1.3- Butadiene microstructure

The relative concentration of the various butadiene microstructures, (1,4 cis, 1,4 trans, and 1,2 vinyl), were determined from the infrared spectra of solid films cast on KC1.(26) The 1,2 microstructure content of all the polymers considered in this paper were between 5-8 mole percent as determined from the IR spectra. Number average and the weight average molecular weight of the polymers were obtained via osmotic pressure and HPLC. The molecular weight of all polymers is around 200,000 g/mole while the polydispersities were about 1.1 thus, all of these polymers have a relatively narrow molecular weight distribution. Note, that both the precursor diene blocks and hydrogenated copolymers... 

In the 1960s, anionic polymerized solutron SBR (SSBR) began to challenge emulsion SBR in the automotive tire market. Organolithium compounds allow control of the butadiene microstructure, not possible with ESBR. Because this type of chain polymerization takes place without a termination step, an easy synthesis of block polymers is available, whereby glassy (polystyrene) and rubbery (polybutadicnc) segments can be combined in the same molecule. These thermoplastic elastomers (TPE) have found use ill nontire applications. 

Effect of Poly butadiene Microstructure on Grafting Efficiency. . 155... 

Random Styrene-Diene Copolymers. Random copolymers of butadiene (SBR) or isoprene (SIR) with styrene can be prepared by addition of small amounts of ethers, amines, or alkali metal alkoxides with alkylhthium initiators. Random copolymers are characterized as having only small amounts of block styrene content. The amoimt of block styrene can be determined by ozonoly-sis or, more simply, by integration of the nmr region corresponding to block polystyrene segments (S = 6.5-6.94 ppm) (180). Monomers reactivity ratios of tb = 0.86 and rs = 0.91 have been reported for copolymerization of butadiene and styrene in the presence of 1 equiv of TMEDA ([TMEDAMRLi] = 1) (181). However, the random SBR produced in the presence of TMEDA will incorporate the butadiene predominantly as 1,2 imits. At 66°C, 50% 1,2-butadiene microstructure will be obtained for copolymerization in the presence of lequiv of TMEDA (134). In the presence of Lewis bases, the amounts of 1,2-polybutadiene enchainment decreases with increasing temperature. 

Little information could be located in the literature, as indicated by the question mark entries in this Table, of the effect of composition, butadiene microstructure, and monomer sequence distribution on tan 6 of rubber networks in the appropriate frequency range. This prompted us to prepare rubbers of controlled structure that incorporate these features, and to study their fundamental and functional properties. 

BUTADIENE MICROSTRUCTURE VINYL/CTRANS + CIS) TRANS/CIS AT CONSTANT VINYL... 

We devoted special attention to characterize all the polymers that were prepared for this study. The characterization techniques listed in Table 4 were used for determining (1) comonomer composition, butadiene microstructure, and sequence distribution of the monomer units (2) molecular weight, molecular weight distribution, and chain architecture ... 

COMONOMER COMPOSITION, BUTADIENE MICROSTRUCTURE AND COMONOMER SEQUENCE DISTRIBUTION... 

Figure 11.6 The influence of the hard-segment content on force required to bring about 100% elongation for CEBC elastomers with 40% 1,2 butadiene microstructure (A) and 60% 1,2 butadiene micro structure (x). The line is a polynomial fit for the data obtained from the 40 % 1,2 butadiene based polymers (y = 0.00698ji3 — 0.23x -I- 3.98, = 0.936).

Cohen and Ramost describe some phase equilibrium studies of block copolymers of butadiene (B) and isoprene (I). One such polymer is described as having a 2 1 molar ratio of B to I with the following microstructure ... 

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>. 

Measurements of polymerization rate and parallel measurements on the resultant polymer microstructure in the butadiene/DIPIP system cannot be reconciled with the supposition that only one of the above diamine solvated complexes (eg. Pi S) is active in polymerization 162). This is probably true of other diene polymerizations and other diamines. The observations suggest a more complex system than described above for styrene polymerization in presence of TMEDA, This result is clearly connected with the increased association number of uncomplexed diene living ends which permits a greater variety of complexes to be formed. 

The yield of cross-linking depends on the microstructure of polybutadiene and purity of the polymer as well as on whether it is irradiated in air or in vacuum. The cross-link yield, G(X), has been calculated to be lowest for trans and highest for vinyl isomer [339]. The introduction of styrene into the butadiene chain leads to a greater reduction in the yield of cross-linking, than the physical blends of polybutadiene and polystyrene [340]. This is due to the intra- and probably also intermolecular energy transfer from the butadiene to the styrene constituent and to the radiation stability of the latter unit. 

The copolymers consist of strictly alternating sequences of diene and olefin. C-NMR measurements Showed the microstructure of the butadiene units in BPR to be exclusively of the trans-1,4 configuration (Figure 8). The isoprene units in isoprene-ethylene copolymer (IER) contain 84 % trans-1,4, 15 % cis-1,4, and 1 % 3,4 structures (Figure 9). Spontaneous crystallization in unstretched BPR samples was detected by dilatometry and confirmed by X-ray diffraction and DSC measurements. The extrapolated equilibrium melting point is about -10 °C. 

