubber) has a more complicated fusion and crystallization behavior . The reported entropy of fusion of the common monoclinic (P2ja) crystal polymorph is only 14.4 J/(K mol), less than half of the expected value. The crystal structure has been reported statistically disordered, but only relative to packing of chains that are mirror images of each other along the crystallographic a-axis. Such geometric disorder cannot account for a 50% decrease in entropy of fusion. A full study of the thermod5mamic functions as available for the polybutadienes would be of value. 

The poly(l,3-dienes) are made by similar chemical processes to the polyolefins  

Natural rubber (pages 5 and 6) is a member of this group of polymers although it is synthesised in the rubber tree by a quite different process. NR is a high member of the terpene family and is made by the same kind of enzyme-controlled chemistry as the terpene squalene, which is the precursor of cholesterol in animals. Squalene is often used as a low Mj model to study the reactions of cis-poly(isoprene) (PI). 

The earliest rubber to be manufactured synthetically was not poly-isoprene but the copolymer of butadiene and styrene (SBR) by random free radical polymerisation. Modern SBR contains styrene and butadiene units in the ratio 1 3. 

The structure of cis-polybutadiene (cis-PB) is more complex than that of cis-PI since some of the double bonds react by 1,2 addition to give a pendant vinyl group which is more reactive toward radical addition than the vinylene group in cis-PI. 

The same ex-chain double bond structure is also present in SBR and leads to cross-linking through oxygen during ageing. 

Polybutadienes with various microstructures crosslink rapidly below their decomposition temperature, which affects the subsequent breakdown behaviour. Oxidation of c/s-1,4-polybutadiene at 90—180 °C gave primarily epoxides as products with smaller quantities of alcohols, peroxides, and carbonyl-containing, structures. C n.m.r. was used to determine the structure of the oxidized products. The oxidation of cis- 1,4-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen, and ozone has also been studied. All the forms of oxygen interact by a 

Shanina, V. A. Roginskii, and V. B. Miller, Fiz-Khlm Protsessv Gaiov Kondens Fazakh, 1979, 96 (Chem. Abstr., 1980, 93, 27 018). 

Polyisoprene has also been pyrolysed in an inert atmosphere and here the main products are isoprene and l-methyl-4-isoprenylcyclohexene. The latter compound can disproportionate to l-methyl-4-isopropyIbenzene and methyl-l-iso-propylcyclohexenes and this reaction is catalysed by Ziegler-Natta catalyst residues or by carbon black. The dominant initiation process is j9-chain scission with the formation of two allylic radicals. The kinetics of thermal decomposition have been studied for cis- and rraiw-1,4-polyisoprene and the copolymer of isoprene with 4-isopropyl- r-methyl styrene and also for isoprene polymers containing 4-CjH4—Z—4-C(H4— and —CjH4—Z—C5H4N=N— units, where Z may be O, CHj, SOj or a single bond.  

Thermo-oxidative studies have also been reported on polyisoprenes and on natural rubber. In the latter case, the effect of relative molecular weight on thermo-oxidation as evidenced by carbonyl group formation was investigated. A mechanism for formation of isoprene during thermal degradation of natural rubber has been proposed. It involves a cyclic intermediate. 

The kinetics of thermal and thermo-oxidativebreakdown of polystyrene continue to be studied. It has again been confirmed that the type of end-group is a 

Thermal anaerobic cyclizations and cis-tram isomerizations of unsaturated hydrocarbon polymers have been reviewed. Three main types are cited chain 

Takamatsu, M. Fukutomi, M. Fukuda, and A. Shimizu, Toyo Soda Kenkvu Hokoku, 1978, 22, 19. Chem. Abs., 1978, 88, 191 739). 

Studies of the thermal decomposition of polystyrene have been made in fluidized or fixed bed reactors with the aim of controlling the yield of products. The course of the reaction may be modified by using different contact particles in a fluidized bed, by adding various halogenated compounds, or by hydrocracking in the presence of zinc chloride.  

Ozonization of polystyrene in solution results in cross-linking as well as chain scission. The major products are carboxylic acids. 

A variety of studies have been made on copolym of styrene. They include the kinetics of degradation of styrene-alkyl methacrylate copolymers the effect 

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Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. Natta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. 

RITCHIE, p. D. (Ed.), Vinyl and Allied Polymers, Vol. 1 —Aliphatic Polyolefins and Polydienes Fluoro-olefin Polymers, Iliffe, London (1968)... 

As with most polyolefins and polydienes the presence of copper has a strong adverse effect and most antioxidants are relatively ineffective. In these instances quite good results may be achieved by the use of 1% of a 50 50 phenol alkane-dilauryl thiodiproprionate blend instead of the 0.1-0.2% of antioxidants more commonly used in polypropylene. 

