ONLINE MONITORING OF THE RAFT POLYMERIZATION OF STYRENE MEDIATED BY VINYBENZYL DITHIOBENZOATE

Andrew J. Heidenreich, Judit E. Puskas

Department of Polymer Science

The University of Akron170 University Ave.

Akron, OH44325

Alina M. Alb, Wayne F. Reed

Physics Department, 2001 Percival Stern Hall

TulaneUniversity, New Orleans, LA70118

Introduction

Inimer-type (initiator-monomer, IM) living carbocationic polymerization led to the successful synthesis of high molecular weight randomly branched polyisobutylenes.1 The architecture of these materials was analyzed by selective link destruction and Size Exclusion Chromatography (SEC).2. Yang and co-workers reported RAFT polymerization of styrene mediated by a chain transfer benzyl 4-vinyldithiobenzoate.3 In this case the inimer forms in situ by using a radical source.

This work reports the first results of monitoring Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization of styrene usingthe styrene-functionalized CTA, 3- and 4- vinylbenzyl dithiobenzoate (VBDB),as the inimer.As with all RAFT polymerizations, a conventional free radical initiator is used.The key to achieving high molecular weight randomly branched polymers is to have similar reactivity of the initiating and propagating radicals (Figure 1).

Figure 1: Structure of initiating and propagating radicals of VBDB.

Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) was used to follow the reaction kinetics and evolution of weight average molar mass Mw, z-average mean square radius of gyration <S2z, and weight average reduced viscosity r,w.5,6

Experimental

Materials. For VDBD synthesis: THF from Aldrich was distilled over calcium hydride prior to use. Magnesium turnings, bromobenzene, vinylbenzyl chloride, and carbon disulfide where all used as received from Aldrich.

For the polymerizations: styrene and n-butyl acetatewere used as received from Aldrich. Dicumyl peroxide from Acros and Luperox® 331(1,1-di(tert-butylperoxy) cyclohexane) from Arkemawere also used as received.

CTA Synthesis. The synthetic procedure for the synthesis of VBDB was followed as per that described by Rizzardo and co-workers.4 A mixture of 3- and 4- vinylbenzyl dithiobenzoate was obtained at 68% yield as a red oil.

Polymerization. Styrene polymerization reactions were carried out in a controlled fashion mediated by VBDB at 116oC in butyl acetate. This CTA possesses a dithioester group and a polymerizable double bond which, in combination with a radical source, expected to lead to formation of branched polystyrene as shown in Figure 2.

Figure 2: Synthetic method for arb-poly(styrene).

Table 1 shows the experimental conditions used in this work.

Table 1: Reaction conditions.

# / [Sty] mol/L / [CTA] mol/L x103 / [I]
mol/L x10-3 / [M]/[CTA] / [M]/[I]
1a / 1.59 / 6.14 / 1.26a / 259 / 1259
2b / 1.44 / 5.39 / 2.26b / 267 / 637
3a / 1.44 / 5.42 / 2.11 / 266 / 681

aLuperox L331; bdicumyl peroxide

ACOMP instrumentation. A five pump dual high and low pressure mixing stage system was used creating a 25-fold dilution of the reactor content with butyl acetate. This stream, flowing at 1ml/min, fed a detector train comprising a custom built single capillary viscometer, a Brookhaven Instruments Corp. BI-MwA multi-angle light scattering photometer (LS), a Shimadzu VP-10 dual wavelength spectrophotometer (UV), a Shimadzu SPM-20 photodiode array UV/Vis detector, and a 410 Waters differential refractive index detector (RI).

Results and Discussion

Figure 3shows raw data from reaction #2 in Table 1. The different steps correspond to the following procedure:(i) Stabilization of the detector baselines by pure solvent,(ii) Styrene baseline accumulation by withdrawal of the starting reactor contentsand the 25 fold dilution mentioned above, (iii) Accumulation of the sty/CTA baseline after theCTA (dissolved in monomer solution) is added, (iv) addition of initiator (dissolved in monomer/CTA solution), followed by the insertion of the reactor in the oil bath at 116oC allows the reaction to start. Nitrogen flows continuously in the reactor before and throughout the polymerization.

The UV signal decreases as polymerization proceeds, and it was used to compute monomer and polymer concentrations, cmand cp, and monomer conversion f. The LS signal increases as polymer is formed andMwand <S2z were computed by combining the LS signal with the cp data using the usual Zimm approximation.r,w was computed from the raw viscometer voltage V(t) and cp dataaccording to eq. (1)

(1)

where Vb is the viscometer’s solvent baseline voltage.

Figure 4 shows fractional monomer conversion f, and Mw vs. t for reaction #2. A first order fit to f is also shown and describes the data well. Mw increases in time, which is characteristic of a controlled radical polymerization. In free radical polymerization Mw normally decreases in time, or stays constant. No specific evidence of branching is evident in the signal per se, but the values of Mw are significantly higher than the predicted values of Mn, for both reactions #1 and #2, as seen in Table 2. SEC shows values of polydispersity, Mw/Mn, much higher than is typical for controlled radical reations. The Mw and Mw/Mn constitute preliminary evidence of branching.

Figure 3. Raw data signals for reaction #2.

Figure 4. Fractional monomer conversion f, and Mw vs. t for reaction #2.

Figure 5 shows raw LS results and Mw vs f for experiment #1. The strong upwards curvature of the LS signal is characteristic of a controlled radical polymerization, as is the increase of Mw. Again, without a specific branching model the ACOMP data cannot be taken as evidence of branching at this point, but the data again show characteristic living behavior and Mw far in excess of the expected Mn.

Figure 5: Scattering intensity from 90° LS and evolution of weight-average molecular weight (Mw)versus conversion for #1 in Table 1.

Table 2: Resultsfrom the experiments listed in Table 1.

# / Rate
к(s-1) x104 / Mn,th (g/mol)
@f=0.35 **(f=0.15) / Mw
,ACOMP (g/mol)
@f=0.35, **(f=0.15) / ηrw,
ACOMP
cm3/g / Mw/Mn
SEC
1 / 5.743 / 9440 / 30870 / 30 / 2.2
2 / 0.211 / 4450** / 11000** / 19 / 2.1
3 / 11.3 / 9700 / 26 / 2.7

Conclusions

In conclusion, aborescent polystyrenes were synthesized using a RAFT polymerization mediated by the RAFT-based inimerVBDB. More precise SECwill be performed in future experiments to better understand the branching and architecture of these polymers. ACOMP signals have yet to be interpreted in terms of a branching model.

Acknowledgements. This material is based upon work supported by the National Science Foundation under CHE#0616834(GOALI).Financial support by LANXESS Inc. Canada is also acknowledged.

References

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(4)Phuong, L.T.; Moad, G.; Rizzardo, E.; Thang, S.H. W.O. Patent 9,801,478, 1998.

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