Star polymers branched

Masuda et a/.[30] reported data collected for a series of polystyrene star polymers that seemingly conflict with the discovery made by Quack and Fetters [27]. They showed that the viscosity of polystyrene star polymers was dependent on the number of arms. Specifically, they showed that viscosity increased with the number of branches for a series of polystyrene stars with Mw, arm = 55 000 g/mol and the number of arms ranging from 7 to 39. However, the level of arm entanglement for the polystyrene stars was far lower than that of the polyisoprene stars studied by Quack and Fetters [27]. 

Clearly, in order for the viscosity of star polymers to be independent of the number of branches, a certain level of entanglement needs to be present. 

Following the initial discovery [27] that rj0 depends on just arm molecular weight for star polymers with sufficiently high levels of branching, this type of dependence was confirmed by others both theoretically [32] and experimentally [33]. Pearson and Helfand [32] predicted that the zero shear viscosity of star polymers should scale with arm molecular weight (Afa) as 

It is not clear why this transition should occur at such a higher level of arm entanglement for polystyrene stars than for other star polymers. This observation is in direct conflict with the standard assumption that through a proper scaling of plateau modulus (Go) and monomeric friction coefficient (0 that rheological behavior should be dependent only on molecular topology and be independent of molecular chemical structure. This standard assumption was demonstrated to hold fairly well for the linear viscoelastic response of well-entangled monodisperse linear polyisoprene, polybutadiene, and polystyrene melts by McLeish and Milner [24]. 

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By water, SPreparation and Properties of Star-branched Polymers. Vol. 30, pp. 89—116. 

Spot tests, 245 Spray elastomers, 204 Star-branched polymers, 187 Star copolymers, 7... 

In a seminal and seemingly forgotten paper, Burchard et al. " discussed the analysis of various polymer architectures based on integrated light scattering (LS) and quasielastic light scattering (QELS). They considered mono- and polydisperse linear and star-branched polymers with/number of arms ( rays ), and random polycondensates of Af or ABC type (identical or different... 

Hancock, D. O. and Synovec, R. E., Rapid characterization of linear and star-branched polymers by concentration gradient detection, Anal. Chem., 60,2812, 1988. 

As we conjectured in the introduction, the fundamental role of topology in this approach to entangled polymer dynamics would indicate that changes to the topology of the molecules themselves would radically affect the dynamic response of the melts. In fact rheological data on monodisperse star-branched polymers, in which a number of anionically-polymerised arms are coupled by a multifunctional core molecule, pre-dated the first application of tube theory in the presence of branching [22]. Just the addition of one branch point per molecule has a remarkable effect, as may be seen by comparing the dissipative moduli of comparable linear and star polymer melts in Fig. 5. 

Branched polymeric materials have different properties than their linear counterparts. In the case of star-branched polymers (multiple branches radiating from a single site), enhanced engineering properties are possible from increased chain entanglements. The initial goal of this research was to create a material with enhanced performance properties via a star-branched network. 

Reactions (3) and (4) described above result in the beginning of star-branched polymer formation. Once the alkylbenzylanion (III) has formed it may further react with any available DVB monomer as illustrated in reaction sequence (5). 

As seen from the above reaction steps, the DVB procedures for star-branched polymer formation consists of several competitive and consecutive reactions. It was the aim of our work to study various reaction variables which effect this process. As mentioned earlier, these include DVB/RLi molar ratio, reaction... 

Figure 6 illustrates the effect of the DVB/RLi ratio. It was observed that as the DVB/RLi ratio increased, the percentage of unlinked linear material decreased. At the lower DVB/RLi ratios, linear coupled two arm stars were formed (see Ref. 15), while at the higher ratios, the linear material was not coupled. Thus, there is a strong influence on the efficiency of the star-branched polymer formation as the DVB/RLi ratio is varied for the case of poly(butadienyllithium). At the low molar ratios of DVB/RLi and two arm coupling reaction competes with the star-formation process. 

The reaction temperature was also found to influence the efficiency of the linking reaction. As summarized in Table I, the increase in reaction temperature from 25°C to 45°C resulted in an increase in star-branch polymer formation. Qualitatively, it was also observed that the formation of vinylbenzylanion occurred more rapidly at the elevated temperatures. Apparently, the increased reaction temperatures render the intermolecular attack on the pendant vinyl groups by the polydienyllithium anions more favorable in comparison to the intramolecular intra-nodule alkylbenzyl anion-vinyl group reaction (Table I on the following page). 

Figure 9. KMK-6/DP GPC detector response for Sample PI-S-16, a star-branched polymer linked with DVB/RLi = 11.9, Sample 3-MM-S-l 6.

G.P.C. as well as the membrane osmometry results. The elution volume behavior for the star-branched materials appear insensitive to the overall star-branched polymer molecular weight, while more dependent upon arm molecular weight. This is what one might expect as the G.P.C. separation process occurs by differences in hydro-dynamic volume not actual molecular weight. As the star-branched arm molecular weight increases so does the hydrodynamic volume, hence earlier elution volumes would be expected with increasing arm molecular weight. 

Low Angle Laser Light Scattering (LALLS) results indicate, that at lower DVB/RLi molar ratios, a relatively narrow molecular weight distribution (1.04-1.1) of star-branched polymers can be synthesized. In contrast, DVB/RLi ratios greater than 11 can lead to rather broad molecular weight distributions (Mw/Mn = 1.3). 

Fig. 5. Diffusion of C PsCl (open symbols) and cispolyisoprene (filled symbols) in solution at SO °C, from dilute solution to the melt. Left linear polymer, M =- 10 right eight-armed star-branched polymer, M = 4x 10 (Ref.32 >, with permission).

However, if both ends of the polymer, or the ends of a star branched polymer should have a high vinyl content and the remaining segments should have a low vinyl content, this method will not work because coupling reactions result in high vinyl in the center of the polymer chain or star branched polymer. 

Bywater, S. Polymerization Initiated by Lithium and Its Compounds. Vol. 4, pp. 66-110. Bywater, S. Preparation and Properties of Star-branched Polymers. Vol. 30, pp. 89-116. 

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