(46f) A Full Exploitation of the Pulsed Laser Polymerization Technique to Assess All Important Rate Coefficients in Acrylate Radical Polymerization

Free-radical
polymerization (FRP) processes are extensively applied for the production of macromolecular
materials, since they allow the synthesis of a large variety of vinyl polymers under
mild reaction conditions and with a high tolerance toward impurities.^[1] Despite the high industrial
importance of FRP, the quantitative understanding of radical polymerization
kinetics is still incomplete. Accurate knowledge of Arrhenius parameters and
rate coefficients for key reactions such as chain initiation, propagation, and
termination is a prerequisite for FRP design, particularly when conducted in large-scale
reactors, in which temperature control is a key issue.

Figure 1. Determination
of k_p via the
characteristic inflection points of the PLP-SEC trace.

One of the most established experimental
techniques to determine intrinsic rate coefficients in radical polymerization is
pulsed laser polymerization (PLP; Figure 1). This technique, as originally
introduced by Olaj and coworkers,^[2] has been extensively used to
obtain the propagation rate coefficient k_p. In PLP,
photoinitiator radical fragments are consecutively formed via laser pulses with
a frequency ν (or dark
time Δt = ν^-1), so that under well-defined
conditions (e.g. limited monomer conversion) in the corresponding size
exclusion chromatography (SEC) trace inflection points (L_j; j
= 1, 2, …) can be identified that are directly linked to k_p
via (Figure 1):

                                                              L_j
= k_p [M]₀ (jΔt)                                               (1)

Currently, the PLP
technique has been used to accurately measure a wide range of k_p
values.^[3-7] For standard monomers such as styrene and methyl
methacrylate, reliable values are already available. A special case is polymerizations
with several radical types, for which the obtained k_p (Equation
(1)) must be seen as an apparent
averaged one (k_p,app). For example, in acrylate radical
polymerization with applications e.g. in the coating and paint industry
both secondary end-chain radicals (ECRs) and tertiary mid-chain radicals (MCRs)
can be present (Figure 2), leading to the need for the determination of k_p,ecr
and k_p,mcr. The tendency of ECRs to switch to MCRs is
expressed by the backbiting rate coefficient k_bb (Figure 2). Currently,
a limited number of experimental methods exists for the determination of k_bb.
One of the most promising methods was introduced by Nikitin and coworkers,^[8] based on the observation that at high ν, Equation (1) relates to k_p,ecr ,
while at low ν, Equation (1) is influenced by both k_p,ecr
and k_p,mcr. From the onset of the sharp decrease in k_p,app
with decreasing ν, Nikitin and coworkers^[8] assessed k_bb. Very recently Wenn
and Junkers^[9] suggested a more simplified procedure, based on the
position of the sigmoidal fit to k_p,app(ν).

Figure 2.
Backbiting reaction leading to a transformation of the radical nature from
secondary to tertiary in acrylate radical polymerization; dominant path via a
cyclic six-membered transition state.^[10]

In the present
work, a novel and highly accurate method to determine k_bb is
presented, using a detailed
kinetic Monte Carlo (kMC) model and considering 2,2-dimethoxy-2-phenylacetophenone
(DMPA) as photoinitiator and n-butyl acrylate as monomer. For different solvent volume fractions (0-0.75), with this model regression
analysis is applied to inflection point data in the low frequency domain (~100
s^-1), which can be easily scanned with less expensive PLP equipment.
The novelty of the method lies in the variation of the solvent volume fraction,
which allows to independently
change the average MCR lifetime and to obtain a high sensitivity toward k_bb.

The developed kMC model is also capable
of the accurate simulation of the complex SEC trace for acrylate PLP, hence,
allowing a kinetic analysis transcending the correct simulation of only
inflection points for k_p and k_bb
determination. In particular, hidden information on chain initiation and
short-long termination can be elegantly extracted. It is for instance confirmed
that inhibition with one of the DMPA radical fragments as formed by
photoinitiation is taking place by detailed analysis of the peak intensities. In
addition, it is demonstrated that the extracted PLP data for short-long
termination (k_t,app,ij; i,j: chain length) can be used
to benchmark FRP diffusion models in the less studied regime of diluted
conditions or low monomer conversions. Such information is crucial to design
not only FRP processes but also novel radical polymerization processes, aiming
at improved microstructural control.

[1]
D. R. D’hooge, P. H. M. Van Steenberge, M.-F. Reyniers, G. B. Marin, Prog.
Polym. Sci. 2016, in press (DOI: 10.1016/j.progpolymsci.2016.04.002).

[2]
O. F. Olaj, I. Bitai, F. Hinkelmann, Macromol. Chem. Phys. 1987, 188,
1689.

[3]
K. B. Kockler, A. P. Haehnel, T. Junkers, C. Barner-Kowollik, Macromol.
Rapid Commun. 2016, 37, 123.

[4]
S. Beuermann,  M. Buback, Prog. Polym. Sci. 2002, 27, 191.

[5]
A. P. Haehnel, M. Schneider-Baumann, L. Arens, A. M. Misske, F. Fleischhaker,
C. Barner-Kowollik, Macromolecules 2014, 47, 3483.

[6] T. Junkers, S. P. S. Koo, C. Barner-Kowollik, Polym. Chem. 2010,
1, 438.

[7] C. Barner-Kowollik, F. Gunzler, T. Junkers, Macromolecules 2008,
41, 8971.

[8] A. N. Nikitin, R. A. Hutchinson, M. Buback, P. Hesse, Macromolecules
2007, 40, 8631.

[9] B. Wenn,  T. Junkers, Macromol. Rapid Commun. 2016,
37, 781.

[10] D. Konkolewicz, S.
Sosnowski, D. R. D'hooge, R. Szymanski, M. F. Reyniers, G. B. Marin, K. Matyjaszewski,
Macromolecules 2011, 44, 8361.