Nitroxide-Mediated Radical Polymerization - American Chemical Society

Chapter 13

Nitroxide-Mediated Radical Polymerization: End-Group Analysis 1

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Downloaded by NANYANG TECHNOLOGICAL UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch013

Yucheng Zhu , I. Q . Li , B. A . Howell , and D. B. Priddy

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Center for Applications in Polymer Science, Central Michigan University, Mount Pleasant, M I 48859 Dow Polystyrene Research and Development, 438 Building, Midland, M I 48667

This paper is devoted to understanding the limitations o f nitroxide mediated polymerization ( N M P ) for making highly end-functional polystyrene. It is believed that NMP virtually eliminates chain transfer and termination resulting i n excellent control (>95%) o f chain-end architecture. T o further study the mechanism o f N M P , we synthesized N M P initiator/mediators having a U V / v i s chromophore attached to either the initiating or terminating fragments. This allowed us to both qualitatively and quantitatively analyze the chain-ends using a G P C – U V / v i s technique. This technique showed that: 1) the chromophores are attached to the chain-ends, 2) initiating fragments give higher chain-end functionalization than the terminating nitroxyl fragment, 3) chain-end purity diminishes with both conversion and MW, 4) chain-end purity o f >90% is only achieved when making low MW (i.e., 7

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© 1998 American Chemical Society

In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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There have been two approaches to the study of the N M R P of styrene: 1) in situ 8

formation of the N M R P i n i t i a t o r "

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and 2) presynthesis of the i n i t i a t o r .

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Most

groups believe that presynthesis is best because the in situ approach leads to the formation of multiple nitroxide initiating species and there is not perfect stoicheometry 1

between the initiating and mediating species. 9 W i t h the presynthesis approach, a pure compound can be used to initiate polymerization which should lead to "cleaner chemistry". However, Georges et al take issue with this conclusion and believe that Downloaded by NANYANG TECHNOLOGICAL UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch013

excess nitroxy radicals is important to achieve narrow polydispersity and good end-group purity.

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There has been debate over the extent that N M R P eliminates termination processes. Priddy et al were the first to synthesize and study the thermal stability of a small molecule (I) that models the propagating chain-end of N M P of s t y r e n e . ' They prepared I by Η-atom abstraction from ethylbenzene and trapping the resultant radical with T E M P O . Thermolysis of I in an E S R spectrophotometer showed continuous formation of T E M P O . Further analysis of the decomposition products revealed the formation of primarily styrene. They went on to study the kinetics of the decomposition and found that I decomposes i n the temperature range utilized for N M R P of styrene at a rate comparable to styrene conversion rate. Based on this observation they concluded that end-group purity achieved by N M R P of styrene should decrease with both M and monomer conversion. They further concluded that this termination process would likely seriously limit the ability of N M R P for the preparation of high molecular weight (> 100,000) polystyrene having a narrow polydispersity. These conclusions have been disputed by other research groups who have attempted to prove that N M R P chemistry virtually eliminates all termination processes. These claims are supported by measuring the amount of nitroxyl moiety on the terminal chain-end. Both N M R " ^ and nitrogen a n a l y s e s have been utilized to determine the nitroxyl content of polystyrene made using N M R P . However, these techniques are not very sensitive and provides only an approximation of the level of nitroxide groups in the polymer. Further, they cannot establish that the nitroxide is at the chain terminus. Furthermore, due to the insensitivity of these analytical techniques, T E M P O - e n d capped polystyrene samples to be analyzed have always had molecular weights 90%. Hawker is the only researcher to date to quantify the initiated end of a polystyrene chain using N M R P . He synthesized a N M R P initiator having a pyrene chromophore attached to the initiating fragment (II). Both N M R and U V analyses were used to quantify the amount of initiator fragment attached to the chain-end and Hawker reported >95% incorporation. ^' ^ However, again the molecular weight of the chromophore tagged polystyrene was again below 10,000. T o date, no one has reported the evolution of chain-end purity with styrene conversion and increasing molecular weight. 20

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Recently Priddy developed a G P C - U V technique to precisely quantitate the level of chromophore attached to p o l y m e r s . The technique also determines the location of a chromophore; i.e., pendant vs chain-end. In this paper we report the results of G P C - U V analyses for polystyrenes made using N M R P . W e have compared the end-group purity of polystyrene made by the in situ and presynthesis of initiator approaches, as well as demonstrating the impact of excess nitroxide on nitroxide end-group yield. 27

Results and Discussion The G P C - U V technique for polymer end-group analysis requires that the initiator/mediator be tagged with a chromophore having a unique absorbance; i.e., absorb at a wavelength at which polystyrene is totally transparent (>280 nm). The chromophore chosen for study was the phenylazophenyl chromophore due to its intense absorption at >300 nm. Scheme 1 shows the synthetic route used to generate nitroxide initiators; one having a chromophore attached to the mediating fragment (IV) and another having the same chromophore attached to the nitroxide initiating fragment (VI). These two tagged initiators/mediators (IV and VI) are orange-red crystalline solids and their structures were confirmed using mass spectrometry and N M R spectroscopy. The U V spectra of IV and VI are almost identical (Figure 1). Furthermore, the relative molar absorbtivities of IV and V I were measured at 320 nm and also were found to be identical. This allows direct quantitative comparison of both the initiating and terminal ends of polystyrene initiated/mediated using these materials. T o compare the relative performance of the in situ (Scheme 2) vs initiator presynthesis approaches for making polystyrene chains having chain-end functionality, we also made T E M P O having the same phenylazophenyl chromophore i n the 4-position (VII) for use i n conjunction with benzoyl peroxide (BPO). Polystyrene initiated using B P O and mediated using VII w i l l place the same fragment on the terminal chain-end as i f the polymer was initiated/mediated using IV. Since the terminal fragment attached to the end of the PS chain would be the same regardless of whether the presynthesis or in situ initiator process was used, direct comparison of the relative end-group purity can be determined. A l s o , VII was added to polymerizations initiated/mediated using IV to see i f the presence of excess nitroxyl indeed enhanced the number of nitroxide functional chain-ends.


