The kinetics of the anionic polymerization of methacrylates and acrylates in THF as well as the MWD of the polymers formed depend on the concentration of active centres and of additives, such as lithium halides and lithium alkoxides. These results are discussed on basis of multiple equilibria between non‐associated, associated, and complexed ion pairs which are supported by viscosity measurements. The position of these equilibria determines the rate of polymerization, whereas the dynamics of interconversion determine the polydispersity. In the absence of additives the rate of monomer addition to non‐associated ion pairs competes with the rate of association. Addition of lithium chloride mainly affects the rate of interconversion between dormant and active species, whereas lithium tert‐butoxide forms adducts which stabilize the active centre.
For
the first time, ATRP was successfully employed for homopolymerization
of a commercial methacrylate-functionalized polyhedral oligomeric
silsesquioxane (POSS) monomer, iBuPOSSMA, to high molecular weights.
It was found that iBuPOSSMA has a low ceiling temperature (T
c); therefore, low temperatures and/or high
initial monomer concentrations need to be used in order to avoid low
degrees of polymerization that had been observed previously. The values
of T
c, as well as of the polymerization
enthalpy ΔH
p and entropy ΔS
p were determined to be 130 °C (at [M]0 = 1 M), −41 kJ mol–1, and −101
J mol–1 K–1, respectively. Under
optimized conditions, poly(iBuPOSSMA) homopolymers having low dispersity
and high M
n, ranging from 23 000
to 460 000, were obtained in a well-controlled ATRP process. Moreover,
various block copolymers having high-M
n poly(iBuPOSSMA) blocks were prepared by copolymerization of iBuPOSSMA
with methyl methacrylate and styrene.
In recent experiments,
the “local pH” near polyelectrolyte
chains was determined from the shift in the effective acidity constant
of fluorescent pH indicators attached to the macromolecules. This
indirect determination raises the question if the analyzed quantity
was indeed the “local pH” and what this term actually
means. In this study, we combined experiments and simulations to demonstrate
that the shift in ionization constant is slightly lower than the difference
between the pH and the “local pH”. This offset is caused
by correlations between fluctuations in chain conformation, small-ion
distribution, and fluorophore ionization.
Interactions of living oligomers of tert-butyl methacrylate (tBuMA) with a Li counterion and of the model living dimer di-tert-butyl 2-lithio-2,4,4-trimethylglutarate (A) with LiCl (B) were studied in tetrahydrofuran-d 8 (THF-d8) solution by 7 Li, 6 Li, 1 H, and 13 C, 1D and 2D, NMR spectroscopy. Di-tertbutyl 2-lithio-2,4,4-trimethylglutarate is shown to form a 1:1 complex with lithium chloride in THF. 1 H, 7 Li NMR results suggest that a rapid equilibrium between free LiCl and the mixed complex takes place when the molar ratio LiCl:lithiated dimer is higher than 1. From 13 C relaxation times, a dimeric aggregation (i.e. A2B2) of the complex is deduced, in full accord with MNDO calculations. Preliminary results are presented for living tBuMA oligomers prepared by initiation with diphenylhexyllithium in the presence of various amounts of lithium chloride. Fast-acquired 13 C NMR spectra of the living ends were recorded using 13 C-enriched monomer added at the end of the polymerization. Their structure is shown to be affected by the amount of LiCl added and to correspond to an equilibrium between free lithium chloride and a complexed end group. A higher excess of LiCl apparently does not change the nature of the complexed living end. The oligomers exhibit marked self-termination in THF at 273 K. Finally, a correlation of the structure of the living chain ends with the macromolecular parameters of the resulting polymer is attempted.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.