Living Radical Polymerization - ACS Publications

Chapter 22

End-Group Control in Catalytic Chain Transfer Polymerization 1

Johan P. A. Heuts , David A . Morrison, and Thomas P. Davis

1

Downloaded by UNIV OF TEXAS AT DALLAS on July 11, 2016 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch022

Centre for Advanced Macromolecular Design, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, New South Wales 2052, Australia

The application of catalytic chain transfer polymerization as a technique to produce end-functionalized low-molecular weight polymers is discussed in terms of simple kinetic models and illustrated by practical examples. Since end-functionalities other than vinyl groups need to be introduced via a functional monomer it is shown to be necessary and possible to control the endgroup by careful selection of a comonomer. Simple kinetic modeling reveals the required properties of this comonomer to be a high affinity of its derived radical to react with Co(II) rather than with other monomers, and that the other present monomers need to possess the opposite properties. The use of α-methyl styrene as a monomer to selectively introduce end-functionalities is discussed in detail. Furthermore it is shown that the use of α-hydroxy methyl functionalized monomers in catalytic chain transfer polymerization lead to the introduction of aldehyde endgroups, which are very versatile building blocks for post-polymerization modifications. The α-methyl styrene analogue, i.e., 2-phenyl allyl alcohol, has great promise for the selective introduction of these aldehyde endgroups.

In certain applications of (free-radical) polymers low molecular weight materials are required, which may or may not be subsequently modified. Good examples of this can be found in the development of high-solids organic coatings, self-reinforced hydrogel materials for contact lenses or even the production of surfactants. In the first example, the reduction of organic solvent content, required by environmental legislation causes an increasing viscosity of the resin, which can be compensated for by using a resin of a lower molecular weight; this resin is subsequently cross-linked. The second example shows the use of hydrophobic macromonomers, which are copolymerized with more hydrophilic comonomers to produce high performance hydrogel materials, and in the last example, a hydrophobic Corresponding authors.

© 2000 American Chemical Society

Matyjaszewski; Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

313


314 oligomer is extended by a hydrophilic block, or vice versa, to produce a surface active molecule. It is clear from the above that there is an industrial need for low molecular weight materials and the chosen examples further illustrate the fact that these materials often need to contain functional groups. Common practices for producing low molecular weight materials are the use of very high initiator concentrations or the use of added chain transfer agents such as thiols. These practices are based on the fact that both a higher initiator concentration and the addition of a chain transfer agent cause an increase in the rate of chain stopping relative to propagation. These effects can be quantitatively described by the Mayo equation (Eq 1), which expresses the reciprocal of the number average degree of polymerization, D P , as the ratio of the rates of all chain stopping events to the rate of propagation:(/,2) n

DP

*

n

' k [MJ p

[M]

In this expression, λ is the fraction of termination by disproportionation, ( k j the average termination rate coefficient, [R] the overall radical concentration, k the propagation rate coefficient, [M] the monomer concentration, C M the chain transfer constant to monomer and C the chain transfer constant to chain transfer agent S, where the latter two chain transfer constants are defined as the ratios of the chain transfer rate coefficients to the respective molecules and the propagation rate coefficient. It is clear upon examination of Eq 1 that an increase in initiator concentration will increase the total radical concentration and hence increase the first term on the RHS of Eq 1. Similarly, the addition of a chain transfer agent leads to the appearance of the last term on the RHS of Eq 1. Both effects thus increase DP,," , and hence decrease D P . Since the use of additional amounts of initiator causes a change in polymerization rate and the associated reaction heat (2), chain transfer agents are more generally used for molecular weight control. These chain transfer agents are often thiols which have chain transfer constants in the order of 10* to 10 for most common monomer systems (J). They act through a hydrogen transfer reaction with the growing radical chain, which abstracts the hydrogen from the - S H group, thus creating a dead polymer chain and a thiyl radical which can subsequently initiate a new chain (4). A major drawback of thiols is that the chain transfer constants are relatively low and that they are consumed during the reaction. This implies that relatively large amounts of thiols, which are odorous and toxic, are required in the reaction mixture, and probably end up in the polymer product and hence need to be removed. p

S

1

n

1

Catalytic Chain Transfer Polymerization. A n interesting alternative to the use of thiols has emerged over the past two decades in the shape of catalytic chain transfer agents, i.e., certain low-spin Co(II)


315


complexes (for example, see Scheme 1) which were found to catalyze the chain transfer to monomer reaction and hence provide a means for molecular weight control (5-7).

