Molecular weight star polymers

The tube model gives a direct indication of why one might expect the strange observations on star melts described above. Because the branch points themselves in a high molecular weight star-polymer melt are extremely dilute, the physics of local entanglements is expected to be identical to the linear case each segment of polymer chain behaves as if it were in a tube of diameter a. However, in... 

The synthesis of extremely high molecular weight star polymers has been achieved by another method. First divlnyl-benzene is reacted at very low concentration with an anionic Initiator (sec. BuLi) to yield a suspension of poly-DVB nodules fitted with numerous initiating sites. Then styrene Is added, and each Initiating site should give yield to a branch. However, the polydispersity of the samples obtained is very high. 

Anionic polymerization of poly(methyl methacrylate) (PMMA) using the Grignard reagent prepared from polymer 3L was also attempted for low molecular weight star polymers. The PMMA segment was essentially syndiotactic. 

The branched architecture has great influence on the packing of molecular chains. In general, dendrimers have smaller hydrodynamic radius and the melt and solution viscosity of a hyperbranched polymer is expected to be lower than that of a parent linear polymer. Viscosity measurements performed with a cone viscometer confirmed the decrease of viscosity of star-shape polymers compared to the respective high molecular weight arms (polymers B-R-4 and C-R-4, Tables 1 and 2). This observation is consistent with the decrease of hydrodynamic volume observed for... 

C. Assume that the GPC curve of Polymer A is that of a polydisperse, tetrafunctionally star-branched polychloroprene (i.e. apolydis-perse polymer made up of different molecular weight star-shaped polymers having four arms of equal length). Calculate the weight and number molecular weight averages, the polydispersity and the intrinsic viscosity of this star-branched polychloroprene. 

Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization using xanthanes and dithiocarbamates is described [266]. Narrow polydispersities and good control of molecular weight for polymers of M < 30 000 are achieved for these polymers. The living nature of RAFT polymerization allows the synthesis of block copolymers, star polymers and gradient copolymers [266]. 

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. 

One polymer is formed for each initiator molecule, so that the number average molecular weight of polymers or block segments can be predicted from the reaction stoichiometry. Multifunctional initiators with functionality n can form stars with n arms. 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

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