Living Radical Polymerization - ACS Publications

Chapter 29

Synthesis of Oligomers by Stable Free Radical Polymerization of Acrylates, Methacrylates, and Styrene with Alkoxyamine Initiators Helmut Keul, D i r k Achten, Birte Reining, and Hartwig H ö c k e r

1

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Lehrstuhl für Textilchemie und Makromolekulare Chemie der Rheinisch-Westfälischen Technischen Hochschule Aachen, Worringerweg 1, 52056 Aachen, Germany

A controlled polymerization of styrene (St) and methyl acrylate ( M A ) yielding oligomers of M ≤ 2000 was achieved using an alkoxyamine as the initiator. For the polymerization of methyl methacrylate ( M M A ) side reactions leading to unsaturated end groups prohibit the control of the polymerization. The extent of the side reactions is explained by a sterically hindered combination of T E M P O with the active P M M A chain end. Copolymerization of M A with St reveals an increase of the polymerization rate compared to those of the homopolymerizations and a good control of the M A / S t copolymer composition within a wide range. The copolymerization parameters were found to be in good agreement with those observed in the free radical polymerization. For the copolymerization of MMA with St a controlled polymerization was achieved at molar fractions of styrene higher than 50 %. n

Introduction Nitroxide mediated living radical polymerization is a process suitable for the preparation of well defined polymers ' , especially on the basis of styrene. Two initiation procedures have been employed: (i) a bimolecular initiating system 1 5

'Corresponding author.

408

© 2000 American Chemical Society

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

409 comprising a free radical initiator, styrene, and a stable nitroxide, e. g., T E M P O (2,2,6,6-tetramethylpiperidin-l-oxyl) and (ii) a unimolecular initiating system comprising an alkoxyamine prepared in advance ' . With unimolecular initiators long induction periods are usually avoided. The molecular weight of the polymers is predetermined by the monomer/alkoxyamine ratio and the conversion. Recently, the syntheses of alkoxyamines with structures resembling the dormant species of styrene, acrylate, methacrylate and acrylonitrile have been performed . The main topics addressed in the publications dealing with alkoxyamine initiators are: (i) the synthesis of alkoxyamines " , (ii) the synthesis of polymers with complex architecture using these alkoxyamines , (iii) the initiation efficiency of these alkoxyamines (R=NO-R) as a function of R and R' > > and (iv) the thermal stability resp. the thermal decomposition of the dormant and active species . In this contribution the results concerning the homopolymerization of methyl acrylate and methyl methacrylate and the copolymerization of these monomers with styrene with alkoxyamine initiators will be presented in comparison to the homopolymerization of styrene with the goal to prepare oligomers. Special emphasis will be given to side reactions. T E M P O mediated living radical polymerization in the ideal case exerts fast initiation followed by propagation with nearly no irreversible termination and 1 8

6


6

8

910

lc6

711

12

?

active species

Scheme 1: Reversible alkoxyamine C-O bond homolysis and propagation nitroxide mediated living radical polymerization.


in


410 transfer reactions. A t elevated temperatures alkoxyamines, the initiators for this polymerization procedure, suffer a homolytical cleavage of the most labile bond the C-O bond - producing two radicals. l'-Phenyl-l'-(2,2,6,6,4etrame^ (HST) upon thermal cleavage results in the nitroxide radical (TEMPO) a stable free radical which does not initiate the polymerization and a phenylethyl radical (Scheme 1). The phenylethyl radical is expected to recombine with the nitroxide radical or react with styrene present i n the system in the sense of an initiation reaction. The newly formed radical (active species) is quenched i n a nearly diflusion controlled process by the nitroxide radical with formation of the dormant species. Active chains and dormant chains are in equilibrium which is strongly shifted to the side of the dormant chains with the consequence of a low stationary radical concentration. The low concentration of radicals and a high rate of exchange between dormant and active species in comparison to the rate of monomer addition results in a living radical polymerization . Because of the persistent radical effect and the occurence of termination a continuous increase in nitroxide concentration is to be expected shifting the equilibrium towards the side of the dormant species, thus decreasing the overall rate of polymerization. The only way to reach high conversion is the removal of T E M P O from the system either by adding an additional radical source or by decomposition of the nitroxide, i.e., by a sulfonic acid . In the case of the polymerization of styrene radicals formed by thermal self initiation compensate the amount of T E M P O produced by termination reactions. The radicals obtained by thermal self initiation control the rate of polymerization since their concentration is higher than the concentration of radicals produced by the homolysis of the alkoxyamine by one order of magnitude . 16

17

18

19

20

Results and Discussion In order to study the influence of high initiator concentrations necessary for the synthesis of oligomers and the nature of the alkoxyamine on the reaction course we have polymerized styrene with alkoxyamine initiators based on styrene (HST), methyl acrylate (HMAT), and methyl methacrylate ( H M M A T ) .

63

These initiators were prepared according to a modified literature procedure .


