Transesterification side reactions

The analogous magnesium alkoxide dimer, (273), is more active than (272), although polydispersities are generally higher (ca.1.5), a result of transesterification side reactions.824 An increased level of control may be acquired by the addition of 1 equivalent of PrOH (Mw/Mn... 

On the other hand, when sCL is copolymerized with dilactones such as GA [38] and (D- andD,L-)LA [39], tapered or pseudoblock copolymers are obtained with a reactivity ratio much in favor of the dilactone. As an example, the reactivity ratios in the copolymerization of eCL and D,L-LA in toluene at 70 °C are r = 0.92 (e-CL) and r2=26.5 (D,L-LA). Very similar reactivity ratios were calculated for copolymerization between eCL and L-LA, other experimental conditions being kept unchanged. However the control over the polymerization is lost due to transesterification side reactions perturbing the propagation step. Such a behav-... 

Scheme 4 Intramolecular and intermolecular transesterification side reactions...

When the general approach for the ROP of lactide and evaluation of numerous catalysts is considered, it is apparent that a proliferation of variables be considered. For example, the type and concentration of catalysts, solution or bulk reaction conditions, polymerization temperature, undesirable transesterification side-reactions and the general classes of initiators all influence the polymerization. To survey these variables, a parallel approach to polymer synthesis, designed to enable rapid screening of optimal catalyst-initiator systems and associated polymerization conditions, was employed. The Quest 210 robotic reactor proved to be an... 

Scheme 10.3 Intra- and inter-molecular transesterification side reactions.

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