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44247-AC7
Complementary Hydrogen Bonding in Highly Branched Macromolecules: Thermoreversible Supramolecular Architecture for Improved Melt Processibility and Performance

Timothy E. Long, Virginia Polytechnic Institute and State University

I. Synthesis and Non-covalent Interactions of Phosphonium Cations for Facile Melt Processing

Introduction

Ionomers are polymers with less than 15 mol% ionic groups that can aggregate and serve as reinforcement.  Ionic aggregates can display a disorder-order transition, allowing thermoreversible non-covalent associations that may positively impact mechanical properties.¹ Low molecular weight polymer with thermoreversible ionic aggregation would have properties similar to high molecular weight derivatives below the dissociation temperature of the aggregates.² Processing of ion-containing polymers must also be maintained for commercial viability.  Synthesis of phosphonium-based ionomers with only weak ionic interactions and sterically hindered cations may maintain facile processing of high impact polyesters and other high performance polymers while impacting mechanical performance.

Recent accomplishments

            A thermally stable phosphonium salt, butyl p-carboxyphenyl diphenylphosphonium bromide, was synthesized through an S_N2 mechanism.³ A large excess of 1-bromobutane facilitated a fast, complete reaction with an aryl phosphine (Figure 1).

Figure 1. Synthesis of butyl p-carboxyphenyl diphenylphosphonium bromide

            Phosphonium-based polyesters were prepared using typical melt polyesterification conditions.  ¹H NMR shifts corresponding to the phosphonium salt at the chain ends were evident in the product, and ³¹P NMR also confirmed that significant degradation did not occur.  the phosphonium end-capping agent controlled  the molecular weight of the final polymers. The glass transition temperatures (T_g) were the same for molecular weights spanning 6,000 g/mol to 15,000 g/mol with 7.7 to 1.3 mol% phosphonium end-capping agent incorporated. The multiplet association of the ionic polymer ends present similar “effective” molecular weights for all of the polymers, and the phosphonium salts dissociate towards the end or just above the T_g.

¹H NMR and ³¹P NMR confirmed quantitative incorporation of butyl p-carboxyphenyl diphenylphosphonium bromide into PET-co-PEI. ¹H NMR provided evidence of ionic aggregation in a PET-co-PEI polymer end-capped with 7.7 mol% butyl p-carboxyphenyl diphenylphosphonium bromide (Figure 2). Experimental procedures provided a dramatic shift in methylene protons next to the phosphonium center, which represents an increased mobility of the phosphonium groups as polar solvents help break ionic aggregation.

Figure 2. Evidence of ionic aggregation in a 7.7 mol% phosphonium end-capped PET-co-PEI

Evidence of ionic aggregation was also apparent in the ³¹P NMR spectra. The dissociation of ionic aggregations with CD₃OD provided a narrowed and split peak differing greatly from the one large, broad aggregated phosphonium peak ³¹P NMR signal. Melt rheology indicated weak ionic association at high temperatures at or exceeding 120 °C.  Only weak ionic association was evident at 120 °C and did not greatly increase the viscosity of these polyesters above respective controls, providing evidence that these salts were processible.

Conclusions

     PET-based polyesters were successfully end-capped with butyl p-carboxyphenyl diphenylphosphonium bromide. Molecular weight was controlled with addition of end-capping agent. ¹H NMR, ³¹P NMR, and DSC provided evidence of ionic aggregation. Weak ionic association could provide a new route to high performance and high strength polymers while maintaining facile melt processing.

II. Controlling Complementary, Thermoreversible Hydrogen Bonding on Surfaces

Introduction

             “Smart surfaces” respond to external stimuli or interface changes. Thermoreversibly hydrogen bonding polymers to a substrate with complementary hydrogen bonding groups like adenine and thymine creates a smart surface.⁴ Thymine-based surface functionalization agents allow for the investigation of hydrogen bonding adenine-containing polymers to a surface.  Patterning thymine groups for non-covalent association with adenine-functionalized polymer can be controlled with bulky nitrobenzyl groups that would prevent H-bonding. Meijer et al. blocked H-bonding functionality with a photocleavable o-nitrobenzyl protecting group.⁵  The nitrobenzyl group is cleavable with UV light, 365 nm, and a photomask can be used to pattern a surface with blocked and unblocked thymine groups (Figure 1). 

Figure 1. Complimentary H-bonding Patterning on a Surface

Recent Accomplishments

            Currently, a nitrobenzyl-protected thymine has been synthesized with alkoxysilane groups for sol-gel reaction to covalently bond the thymine to a silicon wafer (Figure 2).

Figure 2. Synthesis of nitrobenzyl-protected thymine via Karstedt's catalyzed hydrosilylation⁶

Hydrosilylation of a vinyl-containing thymine was completed with Karstedt's catalyst, platinum(0)-divinyltetramethyldisiloxane.⁶  Synthesis of novel adenine-containing block copolymers is still underway.

Conclusions

A silyl-functionalized thymine with bulky, photocleavable nitrobenzyl groups has been synthesized. Adenine copolymers are currently underway, and attachment of the alkoxysilane-containing thymine to a silicon surface via sol-gel reaction has been achieved.

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