Legge, N.R., Holden, G. and Schroeder, H.E., Thermoplastic elastomers based on polystyrene-polydiene block copolymers. Thermoplastic Elastomers, Hanser Publishers, New York, 1987. 

Modern assignment of polydiene structures is based on nmr results, including both 1H and I3C. A compilation of the pertinent data reported in the literature was published by Yudin l24). 

The aggregation of lithium polydienes is disrupted in ethereal solvents and their studies provide information about the conformation of the active centers. The stability of ethereal solutions of polydiene salts is greatly improved at low temperatures, especially in the presence of salts suppressing their dissociation 126). Under these conditions the cis-isomer is the most abundant in equilibrated THF solutions, although... 

An interesting approach to studies of the effects of coordination on the reactivity of lithium polydienes in hydrocarbon solvents was developed by Erussalimski and his colleagues 151 154 The polymerization of lithium polyisoprene in hexane is accelerated by the addition of TMEDA152), the rate levels off at a value of R = [TMEDA]/[li-thium polyisoprene] of 8, its final value giving the absolute rate constant of propagation of the polyisoprene coordinated with TMEDA, namely 0.17 M7l s at 20 °C. 

Finally it should be stressed that the complexation affects the microstructure of poly dienes. As was shown by Langer I56) small amounts of diamines added to hydrocarbon solutions of polymerizing lithium polydienes modify their structure from mainly 1,4 to a high percentage of vinyl unsaturation, e.g., for an equivalent amount of TMEDA at 0 °C 157) the fraction of the vinyl amounts to about 80%. Even more effective is 1,2-dipiperidinoethane, DIPIP. It produces close to 100% of vinyl units when added in equimolar amount to lithium in a polymerization of butadiene carried out at 5 °C 158 159), but it is slightly less effective in the polymerization of isoprene 160>. 

Polydienes. Polydienes that are modifled with organosilicons have been described and find application as antifoaming and/or deaeration agents for oil field treating of crude oil [170]. 

Atomic oxygen oxidation of polymers has been reported by a few authors (46,50). Experiments were limited to the measurements of weight-loss data and changing of the wetteability (46-48), and only two papers were devoted to the study mechanism of atomic oxygen oxidation of polydienes (49,50). 

The low temperature ene reactions of 4-substituted-l,2,4-triazoline-3,5-diones (RTD) were used to modify polydiene surfaces. Hydrophilic surfaces (contact angles with water of 30-50°) were obtained on polybutadiene, poly-isoprene and styrene-butadiene copolymers by first treating the polymer at room temperature with RTD (R=Ph,... 

Of particular interest to us was to find a method to surface modify elastomers. G. B. Butler and co-workers have demonstrated that 4-substituted-l,2,4-triazoline-3,5-diones, RTDs, readily undergo ene reactions with polydienes at ambient temperatures (13). They found that the solubility and solution properties of the modified... 

This investigation was undertaken to study the important variables in the hydrophilization of polydiene surfaces by ene reaction with triazolinediones (Step 1) followed by neutralization (Step 2) as shown below. These variables included the nature of the... 

Surface Modification. A polydiene film (supported on a microscope slide) was immersed in a stirred, room temperature, RTD-acetonitrile solution of known concentration contained in a large glass-stoppered test tube. After a specific reaction time, the film was removed from the solution, washed with acetonitrile, water, and acetonitrile again, and dried under vacuum (Step 1). Films subsequently treated with base were immersed in aqueous solutions for 5-15 min. They were then washed with water and CH3CN, and vacuum dried (Step 2). Some films were aged in air at room temperature. 

Table II. Contact Angles (H2O) of Treated Polydiene Films3...

When the reaction times for Step 1 are 5 min or longer, the samples severely crack, curl, or dissolve. These results suggest that substantial reaction is occurring in the bulk of the polymer. Significant hydrophilization can occur with reaction times as short as 5 s with RTD concentrations of 0.2-0.5 M. However, 0.002-0.02 M solutions of MeTD or PhTD do not allow sufficient reaction rates for surface hydrophilization at the shorter reaction times. Thus, diffusion of MeTD and PhTD into the polymer must occur readily from the acetonitrile solutions. Acetonitrile was used because it does not swell or dissolve the polymer or RTD-polymer adduct, and the RTDs are soluble and stable in it. This solvent is quite polar (dielectric constant, 38) (25), and this is probably a major factor in the partitioning of the relatively nonpolar RTDs between the polydiene film and the solvent. As noted below, more polar RTDs show less tendency to diffuse into the polymer. 

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