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Scheme 1 Synthesis of initiators for N M P bearing a chromophore on either the initiating or mediating fragment



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Wavelength (nm) F i g u r e 1 Comparison of the U V spectrum of I V and V I .

Polymerizations were carried out in glass ampoules at two temperatures (i.e., 120 and 140 °C) and at a concentration of 27 mmolar in nitroxide. Polymerizations initiated/mediated using the in situ approach utilized a mixture of 27 mmolar benzoyl peroxide ( B P O ) and 34 mmolar V I I . To see i f excess nitroxide resulted in higher nitroxide chain end-group purity, a polymerization was conducted by adding 7 mmolar V I I to a polymerization initiated/mediated using 27 mmolar I V . Polymers were isolated by multiple precipitation


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to remove all non-polymeric components. The purified polymers were analyzed using both U V and G P C - U V analyses. The results obtained from the two techniques generally were i n good agreement (Figure 2). Both techniques were calibrated using I V to determine the molar absorbtivity at 320 nm for the phenylazophenyl chromophore. The G P C - U V technique is expected to be more precise because it excludes small molecules and only measures the absorbance of polymer bound chromophores.


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F i g u r e 2 Correlation of measurement of percent chains having a phenylazophenyl chromophore using direct U V vs. G P C - U V analyses.

Another advantage of G P C - U V is that it also demonstrates the point of attachment of the chromophore to the chain (i.e., pendant vs chain-end). If the chromophore is pendant, its concentration is independent of chain length. However, i f the chromophore is attached to chain-ends, its concentration decreases as chain length increases. A n overlay (Figure 3) of the G P C - U V curve collected at 260 nm (pendant phenyl absorbance) with the signal collected 320 n m (chromophore absorbance) shows an offset with the 320 n m absorbing species eluting at a later time (lower M W ) . In other words, the chromophore is more concentrated i n shorter chains confirming that the chromophores are on the chain-ends rather than pendant to the chain. 27


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260 nm

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Retention Time (min) Figure 3 Comparison of the G P C - U V curves collected at 260 and 320 nm

Figure 4 Monomer conversion rates for four initiator/mediator systems (functional group on nitroxyl (IV), functional group on initiating radical (VI), in situ initiator with functional group on nitroxyl (BPO+VII), and functional group on nitroxyl + excess functionalized nitroxyl (IV+VII)) at 120 °C


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T i m e (h) F i g u r e 5 Styrene conversion vs. time at 140 °C

The polymerization temperatures (120 and 140 °C) were chosen because most of the published N M R P work is carried out in the 110 - 140 °C range. A t 120 °C the rate of polymerization is quite slow and it takes many hours to achieve high (>60%) monomer conversion (Figure 4). Polymerizations conducted at 140 °C led to high conversion within a few hours (Figure 5). The presence of excess nitroxide results i n an induction period at both temperatures. The presence of an induction period when using excess nitroxide has been previously observed. 11

The change in number average molecular weight with conversion is quite linear for all four initiator systems (Figure 6). Since industrial polymerization processes normally require the achievement of high conversion within a few hours for economic viability, we have performed most of our previous N M S P studies at temperatures >130 C . ! » ' 0

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The data consistently show (at both 120 and 140 °C) that the number of polymer chains having chromophores attached to a chain-end decreases as number average molecular weight ( M ) (Figures 7 and 8). This trend is not as dramatic i f the chromophore is attached to the initiating radical. Since M and styrene conversion are directly proportional (Figures 3 and 4), it follows that chain-end purity also decreases rapidly with monomer conversion. This was predicted from our earlier work in which we studied the thermal decomposition of a small molecule which models the dormant end during N M S P . 16 During the course of this investigation we never obtained a polystyrene sample that had both a M >10,000 and at the same time >90% of one of its chain-ends having a chromophore. n

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35Ο

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VI BPO IV+VII

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Styrene Conversion (%) Figure 6 Linear increase of M with styrene conversion at 120 °C for four polymerization systems. n

Figure 7 Percent of Chains runctionalized vs. M at 120 °C. n


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F i g u r e 8 Percent of Chains functionalized vs. M at 140 °C n

Conclusions It has been suggested that >90% of the chain-ends of polystyrene prepared using N M R P contain nitroxyl fragments. However, to maximize analytical sensitivity, low M chains have generally been prepared. Our data shows high chain-end purity is only achieved during N M S P when making short chains ( M

Nitroxide-Mediated Radical Polymerization - American Chemical Society

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