Microblock structures

EO is observed to be much more reactive against the hydroxyethyl primary hydroxyl group (chain end derived from EO). That explains the preferential tendency of EO to react with another hydroxyethyl unit, forming microblock structures (...PO-EO-EO-PO-...) with a low tendency of alternation. It is observed, that by the temperature increase, the tendency to alternation increases and the tendency to form poly[EO] microblocks diminishes. Therefore at higher temperatures more uniform random structures are obtained than at lower temperatures. Other authors have proved that between 25-80 °C the reactivity ratios are identical to those obtained in copolymerisation of PO with EO [59]. 

The 13C NMR study of a heteropolyether polyol (MW 3600 daltons) containing 15% EO units proved that 45% of total EO units have a microblock structure (...PO-EO-EO-PO..) and that 55% of total EO units are alternating sequences of PO-EO-PO units [97]. 

A very interesting structure of random PO-EO copolyethers is a structure containing a high EO content, of around 75% EO (Figure 4.19). In this structure the majority of monomer units are EO units (mainly internal EO and terminal), and PO represents minority units of around 25%. This structure consists of microblocks of polyfEO] alternating with PO units and has two important applications ... 

Andronova, N Finne, A. and Albertsson, A.-C. (2003) Fibrillar structure of resorbable microblock copolymers based on l,5-dioxepan-2-one and e-caprolactone. Journal of Polymer Science Part A-Polymer Chemistry, 41, 2412-23. 

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