Polydispersity index values

Silane radical atom transfer (SRAA) was demonstrated as an efficient, metal-free method to generate polystyrene of controllable molecular weight and low polydispersity index values. (TMSlsSi radicals were generated in situ by reaction of (TMSlsSiH with thermally generated f-BuO radicals as depicted in Scheme 14. (TMSlsSi radicals in the presence of polystyrene bromide (PS -Br), effectively abstract the bromine from the chain terminus and generate macroradicals that undergo coupling reactions (Reaction 70). 

The use of a Tp"Cu complex for the ATRP reaction was described with the simple TpMe2( u complex, using styrene or n-butyl acrylate as the monomer. However, high values of the polydispersity were found (MyM =3.82-4.63). Later, the bulkier Tp Cu(NCMe) catalyst provided much better results with acrylates, with polydispersity index values as low as 1.09 and lacking the need for additives (Scheme 28). ... 

By controlling the hydrophobic/hydrophilic equilibrium of the polymers, their selectivity for bacterial cells over red blood cells (RBC) can be improved. This property is related to the effect that the ionic nature and hydrophobic character of these polymers has on cell membrane activity. The length of the alkyl substituents in the polymer repeat units affects antibacterial effectiveness in more hydrophobic polymers (hexyl and higher alkyl chain lengths in the repeat unit), disruption of the membrane integrity occurs more effectively. ROMP is an attractive method of synthesis which is widely used to prepare well-defined polymers with controlled MW and low polydispersity index values (PDl) [27-30]. 

The ATRP mediated synthesis of non-zwitterionic copolymers, wifliout the use of protecting groups, has been also presented in the literature [18]. An interesting example is the case of the poly(N-isopropylacrylamide)-b-poly(4-vinylpyridine) (PNIPAM-P4VP) copolymers. The synthesis of the particular type of copolymer was achieved by using conventional ATRP techniques. The molecular characteristics and the data obtained by size exclusion chromatography (SEC) analysis indicate the successful synthesis of copolymers with polydispersity index values ca. 1.2. 

The catalyst prepared with this particular bridged metallocene provided an ethylene/1 -hexene copolymer at 80 C with an of283,000 and a polydispersity index value (MyMj of 3.1 utilizing commercial polymerization conditions. 

Examination of this catalyst system under solution polymerization conditions carried out at 160°C in 216 ml of cyclohexane as solvent containing 10 or 20 ml 1-octene as comonomer, 140 psig total reactor pressure, the catalyst exhibited a relatively high activity of 2430 g PE/mmol catalyst/hr. Characterization of the polyethylene showed a molecular weight (Mw) of 114,000 with a polydispersity index value of 2.6, indicating near single-site MWD. 

It is useful to note that even so-called monodisperse samples have polydispersities that are rarely smaller than 1.01. But even at this low value, the sample contains some high and low-molecular weight material. This is illustrated in Fig. 2.2, which shows the distributions calculated using the log-normal distribution (Eq. 2.70) for M = 100,000 and polydispersity index values of 1.01, 1.03 and 1.1. We see that even the material with the smallest PI has a small amount of polymer with M 20% higher or lower than the mean. 

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