Polymer polydispersity index

Q = Mw/Mn = Polymer polydispersity index ROT Quenching with tritium labeled alcohol 1 1/mol sec... 

Doi et al. studied the effect of different aluminium alkyls on the polydispersity of syndiotactic polypropylene obtained with the V(acac)3-alkyl aluminium halide soluble catalytic system, at temperatures below —65 °C. These authors found that, by varying the type of aluminum alkyl not only the propagation and transfer rates are changed, but also the polymer polydispersity index decreases in the following order ... 

In other identical conditions, the decrease of monomer initial concentration reduces the number-average molecular mass of polymer. Polydispersity index doesn t practically depend on monomer initial concentration. 

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

The MWD is the ratio of Mw Mn and is called the polydispersity index. This largely varies from one grade of polymer to the other, depending on the polymerization conditions and the type of catalyst used. Figure 14 shows different types of MWD for the polymers. 

Assuming that no intramolecular or side reactions take place and that all groups are equireactive, the polydispersity index, 7P, of hyperbranched polymers obtained by step-growth polymerization of ABX monomers is given by Eq. (2.2), where pA is die conversion in A groups.196 Note that the classical Flory relationship DPn = 1/(1 — pa) holds for ABX monomer polymerizations ... 

The micro-mixed reactor with dead-polymer model was developed to account for the large values of the polydispersity index observed experimentally. The effect of increasing the fraction of dead-polymer in the reactor feed while maintaining the same monomer conversion is to broaden the product polymer distribution and therefore to increase the polydispersity index. As illustrated in Table V, this model, with its adjustable parameter, can exactly match experiment average molecular weights and easily account for values of the polydispersity index significantly greater than 2. 

A micro-mixed, seeded reactor will produce a broad polymer distribution with a high molecular weight tail and polydispersity index that approaches 2 at large degrees of polymerization. 

The effect of dead-polymer and by-passing on the micro-mixed reactor for the same degree of monomer conversion is to broaden the product polymer distribution and thus allow values of the polydispersity index much larger than 2. 

Our theoretical studies [38] showed that the hyperbranched polymers generated from an SCVP possess a very wide MWD which depends on the reactivity ratio of propagating and initiating groups, r=kjk. For r=l, the polydispersity index where P is the number-average degree of polymerization. 

In general, syndiotacticity (rr%) increases with a reduction of the polymerization temperature. In the case of SmH(C5Me5)2, it increased from 78 to 95.2% as the polymerization temperature was reduced from 25 to — 95 °C, but the polydispersity index remained low [3]. Extrapolating the data suggests that syndiotacticity over 97% may be obtained at — 115°C. Polymerization of MMA in both THF and toluene using the organolanthanide initiators produced syndiotactic polymers, despite the fact that the RMgX initiator in toluene led to isotactic polymers [15]. 

The hydroboration/oxidation sequence does not change the molecular-weight distribution. Gel permeation chromatography (GPC) measurements in dimethyl-formamide (DMF) with the resulting polystyrene-ft-polyalcohol polymers show very similar polydispersity indexes (Table 10.2). Here, the hydroboration/oxidation sequence is clearly superior to the epoxidation reaction, which leads to a... 

The development of mass spectroscopic techniques such as matrix assisted laser desorption (MALDI) and electrospray mass spectrometry has allowed the absolute determination of dendrimer perfection [7,8], For divergent dendrimers such as PAMAM and PPI, single flaws in the chemical structure can be measured as a function of generation to genealogically define an unreacted site of or a side reaction producing a loop at a particular generation level. Mass spectromet-ric results on dendrimers, not only demonstrate the extreme sensitivity of the technique, but also demonstrate the uniformity of the molecular mass. The polydispersity index of Mw/Mn for a G6 PAMAM dendrimer can be 1.0006 which is substantially narrower than that of living polymers of the same molecular mass [7],... 

Controversial results are reported in the literature regarding the polydispersity of polyesters produced by SSP, associated with the side reactions in the later stages of the reaction. These are not only dependent on the concentrations of the reactive groups but also on their intramolecular distances [11], Additionally, it has been found that cyclization leads to a different polydispersity. According to theoretical considerations, the polydispersity index of an SSP polymer is generally higher than that of prepolymer produced in the melt phase, which should, in an ideal case have a value of 2 [21-24, 59],... 

The degree of polymerization depends on the duration of the process. After 7 min, the molecular mass is equal to 9400 (the polydispersity index is 5.30). When the reaction is carried out for 15 min, the molecular mass of the polymer increases to 37,000 and the polydispersity index reaches 7.31 (Bauld et al. 1996). Depending on whether cation-radical centers arise at the expense of intramolecular electron transfer or in a stepwise intermolecular lengthening, polymerization can occur, respectively, through a chain or a step-growth process (Bauld and Roh 2002). In the reaction depicted in Scheme 7.17, both chain and step-growth propagations are involved. 

Hence, cation-radical copolymerization leads to the formation of a polymer having a lower molecular weight and polydispersity index than the polymer got by cation-radical polymerization— homocyclobutanation. Nevertheless, copolymerization occnrs nnder very mild conditions and is regio-and stereospecihc (Bauld et al. 1998a). This reaction appears to occnr by a step-growth mechanism, rather than the more efficient cation-radical chain mechanism proposed for poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation-radical Diels-Alder polymerization. ... 

What is the most probable value for the polydispersity index for (a) a monodisperse polymer and (b) a polydisperse polymer synthesized by a condensation technique ... 

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