Elsevier

Polymer

Volume 42, Issue 14, June 2001, Pages 5987-5991
Polymer

Studies on microgels. 5. Synthesis of microgels via living free radical polymerisation

https://doi.org/10.1016/S0032-3861(01)00023-4Get rights and content

Abstract

Statistical and star microgels of t-butylstyrene-1,4-divinylbenzene (TBS-DVB) have been synthesized via living free radical polymerization using tetramethylpiperidin-1-oxyl (TEMPO) as a trapping agent. The alkoxyamines 1 and 2 were used as initiators to prepare statistical and star microgels, respectively. The molecular weight of the resulting microgels was determined by size exclusion chromatography with an on-line multi-angle laser light scattering (MALLS) instrument. It has been found that living free radical polymerization provides a much better control over the formation of statistical microgels than traditional free radical polymerization and can be used successfully for the synthesis of star microgels.

Introduction

Microgels can be defined as intramolecularly crosslinked, high molecular weight soluble polymers and were first described by Staudinger and Husemann [1]. They found that the free radical polymerization of dilute solutions of divinyl or vinyl/divinyl monomers led initially to the formation of macromolecules with pendant vinyl groups. Depending on the reaction conditions, these vinyl groups can undergo random intramolecular/intermolecular crosslinking reactions forming statistical microgels (Fig. 1). Therefore, statistical microgels have a random distribution of crosslinks and lack structural control due to the irreversible termination steps. Various systems have been studied including styrene/1,4-divinyl benzene (DVB) [2], [3], [4], [5].

Alternatively, star microgels are characterized by a very high degree of structural control. Typically star microgels are composed of a series of linear arms held together by a central crosslinked core (Fig. 1). They are generally prepared via anionic polymerization, relying on the “living” nature of the polymer chain for the introduction of the crosslinker [6], [7].

Solomon et al. [8] first reported the process based on alkoxyamines/nitroxides known as living free radical polymerization. Since then, it has been employed by different authors [9], [10], [11] to synthesize well-defined linear and block copolymers of low polydispersity. This technique utilizes a stable free radical such as nitroxyl radicals as the trapping agent. These radicals combine with the propagating radicals and form a labile bond, which can undergo reversible homolytic dissociation at higher temperatures.

We have previously briefly described a new technique for the formation of microgels by living free radical polymerization [12]. We now report the full detail of this methodology to synthesize both statistical and star microgels using t-butyl styrene (TBS)/DVB monomers, the influence of reaction conditions and the characterization of these new materials.

Section snippets

Materials

All chemicals were obtained from Aldrich. TBS was distilled prior to reaction. Purification of DVB was carried out by the method of Wiley [13]. Tetramethylpiperidin-1-oxyl (TEMPO) radical was used without further purification. Benzene was refluxed over sodium for 16 h and then distilled prior to use.

Polymerization

The aminoxy terminated initiators 1 and 2 (Fig. 2) were synthesized as we previously described [12]. The chain growth capability of initiator 1 to produce linear, well-defined poly(TBS) was examined

Synthesis of linear polymer arms

The polymerization of TBS using a living free radical technique was carried out to examine the performance of initiator 1 under the conditions to be used for the microgel synthesis. Linear aminoxy terminated poly(TBS) polymers were prepared using alkoxyamine 1 as a function of reaction time. The samples were analyzed by SEC-DRI for molecular weight properties and the yield determined (Table 1). It was observed that both the molecular weight and the yield increased as a function of reaction

Conclusion

The nitroxide mediated free radical polymerization has successfully been applied to the synthesis of statistical and star microgels. This new methodology overcomes the problem of gelation associated with the polymerization of divinyl monomers by traditional free radical processes for the synthesis of statistical microgels. It provides better control of molecular weight properties of the polymers and is applicable to solutions with much higher divinyl monomer concentration. The formation of star

Acknowledgements

S. Abrol would like to gratefully acknowledge The University of Melbourne for financial support in the form of a Melbourne University Postgraduate Scholarship.

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