Back to Table of Contents

40382-GB4
Noncovalent Modification of Lewis Basic Copolymers Using Arylboroxine Condensation Reactions

Peter Iovine, University of San Diego

2006-2007 Summary of Research Results.  In last year's progress report and in the 2006 literature, we detailed a new synthetic procedure wherein 4-vinylpyridine-containing random copolymers are noncovalently functionalized with arylboroxine-containing moieties.  The experimental process involves combining arylboronic acids in the presence of mild dehydrating agent in organic solvent.  A two-step condensation-ligation reaction takes place where the newly formed arylboroxine binds to the Lewis basic sites on the random copolymer.  The noncovalent functionalization reaction is efficient for a variety of polystyrene –co-4-vinylpyridine (PS/4-VP) copolymers and the solid-state materials show thermal properties indicative of side chain modification.  These results were first published in Macromolecules in the fall of 2006.¹  In addition, the PI gave an invited lecture at the fall 2006 ACS national meeting at the "Polyfunctional Organoboranes:  From Molecules to Materials".²

Scheme 1.  Noncovalent functionalization of Lewis basic copolymers using arylboroxine ring structures.

            In the current grant year we have expanded the scope of our investigation into noncovalent functionalization of Lewis basic copolymers.  In collaboration with Dr. Frieder Jaekle's group at Rutgers Newark, we investigated the functionalization of PS/4-VP copolymers with boronic acid end-functionalized polystyrene.  Unlike our previous experiments involving small molecule boron functionalizing agents, this approach involves a end-capped polymeric modifying agent.  The resulting materials can be considered polymeric brushes, however, unlike conventional polymeric brushes, these materials are held together via noncovalent Lewis acid-base interactions. In turn, the arylboroxine rings are formed through the dehydration-condensation of individual end-functionalized polystyrene strands.  Characterization data for a series of these novel materials is presented in Table 1. 

One especially interesting aspect of the characterization data is the ¹¹B solution NMR data. Interestingly, compositions having molar ratios of 4-VP:arylboroxine between 1.9:1 through 1:3 show two ¹¹B signals at room temperature.  This result indicates that there are two different types of boron species present in solution.  As indicated by the chemical shift, one of those species is sp² hybridized while the second is sp³ hybridized.  There are numerous small molecule arylboroxine¥pyridine adducts in the literature and their solution dynamics are well understood.  In the case of phenylboroxine¥pyridine adduct the dissociation/coordination is fast at room temperature with a reported activation barrier of just 39 kJ/mol.³  Based on this relatively low exchange barrier for the small molecule model system, one would expect to see one averaged ¹¹B NMR signal in the polymeric brushes.  This is not what is observed and for that reason we are currently repeating the experiments and looking at a wide range of compositions.  At this point, we cannot conclusively say whether the two signals observed in the ¹¹B NMR are a result of ligated and unligated arylboroxine rings having polystyrene substitutions.  

		Molar Ratio 4-VP:arylboroxine
	Parent PS/4-VP	4:1	1.9:1	0.9:1	1:3	BA end-functionalized PS
Tg (ûC)	101	103	101	107	106	104
11B NMR (ppm)	-	14.2	27.4; 14.2	30.7; 14.0	28.6; 14.0	29.4

Table 1.  Polymer characterization data for a series of functionalized 22 mol% 4-VP PS/4-VP random copolymers.  The functionalizing agent was a boronic acid end-functionalized polystyrene polymer.  

The data shown in Table 1 corresponds to random PS/4-VP copolymers functionalized with boronic acid end-functionalized polystyrene.  We are currently expanding the scope of these studies to include block copolymers of PS-b-4-VP reacted with the same end-functionalized polymer.  We are also very interested in increasing the proportion of 4-vinyl pyridine present in the base polymer that is ultimately functionalized by the arylboroxine appendages.  Increasing the amount of 4-VP may provide greater contrast, in terms of change in physical properties, when these sites are subsequently capped by arylboroxine rings.  Of particular interest will be to study the 4-VP homopolymer and its phase behavior following functionalization with polystyrene-based arylboroxines. 

References

 ADDIN EN.REFLIST (1)       "Condensation of Arylboroxine Structures on Lewis Basic Copolymers as a Noncovalent Means of Polymer Functionalization"

Iovine, P. M.; Fletcher, M. N.; Lin, S. Macromolecules 2006, 39, 6324-6326.

(2)  Peter M. Iovine, Matthew Arnold, Jason Loera, Emily K. Perttu, Matthew Fletcher and Shirley Lin.  Invited Lecture at the 232^nd ACS National Meeting San Francisco 2006.  Symposium Title:  Polyfunctional Organoboranes-From Molecules to Materials.  Abstract Title: "Synthesis and characterization of arylboroxine-containing copolymers and boroxine core molecular assemblies"

(3)       "B₃O₃Ph₃(7-Azaindole):  Structure, Luminescence, and Fluxionality"

Wu, Q. G.; Wu, G.; Brancaleon, L.; Wang, S. Organometallics 1999, 18, 2553-2556.

Back to top