Controlled Radical Polymerization - American Chemical Society

The addition of radicals to alkenes is largely governed by polar and ... 2.0. 1.0. 0.0. CN. C 0 2 C H 3 V. • C H 2 C 0 2 R. / C H 3 r C(CH3 )3. /· ...
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Chapter 3

Factors Influencing the Addition of Radicals to Alkenes Anne Ghosez-Giese and Bernd Giese

Downloaded by HARVARD UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch003

Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland

The addition of radicals to alkenes is largely governed by polar and steric effects as well as, in some cases, stability effects. These factors play also an important role on the regio- and the stereoselectivity of the additions and in radical polymerization reactions. A n overview is presented on the basis of the data collected over the past 20 years.

The question of the factors influencing the radical addition to alkenes is a major one in polymer chemistry since it concerns the parameters governing the propagation step during radical polymerization. In this report, we have summarized the rules controlling the radical addition to alkenes using data that were obtained mostly in the seventies and eighties. Propagation:

Method The measurements of the radical additions to alkenes were performed using the „mercury method" (1) a chain reaction in which the radical precursor is an alkylmercury hydride 2 (2). Indeed, alkylmercury halides 1 react with N a B H giving alkylmercury hydride 2 which, after loss of metallic mercury, yields alkyl radical 3. In the presence of an alkene 4, radical 3 adds to the double bond and forms radical adduct 5 which abstracts a Η-atom from the alkylmetal hydride 2 to give the 1:1 addition product 6 and regenerates alkyl radical 3. The formation of polymers is observed only in special cases (3) because alkylmercury hydrides 1 trap alkyl radicals 3 with rate coefficients of at least 10 M ' V at room temperature (4). 4

7

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© 1998 American Chemical Society

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

Downloaded by HARVARD UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch003

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Scheme 1 This method is very useful in synthesis (5) as well as for kinetics studies (6) since the yields are high, the reaction conditions are mild (room temperature, no light) and the reaction times are short (in the order of minutes). Moreover, the separation of the products from metallic mercury is easy. The relative rates of radical addition to alkenes were determined by pseudofirst order competition kinetics (6). The alkyl radicals 3 were treated with a pair of alkenes ( A A ) and this led exclusively to the products P and P . Since dimerization, polymerization, disproportionation, or oxygen trapping reactions can be suppressed, the quantitative conversion of the adduct radicals into the products P p P allows the measurement of the relative addition rates. The competition constants k/k can be determined following a pseudo-first order kinetics i f one works with a large excess of alkenes so that their relative concentration [Aj]/[A ] vary only negligibly during the reaction. 15

2

1

2

2

2

2

[AJ/tAJ Scheme 2 The relative rates that we have measured are in accordance with the absolute rates obtained by H . Fischer in later work (7).

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

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Polar and Stability Effects (8) The stability of the adduct radical 5 plays only a modest role in the addition rate. Not stability effects but polar effects are dominant when nucleophilic or electrophilic radicals 3 are involved. This can be illustrated by the correlation of the relative reactivities of raonosubstituted alkenes 4 (towards cyclohexyl radical 3a) with the Hammett σ " parameters of the substituent Y at the double bond (Figure l)(8). This plot shows that the variation of Y has a strong effect on the rate of addition: the better the electron withdrawing group Y , the faster the addition. Thus, the rate of addition is increased by 8500 in going from the electron rich 1-hexene ( Y = C H ) to the electron poor acrolein ( Y = CHO)(9). However, substituents Y which stabilize radicals very strongly, e.g. a phenyl group, deviate from linearity (9). Thus, styrene ( Y = Ph) reacts faster as would be expected from the σ ~ values (Figure 1). However, it still reacts much more slowly than acrolein ( Y = C H O ) , for instance, although radicals are better stabilized by a phenyl group than by a carbonyl substituent.

Downloaded by HARVARD UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch003

4

9

Figure 1. Correlation of the relative reactivities o f cyclohexyl radical 3a with the Hammett σ " parameters of the substituents at the monosubstituted alkene 4 (20°C). The variation of geminal substituents at 1,1-d/substituted alkenes 7 shows a good correlation of the relative reactivities with the Hammett parameters and no deviation is observed for stabilizing substituents like phenyl or capto-dative groups (Figure 2). This can be explained by a slight twisting of the substituent (especially the phenyl group) out of conjugation because of the presence of the geminal group. Thus, the stabilizing effect of the phenyl group is reduced and only the polar effects remain effective in these cases as well.

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

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CN C0 CH V 2

3

2.0

C H . 6

n

C0 Et

= ζ

+

2

C0 Et

1.0

Ζ

2

Downloaded by HARVARD UNIV on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch003

3a • CH C0 R

0.0

2

/ C H r

2

3

C(CH ) 3

3

/· OCH, 0.5 σ -Figure 2. Correlation of the relative reactivities of cyclohexyl radical with the Hammett σ " parameters of the substituent Ζ at the 1,1-d/substituted alkene 7 (20°C). 0.0

Substituents at the radical center also influence to a great extent the absolute (7) and the relative rates of the addition to alkenes. Radicals behave either as nucleophiles(4) (e.g. alkyl radicals) or as electrophiles (e.g. malonyl (4) radical 3b, malononitrile radical (10), perfluoroalkyl radicals (11)). Table I illustrates this phenomenon: the differences in relative rates for both nucleophilic (3a) and electrophilic (3b) radicals with alkenes of various polarities are very large and depend on the polarity of the alkene. Thus, cyclohexyl radical 3a react 5900 times faster with the electron deficient α-phenyl-acrylonitrile ( X = C N ) than with an electron rich enol ether ( X = OEt). On the other hand, the relative rate of addition of malonyl radical 3b to a nucleophilic enamine ( X = morpholino) is 34 times larger than to the electrophilic α-benzoyl styrene ( X = COPh)(72). Table I: Relative Rates of Addition of Radicals 3a-d to α-substituted Styrenes 9 (20°C)(72). R*

+

=