Bimodal polymer group

Most bimodal networks synthesized to date have been prepared from PDMS [88], One reason for this choice is the fact that the polymer is readily available with either hydroxyl or vinyl end groups, and the reactions these groups participate in are relatively free of complicating side reactions. These ideas can obviously be extended to higher modalities (trimodal, etc., eventually approaching an extremely broad, effectively-unimodal distribution) [102-104],... 

Conclusions. Table II compares some characteristics of the semicontinuous and batch techniques. According to 1H NMR and GPC data the semicontinuous technique produces polymers with well defined symmetrical end groups, while the batchy polymerization may yield once-fired and unfired chain ends. The M Mn values of polymers obtained in semicontinuous runs are close to theoretical, whereas those harvested in conventional batch polymerization exhibit broader molecular weight distributions due to changing [I] and [M], presence of once-fired and/or unfired chain ends, and insufficient reactor control. The latter circumstance may even result in bimodal distributions. 

An important group of surface-active nonionic synthetic polymers (nonionic emulsifiers) are ethylene oxide (block) (co)polymers. They have been widely researched and some interesting results on their behavior in water have been obtained [33]. Amphiphilic PEO copolymers are currently of interest in such applications as polymer emulsifiers, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. Examples are block copolymers of EO and styrene, graft or block copolymers with PEO branches anchored to a hydrophilic backbone, and star-shaped macromolecules with PEO arms attached to a hydrophobic core. One of the most interesting findings is that some block micelle systems in fact exists in two populations, i.e., a bimodal size distribution. 

Bulky crosslinks or side-groups in the network chains, e.g., dendritic wedges [73], may also influence molecular mobility and viscoelastic properties of polymer networks. For example, UV curing of difunctional acrylates results in the formation of zip-like network junctions, which may be regarded as extreme cases of bimodal networks [52], Results obtained with the NMR T2 relaxation method agree well with those of mechanical tests... 

The yields of high polymer from this synthesis depend on the exact conditions and the nature of the substituent groups, but are typically low, from a few percent up to 60-70% in the most favorable cases. The by-products are mainly cyclosilanes, (RR Si) where n = 4 6. The molecular weight distributions of the polymers obtained by this procedure are usually bimodal, as shown in Figure 5 Various attempts have been made to improve the sodium condensation, including sonication and the addition of crown ethers and other additives, with some success. ... 

The copolymer prepared by Hammond et al. 33), under essentially the same conditions, had different characteristics. GPC analysis showed bimodal molecular weight distribution, besides a high-molecular-weight polymer an oligomer fraction was also formed. The latter consisted mainly of cyclic PO tetramer but mixed (PO and THF) tetramers and pentamers were also identified. The M determined by GPC was low (usually below M = 500). The OH end-groups were determined by acetylation with acetic anhydride in pyridine with HC104 catalyst. 

Blanchard et al. have tested a variety of catalytic systems including BF3 OEt, and SnCl4 with diols and trityl salts 34,35). Working under similar conditions as the previous two groups they prepared copolymers with Mn in the Mn = 500-750 range (vpo). GPC analysis showed bimodal distribution with a low-molecular-weight polymer (Mn 300) also present. 

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