Polymerization Temperature. The stereoregularity of polybutadienes prepared with the BuLi-barium t-butoxide-hydroxide catalyst in toluene is exceedingly temperature dependent. Figure 6 compares the trans-1,4 dependence for polybutadiene prepared with BuLi, alone, and with the BuLi-barium t-butoxide-hydroxide complex in toluene (the molar ratio of the initial butadiene to BuLi concentration was 500). The upper curve demonstrates that the percent trans content increased rapidly from 627. to 807. trans-1,4 as the temperature decreased from 75°C to 22°C. From 22°C to 5°C, the microstructure does not change. The increase in trans-1,4 content occurred with a decrease in cis-1,4 content, the amount of vinyl unsaturation remaining at 5-87.. For the polybutadienes prepared using BuLi alone, there is only a very slight increase in the trans-1,4 content as the polymerization temperature is decreased. 

The synthesis and characterization of a series of dendrigraft polymers based on polybutadiene segments was reported by Hempenius et al. [15], The synthesis begins with a linear-poly(butadiene) (PB) core obtained by the sec-butyllithium-initiated anionic polymerization of 1,3-butadiene in n-hexane, to give a microstructure containing approximately 6% 1,2-units (Scheme 3). The pendant vinyl moities are converted into electrophilic grafting sites by hydrosilylation with... 

C.E. Miller, B.E. Eichinger, T.W. Gurley and J.G. HermiUer, Determination of microstructure and composition in butadiene and styrene-butadiene polymers by near-infrared spectroscopy. Anal. Chem., 62, 1778-1785 (1990). 

Quantitative analysis of the 22.6 MHz C NMR spectrum in Figure 8 yields the following information concerning the microstructure of the hydrochlorinated 1,4-polydimethyl butadiene sample. First, the population of one of the diastereoisomers, threo or... 

The yield of cross-links depends on the microstructure and purity of the polymer as well as whether it was irradiated in air or in vacuo2 The rate of degradation was found to be essentially zero when polybutadiene or poly(butadiene-styrene) was irradiated in vacuo, but increased somewhat when irradiated in air. 

The information on physical properties of radiation cross-linking of polybutadiene rubber and butadiene copolymers was obtained in a fashion similar to that for NR, namely, by stress-strain measurements. From Table 5.6, it is evident that the dose required for a full cure of these elastomers is lower than that for natural rubber. The addition of prorads allows further reduction of the cure dose with the actual value depending on the microstructure and macrostructure of the polymer and also on the type and concentration of the compounding ingredients, such as oils, processing aids, and antioxidants in the compound. For example, solution-polymerized polybutadiene rubber usually requires lower doses than emulsion-polymerized rubber because it contains smaller amount of impurities than the latter. Since the yield of scission G(S) is relatively small, particularly when oxygen is excluded, tensile... 

Kinetics in Non-Polar Media. Polymerization of vinyl monomers in non-polar solvents, i.e., hydrocarbon media, has been almost entirely restricted to the organolithium systems (7), since the latter yield homogeneous solutions. In addition, there has been a particularly strong interest in the polymerization of the 1,3-dienes, e.g., isoprene and butadiene, because these systems lead to high 1,4 chain structures, which yield rubbery polymers. In the case of isoprene, especially, it is possible to actually obtain a polymer with more than 90% of the eis-1,4 chain structure (7, 8, 9), closely resembling the microstructure of the natural rubber molecule. 

Figures 1 and 2 show the dependence of polymer microstructure on the molecular weight of the polymer and therefore on the initial initiator concentration. The polymerization temperature also has an effect on the microstructure as can be seen in Figure 3 for polybutadiene. The overall heat activation energy leading to 1,2 addition is greater than that leading to 1,4 addition.2 IZ In summary, the stereochemistry of polymerization of butadiene and isoprene is sensitive to initiator level, polymerization temperature and solvent. The initiator structure (i.e., organic moiety of the initiator), the monomer concentration and conversion have essentially no effect on polymer microstructure.

Polymerization of butadiene with lithium morpholinide, an initiator with a built-in microstructure modifier, has been carried out in hexane. In general, the vinyl content of the polymers prepared with this initiator is dependent on the initiator concentrations and on the polymerization temperatures. This dependence is identical to that observed in a THF-modified lithium diethylamide polymerization initiator system. A comparison of these initiator systems for polymerization of butadiene is presented. In addition, a study of the effect of metal alkoxides on the vinyl content of lithium morpholinide initiated butadiene polymerization is included. 

Copolymerization of butadiene and styrene in hexane with a number of initiators, such as lithium morpholinide, lithium dialkylamide, lithium piperidinide, etc., has also been examined. In general, the microstructure and styrene content of the polymers are dependent on the type of initiator and the polymerization conditions. Detailed results including a postulated mechanism for these polymerizations are discussed. 

The polymerization of butadiene with lithium diethylamide was conducted at several different temperatures. In general, the conversion to polymer was reasonable (75-89%), and the microstructure was independent of polymerization temperatures and initiator levels over the range investigated. These results are shown in Table IV. 

The effect of THF on the microstructure of lithium diethylamide initiated polymerization of butadiene-1,3 was studied. 

The temperature dependency of 1,2 content shown in Table II is also consistent with complex formation between polybutadienyl-lithium and the oxygen atom in the lithium morpholinide moleculre. One can visualize an equilibrium between noncom-plexed and complexed molecules which would be influenced by temperature. Higher temperatures would favor dissociation of the complex and, therefore, the 1,2 content of the polymer would be lower than that from the low temperature polymerization. This explanation is supported by the polymerization of butadiene with lithium diethylamide, in which the microstructure of the polybutadiene remains constant regardless of the polymerization temperature (Table IV). This is presumably due to the fact that trialkylamines are known to be poor... 

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