End-group fidelity

A class of improved nitroxides (compared with TEMPO) for NMRP without a /7-hydrogen are the imidazolidone nitroxides (IX) developed by CSIRO researchers [13]. Both vm and IX are claimed to give improved performance and higher end-group fidelity in NMRP, especially for acylates, compared with TEMPO. 

Docking studies by Veld et al. suggest that for the cisoid lactones, competitive inhibition by wrongly bound substrate is responsible for the low experimentally observed reactivity (Fig. 2b) [38]. This finding indicates that in ROP of lactones to polyesters where a monomer activated mechanism is operational, the polyester product can be more reactive in the activation step than the lactone substrate. This has important implications for the ability to control the polydispersity, the molecular weight and the end-group fidelity in lipase-catalyzed ROP. 

CareM selection of the reaction conditions and working under thermodynamic control i.e., performing reactions under equifibrium condition allow high end-group fidelity in a lipase-catalyzed ROP. For example, vinyl(meth)acrylates were found to be suitable end-cappers during the polymerization reactions [47, 48]. Using hydroxyl compounds like hydroxyethyl (meth)acrylatc (HEA or HEMA) as... 

The question of end-group fidelity is critical to evaluate the efficiency of any jROP catalyst system. All polymer samples were characterized by NMR spectroscopy (Fig. 28.5), which attested to the presence of the expected termini, -C(CH3)(H)0H together with HC=CCH20-C(0)C(H)(CH3)-, Ci5H 0-C(0)C(H)(CH3)-, or Bn0-C(0)C(H)(CH3)-. MALDI-ToF mass spectrometry corroborated the identity of the polymer end groups and established that intermolecular transesterification reactions occurred at high monomer conversion. 

Thus, in the particular system chosen, chain breaking is by spontaneous chain transfer, and there is no dormant-active equilibrium (all centers are active all of the time). The rate constant for propagation represents the composite for paired and free ions. Eqn (2.10) (with K=oo and kapp = kp) shows that for this system, 99% conversion is reached within about 9ms. Moreover, eqn (2.20) shows that 99% of the original chains have decomposed via spontaneous chain transfer within about 46 s. Clearly, such a system would not be considered living and it would offer no control from the point of view of end group fidelity or block copolymer synthesis. 

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