Polybutadiene polymer microstructure

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.

As may be seen from Fig. 3, there are no resonance peaks at 120-128 ppm characteristic of 1,4-microstructure in polybutadiene polymer. However, on addition of methanol to the chain live ends, resonance peaks at 120-128 ppm appear in ratios of 60% trans-1,4, 14% m-1,4, and 26% 1,2. This suggests that the protonation of the chain live ends with methanol is an independent reaction and does not relate to the actual structure of the propagating species. It may be said that the structure of the allylic lithium of polybutadiene (DP > 1) is postulated to exist in the 1,2-form (13). Yet hydrolysis of 13 gives mixed 1,4- and 1,2-microstructures. 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Since butadiene can also undergo coordinated anionic polymerizations, some of the differences in polymer microstructure are attributable to changes in mechanism. Based on the catalysts reported to date, the isotactic and syndiotactic 1,2-polybutadienes appear to arise from coordinated anionic mechanisms. Qs and trans 1,4-polybutadienes can probably be made by all mechanisms, with cis arising from soluble catalysts which are capable of multi-coordination at one metal site. Trans structure is favored by cationic mechanism and by anionic mechanism involving coordination at two metal centers. 

The dependence of the polymer microstructure on the ratio of catalyst components is related to the nature of these components. The structure of polybuta-diene obtained with an aluminum triisobutyl (AIBU3)-titanium tetrachloride catalyst system is a function of the Al/Ti molar ratio (Table II). Polybutadiene prepared at Al/Ti ratios of 0.5 to 8 in benzene or heptane and at 3° or 25° C. contain at least 90% 1,4- units. Polymerizations carried out at ratios of 1.0 and less at 25° C. in heptane and at ratios of 1.25 or less at 3° C. in heptane or benzene give crystalline polymers containing more than 96% trans-, A- structure (6). A similar temperature dependence of polymer structure has been reported in the polymerization of butadiene with a diethylcadmium-titanium tetrachloride catalyst system (3). Polybutadiene obtained with a triethylaluminum-titanium tetrachloride catalyst system at a 0.9 Al/Ti ratio at 30° C. in benzene is reported to contain 67% cis-1,4- units (19). 

The polymers in Table III catalyzed by sodium-mercury show structures identical with sodium polybutadienes. Because mercury, alone, does not catalyze the polymerization, these results should be compared with previous work (2) using sodium hydride which gave similar results. Both of these sets of experiments show merely that the crystalline structure of the sodium metal, or some other constitutive property, is not the deciding factor in the determination of polymer microstructure. 

When considering only solution polymers, polymer microstructure has a greater effect on tire tread compound performance. Table 9.11 illustrates the impact on tire traction, rolling resistance, and tread wear of a polybutadiene tread on which the vinyl-1,2-butadiene level had been increased from 10% to 50% (Brantley and Day, 1986). The corresponding drop in wear and increase in tire rolling resistance are in agreement with the empirical rules presented by Nordsiek (1985), who attributed such tire property trends to the polymer Tg. 

Polybutadiene polymers that have a 1,2 microstructure varying from 60 to 90% offer potential as moldings, laminating resins, coatings, and cast liquid and formed sheet products. Outstanding electrical and thermal stability results from the structure that is essentially prue hydrocarbon. 

An example of how polymer microstructure and polymer Tg impact performance is when vinyl-butadiene is increased from 10 to 50% in polybutadiene (Table 4.5) [8]. The glass transition temperature increases from —90 to —60 °C, with a corresponding shift in the tan-delta curve. Traction performance has improved significantly, but tread wear and rolling resistance rating drop. 

Figure 11.1 Polymer microstructures of c/s-1,4 polybutadiene, trom-1,4 polybutadiene, 3,4 polyisoprene, 1,2-polybutadiene (syndio- and isotactic).

In this section, the correlation between IR/Raman frequencies of functional groups relevant to polymer analysis and polymer microstructures is described. The functional groups considered are the C=C double bond and substituted benzenes. The microstructure featuring the C=C double bond is important because the physical properties of materials such as polybutadiene and related polymers depend on the double-bond content of the overall structure. Also, a small percentage of residual C=C double bonds are formed in polyethylene chains as a result of side reactions. Substituted benzene compounds are used as starting materials such as bis-phenol A, phthalates, and benzoates. 

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. 

Polymer Characterization. The copolymer composition and polybutadiene microstructure were obtained from infrared analysis and checked for certain copolymers using 13C NMR. 

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... 

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... 

Because of the coalescence of the bands belonging to the polyacrolein and polybutadiene blocks, the (1,4) and (3,4) units of acrolein cannot be estimated separately. The results of the acrolein microstructure in the PABj4 and PAB15 block polymers are summed up in Table 3. 

The unique feature about anionic polymerization of diene to produce homopolymer was that the microstructure of the homopolymer could be altered and changed at will to produce unique physical and chemical properties. These microstructural changes can be introduced before, after or during the polymerization. For example, chelating diamines, such as tetramethyl ethylene and diamine (TMEDA) (18), with the alkyl-lithium catalyst have been used to produce polymer with 80 1,2 addition products, while the use of dipiperidine ethane (DPE),with same catalyst has produced polybutadiene with 100 1,2 addition product. 

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... 

On the basis of this finding, a two-step (stepwise) development technique12 was applied to separations of possible three binary mixtures of polybutadienes with different chain microstructure, namely, those of cis-1,4 + tram-1,4, tram-1,4+1,2-1,2-vinyl, and 1,2-vinyl + cis-1,4. The principle consisted of a utilization of the different development characteristics exhibited by carbon tetrachloride and amyl chloride. An example of this procedure applied by these authors83 will be described below. A mixture cis-1,4 + tram-1,4 was developed primarily with amyl chloride until the solvent front reached a distance, e.g., 10 cm from the starting point by this development only the cis-1,4 polymer should have migrated up to the solvent front (cf. Table 4). In order to identify the immobile component, the chromatogram was dried in vacuum at room temperature and treated with carbon tetrachloride until the solvent front reached an intermediate distance, e.g., 5 cm. It is obvious that this procedure can be alternatively used for identification of any unknown binary mix-... 

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