Polybutadiene star-branched

Tristar polybutadienes prepared by the intermediacy of lithium acetal initiators were also converted to three dimensional networks in a liquid rubber formulation using a diisocyanate curing agent. Table IV shows normal stress-strain properties for liquid rubber networks at various star branch Hn s. It can be seen that as the branch Mn increases to 2920, there is a general increase in the quality of the network. Interestingly, the star polymer network with a star branch Mn of 2920 (Mc=5840) exhibits mechanical properties in the range of a conventional sulfur vulcani-zate with a Me of about 6000-8000. 

A Chromatix low-angle light scattering G.P.C. detector was also employed for the determination of the weight average molecular weight (Mw), as well as a sensitive detector, in order to assay absolutely the Mw versus elution volume profile for a series of star-branched polyisoprenes and polybutadienes. The results indicate that under optimum conditions a relatively well defined number of arms can be achieved with DVB linking. 

Figure 5. GPC of star-branch polybutadiene linked for 72 h at 25°C Sample...

Figure 8. GPC trace for a star-branched polybutadiene linked for 48 h at 50°C with DVB/RLi = 12.1, Sample 3-MM-S-12.

In comparison to the polybutadiene stars under similar reaction conditions, the polyisoprene stars showed slightly lower degrees of branching. The added steric hindrance from the methyl group on the polyisoprene anion perhaps makes entry into the DVB "microgel" nodule difficult. 

We have reported on Fig. 16 the complex shear modulus of two star-branched polybutadiene samples at 25°C. The hill lines have been calculated using the Ball and Mac Leish model for the terminal relaxation region, whereas the same relaxation functions as for the linear polymers have been used regarding the A, B and glass transition domains. 

The data for the three types of star-branched polymers show a decrease in rj on branching, with the exception of the trichain and tetrachain polybutadiene of longer branch length, for which the reverse is true. The data for most of the randomly branched and comb-type poly-vinylacetates nearly all show higher values than the linear polymer, excepting for two samples containing short branches, for which the reverse is true. 

Raju, V.R. Rachapudy, H. Graessley, W.W. Properties of amorphous and crystallizable hydrocarbon polymers. IV. Melt rheology of linear and star-branched hydrogenated polybutadiene. J. Polym. Sci. Polym. Phys. Ed. 1979, 17 (7), 1223-1235. 

The efficiency of the linking reactions of polychlorosi-lanes with poly(dienyl)lithium compounds has been documented by synthesis of well-defined, narrow molecular weight distribution, 18-armed star-branched polyisoprenes, polybutadienes, and butadiene end-capped polystyrenes by linking reactions with a decaoctachlorosilane [(SiCl)ig] [256, 257]. The linking reactions of poly(butadienyl)lithium (Mjj = 5.3-89.6 x 10 g/mol) with carbosilane dendrimers with up to 128 Si-Cl bonds have been reported to proceed... 

INTRODUCTTON Polymer molecules may have a variety of architectural structures such as linear, ring, star, branched, and ladder chains as well as three-dimensional network structures. The first synthetic cychc jxjlymers to be prepared and characterized were the poly(dimethylsiloxanes) (PDMS), which were reported in 1977. Since that time a number of other cychc pwlymers have been synthesized including cyclic polystyrene, cyclic poly(phenyhnethylsiloxane), cyclic poly(2-vinylpyridine), cyclic polybutadiene, and cychc poly(vinyhnethylsiloxane). ... 

V. S. Au-Yeung, C. W. Macosko, and V. R. Raju, Extensional flow of linear and star-branched hydrogenated polybutadiene with narrow molecular weight distribution, J. Rheol. 25, 445-452 (1981). 

Experimentally, is approximately 0.6. Thus, for branched polymers in the entanglement region, both rjo and may be quite large compared with the values for linear polymers of the same molecular weight. The terminal zone is inherently broader for well-entangled branched polymers than it is for linear polymers of comparable polydispersity [49]. The complex viscosities for a nearly monodisperse linear polybutadiene and three-arm polybutadiene star, shown in Fig. 3.24 for other purposes, exemplify the more gradual transition from Newtonian to power-law... 

In order to promote the efficient crossover reaction of the coupled product, 104, with butadiene monomer, the addition of lithium alkoxide (lithium sec-butoxide [LiOR]/[RLi]=1.0) was found to be useful analogous to the effect of lithium alkoxide with the dilithium initiator, 90 [88]. In the presence of lithium sec-butoxide, well-defined, monomodal, heteroarm, star-branched polymers (107) were obtained with high 1,4-microstructure of the polybutadiene blocks [203]. In the absence of the lithium alkoxide, bimodal molecular weight distribution polymers were obtained and residual UV absorption corresponding to the diphenylalkyllithium initiator groups at 438 nm was still observed after all of the monomer had been consumed. 

Figure 6.38 Log G versus log G" plots for three 4-arm star-branched polybutadienes at various temperatures 88S4 at 24.5 "C (A) and 50 °C (A), 173S4 at 24.5 (D) and 50 °C (H),...

Table 6.7 Molecular weights of star-branched polybutadienes...

With the exception of a few commercial polymers such as polyisobutylene, polybutadiene and styrene-butadiene block copolymers, living polymers are prepared in small quantities under stringent conditions. Larger amounts can only be prepared by repeating the synthesis many times, and this is a costly and time-consuming process. In the case of hydrogenated polybutadiene, to prepare samples that resemble polyethylene, the need for a secondary reaction step renders the preparation even more costly. This has so far limited the extent to which it has been possible to use these materials to test models. Cell et al. [ 18] prepared asymmetric stars with structures similar to ethylene-propylene copolymers by hydrogenation of star-branched polyisoprene. The reactions to produce these materials took up to three weeks, and... 

Fig. 5.19showsdataofKrausandGruver [89] for linear, three-arm and four-arm polybutadiene stars. At moderate molecular weights the data for the stars lie below, but parallel to, the line for linear, entangled polymer with a = 3.4, in accord with Eq. 5.2, but when the branch length reaches three (/= 3) or four (/= 4) times M, the data for the stars rise sharply and cross the line for linear polymers described by Eq. 5.2. 

Figure 5.31 Van-Gurp Palmen plot (loss angle versus magnitude of complex modulus) of data for linear (circles), star-branched (triangles), and comb (squares) polymers prepared by the hydrogenation of anionically polymerized polybutadienes.The branching structure hasadistinct effect on the shape of the curve. From Lohse etal. [119].

Rochefort, W. E., Smith, G.G.,Rachapudy,H.,Raju,V. R.,Graessley,W.W. Properties of amorphous and crystaUizable hydrocarbon polymers. II. Rheology of Linear and Star-Branched Polybutadiene. J. Polym. Sci.-Phys. (1979) 17, pp. 1179-1210... 

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