Polypropylene and all industrially produced polyolefins are linear molecules containing chain ends. Employing a tungsten catalyst, Veige and co-workers polymerize propyne to give cyclic polypropyne followed by hydrogenation to produce atactic polypropylene. Evidence for a cyclic topology comes from dynamic and static light-scattering techniques and rheology. Compared with linear polypropylene, the atactic cyclic polypropylene exhibits a >20 C increase in its glass transition temperature (T g ).
This report describes an approach
for preparing tethered tungsten-imido
alkylidene complexes featuring a tetra-anionic pincer ligand. Treating
the tungsten alkylidyne [
t
BuOCO]WC
t
Bu(THF)2 (1) with
isocyanates (RNCO; R =
t
Bu, Cy, and Ph)
leads to cycloaddition occurring exclusively at the CN bond
to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery
of [(O2C
t
BuC)W(η2-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(
t
BuOCO)W(NR)(
t
BuCCO)] (14-R), on the pathway to the formation
of 6-NR. Complex 11-R is kinetically stable
for sterically bulky isocyanate R =
t
Bu
(11-
t
Bu) and is isolated
and characterized by single-crystal X-ray diffraction. Finally, adding
to the short list of catalysts capable of ring expansion metathesis
polymerization (REMP), complexes 6-NR and 11-
t
Bu are active for the stereoselective
synthesis of cyclic polynorbornene.
Cyclic polymers possess properties
that are significantly different
from their linear analogs, such as higher densities, smaller hydrodynamic
volumes, and higher glass transition temperatures. Poly(4-methyl-1-pentene)
(PMP), a linear polyolefin, is a commercial transparent
thermoplastic and has applications in packaging materials and release
membranes. Polymerizing 4-methyl-1-pentyne with a tungsten alkylidyne
catalyst and subsequent hydrogenation (>99%) provided cyclic poly(4-methyl-1-pentene)
(
c-PMP). Evidence of a cyclic topology
comes from rheology/viscosity studies, light scattering measurements,
and size-exclusion chromatography. Importantly, atactic
c-PMP exhibits a T
g (39
°C) 10 °C higher than the linear analog. A 15 g-scale cyclic
polymerization was also achieved with 1-pentyne. Subsequent hydrogenation
yielded 10 g of cyclic poly(1-pentene). Measurements of initial rates
during the polymerization of 1-pentyne reveal a catalyst activity
of 180,000,000 g/molcat/h.
Cyclic polymers possess different properties compared to their linear analogues of the same molecular weight, such as smaller hydrodynamic volumes and higher glass transition temperatures (T g ). Cyclic poly(4-ethynylanisole) (cPEA) was synthesized via a catalytic ring-expansion of 4-ethynylanisole. The catalyst employed was a tungsten complex supported by a tetraanionic pincer ligand. Evidence of the cyclic topology comes from gel permeation chromatography, dynamic light scattering, static light scattering, and solution viscometry. Demethylation of cPEA with boron tribromide affords cyclic poly(4-ethynylphenol) (cPEP-OH). cPEP-OH exhibits pH-responsive water solubility, being soluble in aqueous solutions at elevated pH and becoming insoluble under acidic conditions. The linear equivalent of cPEP-OH was also synthesized, and it exhibits similar pH responsiveness.
Polymer
bottlebrushes are complex macromolecular nanostructures
with polymeric side chains densely grafted to a polymer backbone.
In this work, a synthetic strategy for the synthesis of cyclic bottlebrush
polymers was exhibited by combining ring-expansion polymerization
(REP) and atom transfer radical polymerization (ATRP) by a grafting-from
approach. A variety of ultra-high-molecular-weight (on the order of
MDa) macrocyclic bottlebrushes were generated by employing this method.
Direct visualization of the macrocyclic bottlebrushes was achieved
by atomic force microscopy. Furthermore, a linear bottlebrush polymer
was synthesized independently by a similar synthetic route to investigate
topological differences between cyclic and linear architectures.
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