Elucidation of the intra-chain radical mechanism in poly(norbornene imide) single-chain nanoparticle formation

1
Elucidation of the intra-chain radical mechanism
in poly(norbornene imide) single-chain
nanoparticle formation
Justin P. Cole, Jacob J. Lessard, Christopher K.
Lyon, Bryan T. Tuten, and Erik B. Berda.
Department of Chemistry, University of New
Hampshire.
An Unexpected Mechanism for SCNP Formation
Introduction
Oxygens Role in Collapse
Attempting to design an efficient and scalable
method for producing functionalized single-chain
nanoparticles, we investigated intra-chain
radical polymerization of pendant methacryloyl
decorated poly(norbornene imides). Polymer P1 was
synthesized via ROMP. To initiate intra-chain
polymerization of the pendant methacryloyl
groups, we subjected P1 to a range of radical
initiator (AIBN) concentrations under
ultra-dilute conditions as is typical in the SCNP
literature.
As a control experiment we synthesized polymer
P2, which contains no pendant methacryloyl units,
and subjected it to the same cross-linking
conditions as P1. We were quite surprised to
observe once again shifts in retention time
consistent with the formation of single-chain
nanoparticles. SEC-MALS traces for the parent
chain P2 and corresponding single-chain particles
NP2 are shown in Fig. 2. Here again there is no
evidence of large multi-chain aggregates. Our
initial thought was that polymerization through
the backbone olefins could be driving SCNP
formation here, however examining the 1H NMR
spectra again revealed no discernable loss of
olefin resonances or other changes between P2 and
NP2.

Figure 5 Exposure of P1 (A) and P2 (B) to
radical cross-linking conditions after oxygen
exclusion via freezepumpthaw cycles.
To understand the role of adventitious oxygen in
this system we examined the polymers described
above after rigorous degassing via multiple
freezepumpthaw (FPT) cycles. Fig. 5A compares
the methacryloyl-functionalized polymer P1-FPT
before and after exposure to radical initiation
with rigorous oxygen exclusion. Here, a mix of
intra-chain and inter-chain cross-linking are
observed. Fig. 5B highlights similar experiments
on the hexyl-functionalized homopolymer P2-FPT.
This result confirms that polymerization or
coupling through the methacrylate pendants is
occurring and that without oxygen present the
backbone olefins do not participate in this
process.
Characterization of NP1
Figure 2 SEC overlay of P2 and NP2.
Summary and Conclusions
We found that poly(norbornene imides) decorated
with polymerizable methacryloyl side chains can
be converted to single-chain nanoparticles by
polymerizing through the methacryloyl side
chains, however this process is not as
straight-forward or effective as we originally
hoped. We also discovered that small
concentrations of adventitious oxygen in this
system, when exposed to a radical source, can
react with sites of unsaturation in the NBI
backbone to promote intra-chain cross-linking,
presumably by oxygen bridging. Our work to unveil
this mechanism more fully and exploit this
chemistry to create functional nanostructures is
ongoing.

Acknowledgements
Figure 3 Effect of backbone hydrogenation on
pNBI radical cross-linking.
Figure 4 Intra-chain cross-linking experiments
on P3.
Subjecting a hydrogenated polymer (P2h) to the
previously described radical cross-linking
conditions resulted in no change in the SEC trace
(Fig. 3). This result strongly supports the
involvement of backbone olefins in the
intra-chain radical cross-linking chemistry we
are seeing in this system. We again applied this
hydrogenation strategy to synthesize a similar
material, P3, with a fully saturated backbone.
Doing so permits the isolation of any
cross-linking behavior that may have occurred due
to the methacryloyl groups in our original
example. Fig. 4 shows these results a broadening
of the SEC-MALS trace, indicating a competing
intra-chain collapse and interchain coupling. 1H
NMR shows a decrease in the methacryloyl
resonances in the photochemical reaction but not
in the thermally initiated case.

• The author would like to graciously thank the
  Army Research Office for support through award
  W911NF-14-1-0177, and NIST for support through
  award 70NANB15H060 as well as Dr. Erik Berda, Dr.
  John Tsavalas, and Dr. Gary Weisman for sharing
  their time and expertise.

References
 1. Cole, J. P. Lessard, J. J. Lyon, C. K.
Tuten, B. T. Berda, E. B., Intra-chain radical
chemistry as a route to poly(norbornene imide)
single-chain nanoparticles structural
considerations and the role of adventitious
oxygen. Polymer Chemistry 2015.  
Figure 1(A) SEC overlay of P1 and corresponding
NP1, (B) NMR overlay of N1 and NP1 highlighting
the olefinic region.