Diene microstructure, effect

As a final point regarding compatibility in BR/IR systems, we consider the effect of diene microstructure. In all of the experiments described above, microstructure was not a variable (Table I), with predominantly 1,4-addition appearing in both polymers. In this case it is possible that the incompatibility of the homopolymers might arise from the presence of the methyl group in the isoprene repeat unit and/or... 

An interesting effect of Lewis bases on diene microstructure is the fact that in the presence of strongly coordinating bases such as TMEDA, 1,2 units are observed for polyisoprene. For example, the microstructure of polyisoprene formed in the presence of TMEDA ([TMEDA]/[Li] = 1) in cyclohexane corresponds to 21% 1,4, 12% 1,2, and 67% 3,4 (137). The formation of 1,2 units requires the formation of the less stable 1,4 chain ends versus 4,1 chain ends as shown in equation 52 ... 

It is explained that most rubber products due to the development of cracks, as a result of being subjected to environmental factors. This paper reviews in depth the underlying failure mechanisms, and discusses ways and means of minimising them. It presents the effects of polymer molecular weight, the styrene content of SBR, and the diene microstructure on fracture and fatigue life of the vulcanisates. It also discusses the role played by carbon black and vulcanising systems on fracture and fatigue. 56 refs. 

The properties of some polymers are dependent on their microstructure for example isotactic polypropylene is crystalline whereas atactic polypropylene is amorphous. Microstructure effects are also exemplified by polybutadienes, where the mode of addition of the diene to the growing chain leads to 1,2-addition, 1,3-addition and 1,4-addition, which may be as or trans. The fraction of different addition species changes the mechanical properties of the polymer. Another example is provided by the chemical composition of a copolymer and its sequence distribution, which together determine its ultimate properties. It is thus of great importance to be able to characterize polymer micro structure. This is generally done using spectroscopic methods, specifically infrared spectroscopy and nuclear magnetic resonance spectroscopy. 

Other commercially relevant monomers have also been modeled in this study, including acrylates, styrene, and vinyl chloride.55 Symmetrical a,dienes substituted with the appropriate pendant functional group are polymerized via ADMET and utilized to model ethylene-styrene, ethylene-vinyl chloride, and ethylene-methyl acrylate copolymers. Since these models have perfect microstructure repeat units, they are a useful tool to study the effects of the functionality on the physical properties of these industrially important materials. The polymers produced have molecular weights in the range of 20,000-60,000, well within the range necessary to possess similar properties to commercial high-molecular-weight material. 

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>. 

Termination of these polymerizations with dichlorodimethylsilane followed by hydrolysis of the protecting group generated polymeric diamines with functionalities of 1.7-1.9 and relatively broad MW distributions (1.49-2.22). The authors considered that the titrimetric method was less reliable for the higher molecular weight polymers and perhaps is a reason for the apparent ineficiency of this amination procedure. A major limitation of this method is the fact that the initiator is insoluble in hydrocarbon solvents and therefore most of the diene polymerizations were carried out in mixtures of hexane and ether which has a deleterious effect on the microstructure of the diene polymers. 

The kinetics of copolymerization and the microstructure of copolymers can be markedly influenced by the addition of Lewis acids. In particular, Lewis acids are effective in enhancing the tendency towards alternation in copolymerization of donor-acceptor monomer pairs and can give dramatic enhancements in the rate of copolymerization and much higher molecular weights than are observed for similar conditions without the Lewis acid. Copolymerizations where the electron deficient monomer is an acrylic monomer e.g. AN, MA, MMA) and the electron rich monomer is S or a diene have been the most widely studied." Strictly alternating copolymers of MMA and S can be prepared in the presence of, for example, dictliylaluminum scsquichloridc. In the absence of Lewis acids, there is only a small tendency for alternation in MAA-S copolymerization terminal model reactivity ratios are ca 0.51 and 0.49 - Section 7.3.1.2.3. Lewis acids used include EtAlCT, Et.AlCL ElALCL, ZnCT, TiCU, BCl- LiC104 and SnCL. 

A comprehensive hypothesis has been proposed to explain the effects of the concentrations of active chain ends and monomer on polydiene microstiucture [163], Based on studies with model compounds and the known dependence of polydiene microstructure on diene monomer... 

The stereoregularity of polystyrenes prepared by anionic polymerization is predominantly syndiotactic (racemic diad fraction P = 0.53-0.74) and the stereoregularity is surprisingly independent of the nature of the cation, the solvent, and the temperature, in contrast to the sensitivity of diene stereochemistry to these variables [3, 156]. The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., atactic) microstructure for example, was 0.53 for the butyllithium/toluene system [3, 191, 192]. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium f-butoxide to 58% isotactic triads with potassium f-butoxide [193]. 

Another feature of the emulsion polymerization of chloroprene that distinguishes it from that of the other dienes is the fact that it leads to a predominantly trans-1,4 chain microstructure. Thus, even at ambient polymerization temperature, the polychloroprene contains over 90% trans-1,4 units, as shown in Table X, which illustrates the effect of polymerization temperature on stereoregularity of the chain [87]. As expected, lower polymeriza-... 

Y. Mohammadpour and A. A. Katbab, Effects of the Ethylene-Propylene-Diene Monomer Microstructural Parameters and Interfacial Compatibilizer Upon the EPDM/Montmorillonite Nanocomposites Microstructure Rheology/Permeability Correlation, Journal of Applied Polymer Science, 2007, 106, 4209. 

While they are not strictly functionalized polyethylenes, our model ethylene/propylene copolymers represent our first success at making precise microstructures through the use of symmetric a,o>diene monomers. They also demonstrate the effect of regularly spaced pendant groups (the methyl group) on the thermal behavior of polyethylene, and serve as a basis for comparison of the functionalized models that follow. 

Much less recognized is the possible influence of tacticity on copolymer properties when a-olefin monomer units are a minor component, and crystallinity is not based on a tactic a-olefin sequence but on a different comonomer such as ethylene. In this chapter, this tacticity effect is shown for ethylene-rich ethylene/propylene (EP) copolymers, where the crystallizable sequences are based on ethylene, that is, a comonomer that does not have tacticity requirements. In particular, this chapter describes in detail the microstructure of EP copolymers having industrially relevant compositions (ethylene content 80-55 mol%), with particular focus on the placement of propylene units along the ethylene-based macromolecular chains and their influence on copolymer properties. This subject is, of course, related to the industrial relevance of EP copolymers and ethylene/propylene/diene monomer terpolymers (EPDMs) (collectively referred to as EP(D)Ms), which presently represent the most widely produced saturated rubbers. ... 

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