R

R'

Cobalt(!l)porphyrin

Cobaloxime

Cobaloxime Boron Ruoride

Scheme 1. Low-spin cobalt catalysts

These Co(II) complexes act by facilitating the hydrogen transfer reaction from the growing polymeric radical to the monomer, very likely by forming an intermediate Co(III)H complex (5-5). The overall reactions are clearly illustrated by Scheme 2, which shows that i f an α-methyl group is present in the radical, the hydrogen is abstracted from this α-methyl group, whereas if such group is not present, it w i l l be abstracted from the α-carbon in the polymer backbone. Furthermore, it is clear that chains terminated by this process are characterized by a vinyl endgroup and that those chains initiated by this process are characterized by a hydrogen "initiator" fragment. It is thus possible to produce macromonomers consisting solely of monomer units (9,10). α-Methyl group present H

,CH,

R-Ç—ς ~~ΟΗ -(!:Η^2 X

R

3

0

2

Scheme 4. Versatility of aldehyde chemistry in crosslinking reactions

The aldehyde end-functionality is introduced when a monomer containing an α-hydroxymethyl group is used in catalytic chain transfer polymerization. When a propagating radical containing a terminal unit of this nature undergoes the chain transfer reaction, the primary endgroup will be an enol, which can subsequently tautomerize to an aldehyde (see Scheme 5). This principle has been demonstrated by us previously using the monomer ethyl α-hydroxymethacrylate ( E H M A ) (52). In Figure 7, the relevant region in the Ή-ΝΜΙΙ spectrum of the formed polymer is enlarged, and the signals arising both from the enolic and the aldehydic proton are clearly visible. Since the polymerization behavior of E H M A is likely to be similar to that of the other methacrylates, its use to selectively introduce aldehyde endgroups under ordinary polymerization conditions will in many cases not be conceivable. It is likely that in copolymerizations E H M A will behave similarly to M M A , and may therefore not become the dominating radical species. Preliminary studies in our laboratory on the copolymerization of E H M A and styrene indeed failed to show the presence of a significant amount of aldehyde or enol endgroups. Our earlier discussions on α-methyl styrene (vide supra), however, immediately identify a more suitable candidate to selectively introduce aldehyde endgroups, i.e., the α-hydroxy methyl analog of α-methyl styrene. This monomer, i.e., 2-phenyl allyl alcohol, should also display a very low propagation rate coefficient and a very large chain transfer constant with the cobaloximes. Preliminary studies in our laboratory confirmed these expectations; attempts to polymerize 2-phenyl allyl alcohol in bulk at 50 C using 0.144 M A I B N did not lead to any detectable amount of polymer formation, even after 9 days of heating. Similar attempts using pulsed laser polymerization with total pulsing times of up to 13 hours did not lead to polymer formation. However, heating e


328 e

a 50% (v/v) solution of 2-phenyl allyl alcohol in toluene for 200 hours at 45 C in the presence of re-initiation OH

H

CH

2

Polymer—C

r


Η

\H

+

Polymer—C

Co(ll)

H

+

J—0 \>H

5

Co(lll)H

S

HQ

Ϊ )Polymer—Ç Η

1

H

Polymer—C

C ^=0

\

C2H5

Scheme 5. Catalytic chain transfer isomerism reaction

«

500-

|40φ

€ sod ω « 200o

•HO /

π

ω -100û -20011.5

11.0

10.5

10.0

9.5

$ppm Figure 7. Expansion of'H-NMR and aldehyde signals. (32)

spectrum of polyEHMA clearly indicating the enol


329 2


U - I O ^ M cobaloxime boron fluoride and H O " M A I B N clearly resulted in the formation of aldehyde groups (see Figure 8). This result indicates that the catalytic chain transfer process indeed takes place, but that the formed monomeric radicals do not propagate, but immediately undergo the chain transfer reaction. Investigations into the copolymerization behavior of this very promising monomer are currently underway.

Figure 8. ^-NMR spectrum of the product of 2-phenyl allyl alcohol after heating at 45 °C for 200 hours in the presence of AIBN and COBF. The aldehyde signal is clearly present.

Conclusions Catalytic chain transfer polymerization using low-spin Co(II) complexes is an efficient, relatively simple and clean procedure for the preparation of lowmolecular weight end-functionalized polymers. B y default, the procedure introduces a vinyl endgroup into the polymer chain, and any other desired end-functionalities need to be introduced by die use of monomers containing these functional groups. Using conventional free-radical (co)polymerization kinetics, the requirements for the selective introduction of a particular (functional) monomer as an endgroup were discussed; in a comonomer mixture, the monomer required as the endgroup needs to display a very low reactivity towards addition to another monomer, but a very high reactivity towards the Co(II) complex. The monomers making up the main chain of the polymer should be highly reactive towards addition to monomers (including the "endgroup" monomer) and be relatively unreactive towards the Co(ll) complex. A (semi-)quantitative prediction of whether a particular monomer mixture is suitable for


330


this purpose is simple and straightforward, and only reqires knowledge of some basic, and often readily available, kinetic parameters. It was also shown that α-hydroxy methyl substituted monomers yield the very versatile aldehyde building block when used in catalytic chain transfer polymerization. This was clearly demonstrated for ethyl a-hydroxymethacrylate, which shows a polymerization behavior largely similar to other methacrylates, and for 2-phenyl allyl alcohol, which shows similar polymerization characteristics to α-methyl styrene. This latter functional monomer shows great promise for the selective introduction of an aldehyde endgroup in a wide range of free-radical polymers.

Acknowledgments Discussions relating to this work with Michael Gallagher, Alexei Gridnev, Dave Haddleton, Dax Kukulj, Graeme Moad and Keith Moody, experimental input by Michael Zammit and Darren Forster, and financial support by ICI Pic and the Australian Research Council are all gratefully acknowledged.

References 1. 2. 3. 4. 5.

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