Reaction, chain, copolymer polydispersity

For analyzing structure-property relationships, a variety of PEO-g-PVA copolymers were prepared, differing in the VAc-to-PEO ratio and the molar mass of PEO. The analysis of the copolymers by IR and 1H- and 13C-NMR showed the presence of both PEO and PVA. A small C=0 absorption was still present and was explained by a nonquantitative saponification. SEC showed polydispersities (Mw/M ) of around 5, with a small tailing to the low molar mass side. The latter was probably caused by the relatively low molar mass PVA homopolymer formed by the chain transfer reaction of VAc, both to the PEO and its acetate functionality. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

The discovery that Ti complex (4) was an effective catalyst for living ROMP chemistry resulted in the synthesis of several new types of polymers that were inaccessible with conventional catalysts. Narrow polydispersity polymer and di- and triblock copolymers were synthesized soon after living ROMP was discovered. Because polymer chains that are formed in a living ROMP reaction are terminated with metal carbene groups, fimctionalization of the chain ends is possible. Reaction of the carbene with an aldehyde occurs in a Wittiglike fashion, making a metal oxo complex that is inert to finther metathesis chemistry and terminates the chain with the alkylidene group of the aldehyde. 

NMRP is not a true living polymerization but it has some attributes of a living polymerization, e.g. rather narrow polydispersity polymers can be produced, polymer molecular weight increases linearly with monomer conversion, and sequential addition of monomers leads to block copolymers. However, no one has yet produced truly monodisperse polymer using NMRP. Therefore, there are likely side reactions going on during chain growth that lead to adventitious termination. 

RAFT is effective with a wide range of monomers, but distinguishes itself from SFRP and ATRP in that it can polymerize carboxylic acid-containing monomers such as methacrylic acid [46]. The polymerizations are performed at temperatures of 100 °C or less with typical polydispersities in the 1.1 1.25 range under either bulk, solution or emulsion conditions. Initially formed homopolymers can readily be chain extended or transformed into block copolymers by reaction with a second monomer [47]. 

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