Linked Life Data

19334780

Source:http://linkedlifedata.com/resource/pubmed/id/19334780

Statements in which the resource exists as a subject.
PredicateObject
rdf:type
lifeskim:mentions
pubmed:issue
13
pubmed:dateCreated
2009-4-1
pubmed:abstractText
The zwitterionic ring-opening polymerization of lactide initiated by N-heterocyclic carbenes generates cyclic polylactides with well-defined molecular weights between M(n) = 5000 and 30,000 g/mol with narrow polydispersities (M(w)/M(n) < or = 1.31). These zwitterionic polymerizations are extremely rapid (k(p) = 48.7 M(-1) s(-1)), but also exhibit exceptional control of molecular weight and molecular weight distribution. The unusual kinetic features of these zwitterionic polymerizations are illuminated with kinetic and mechanistic investigations, which implicate a mechanism that involves a slow initiation step (second order in [M]), a propagation step (first order in [M]) that is much faster than initiation (k(i) = 0.274 M(-2) s(-1)), cyclization (k(c) = 0.0575 s(-1)), and depropagation (k(d) = 0.208 s(-1)). Numerical and stochastic simulations of the kinetic data provide a kinetic rationale for the evolution of molecular weight with monomer conversion: the molecular weights increase with increasing monomer conversion, exhibit a nonzero intercept near 0% monomer conversion, and are relatively insensitive to the initial monomer-to-initiator ratio. The observed narrow molecular weight distributions are due to a high rate of propagation relative to cyclization and chain transfer. Kinetic simulations define the kinetic criteria under which the active zwitterions remain in solution; these simulations were substantiated by chain-extension experiments, which provide experimental evidence for chain extension of the zwitterions and reinitiation by the N-heterocyclic carbenes liberated upon macrocyclization. The kinetic model rationalizes some of the unique features of zwitterionic ring-opening polymerization and provides a useful mechanistic framework to optimize these polymerizations as a strategy to generate well-defined cyclic polyesters.
pubmed:language
eng
pubmed:journal
pubmed:citationSubset
IM
pubmed:chemical
pubmed:status
MEDLINE
pubmed:month
Apr
pubmed:issn
1520-5126
pubmed:author
pubmed:issnType
Electronic
pubmed:day
8
pubmed:volume
131
pubmed:owner
NLM
pubmed:authorsComplete
Y
pubmed:pagination
4884-91
pubmed:meshHeading
pubmed:year
2009
pubmed:articleTitle
Zwitterionic polymerization: a kinetic strategy for the controlled synthesis of cyclic polylactide.
pubmed:affiliation
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
pubmed:publicationType
Journal Article, Research Support, U.S. Gov't, Non-P.H.S., Research Support, Non-U.S. Gov't