411 HST. was obtained analytically pure by fractional destination of the crude product. A waxy solid with a melting point of 41-43 °C was obtained. Ή NMR / CDC1 : δ = 0.66, 1.03, 1.17, 1.37 (br. s, CH , 12H), 1.29-1.48 (m, CH , 6H), 1.47 (d, J = 6 Hz, 3H, CH ), 4.77 (q, J= 6 Hz, CH, 1H), 7.20-7.37 (5H, CHU,,). HMAT was obtained analytically pure by the same procedure as a waxy solid with a melting point of 28°C. *H NMR /CDC1 : δ= 1.04, 1.13, 1.20, 1.47 (br. s, C H , 12H), 1.33-1.47 (m, CH , 6H), 1.40 (d, J= 9 Hz, CH , 3H), 3.7 (s, CH , 3H), 4.39 (m, CH, 1H). In analogy HMMAT was obtained as a waxy solid with a melting point of 32-34 °C. Ή NMR /CDC1 : δ = 0.99, 1.15, (s, C H , 12H), 1.27-1.57 (m, C H , 6H), 1.47 (s, CH , 6H), 3.71 (s, CH , 3H) (Figure l). 3

3

3

2

3

3

3

3

3


2

3

3

3

3

2

21

3

3

7,8,13,14

ιI I t ι ιI 2.5 2.0 (ppm)

I I I I I I I 1 I I I I t ι ι I ι ι ι ι I ι ι ι ι

4.5

4.0

3.5

3.0

I I If

1.5

ιι 1.0

I r r ι

ιι 0.5

ι ι ι

ιI 0.0

ι ι ι

I I I

}

Figure 1. HNMR spectrum of HMMAT in CDCl . 3

The polymerization of styrene was performed i n Schlenk tubes at 130 °C with the various indicated alkoxyamine initiators. The conversion was determined gravimetrically after evaporation of residual monomer i n high vacuum. These samples were used for the molecular weight determination by means of G P C using polystyrene standards. The conversion index vs. time plots obtained for a monomer to initiator ratio of 40 at 130 °C are shown i n Figure 2a. Compared with H S T as initiator, the polymerization with H M A T as initiator shows an induction period probably caused by slow initiation. This results in a higher polydispersity index obtained with H M A T as initiator than with H S T (Figure 2b).


412

1,21-

Ο

Ο

Δ

Δ Downloaded by UCSF LIB CKM RSCS MGMT on August 15, 2014 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0768.ch029

Δ Ο

Ο Δ

Ο Ο Ο

•a o,4

Ο Ο

0,2 H 0

a)

OO Ο Ο Ο

20

15

10

0

time in h

1,5

1,4 Ο Ο Ο

Ο Ο

1,2

Ο

fx

Ο

1,1

Ο

Ο Δ

Ο

Δ

Δ

Δ

Δ ê b)

Δ

—ι

0

10

20

15

time in h

Figure 2. Polymerization of styrene with the alkoxyamine initiators HST (O), HMAT (O), and HMMAT (A). Polymerization conditions: [St] /[I]ο = 40, Τ = 130°C. (a) conversion index vs. time; (b) polydispersity index (My/My) vs. time. 0



413 The alkoxyamine initiator H M M A T shows a high rate for the homolytic cleavage leading to a high initial radical concentration (the reason will be discussed later). This has two consequences: (i) a high initial monomer conversion and (ii) afterwards a reduced polymerization rate due to excess T E M P O produced i n side reactions. The polymerization is in effect delayed until additional radicals formed by thermal self initiation of styrene have consumed the excess T E M P O . The fast initiation reaction leads to polymers with polydispersity indices comparable to those produced with H S T as initiator. The delay of the polymerization by excess T E M P O does not influence the polydispersity indices since all chains are equally affected. The plot of M vs. conversion shows for all the initiators applied a linear dependence with values close to the theoretical ones (Figure 3). n

3000

2500

2000

/ot> / Δ

ο

Δ

/Oç Ο/

1500 '

1000 1

' Ο Δ

500 4

0+^

-γ-

10

20

30

40

50

60

70

conversion in %

Figure 3. Plot of number average molecular weight Μ» vs. conversion for the polymerization of styrene with the alkoxyamine initiators HST (O), HMAT (O), and HMMAT (Δ).Polymerization conditions: [StJ /fIJ = 40, Τ = 130 °C. 0

0

From these results we concluded that for the further investigations H S T is the best suited alkoxyamine initiator.



414 Polymerization of Methyl Acrylate and Methyl Methacrylate with HST as the Initiator The polymerization of methyl acrylate was performed i n glass ampules i n bulk at 130 °C with H S T as the alkoxyamine initiator. The initial monomer/initiator ratio applied was 40. For comparison reasons, the results obtained with styrene under the same conditions are presented. After 20 h for styrene as the monomer a conversion of 65 % and for methyl acrylate a conversion of 45 % was reached. For both monomers a linear increase of the molecular weight with conversion and a narrow molecular weight distribution is observed (Figure 4). The polydispersity index of the poly(methyl acrylate), however, is higher than that of poly(styrene) (Figure 4b). This higher value of the polydispersity index can be explained by the addition of more than one monomer unit during the active period of a polymer chain . In contrast to styrene M A and M M A do not significantly enhance the rate of polymerization by radicals produced by thermal self initiation. We believe that i n the case of M A an acceptable rate of polymerization is only due to the high rate constant of monomer addition which is about one order of magnitude higher than that of St and M M A . The polymerization of methyl methacrylate reveals completely different results. A monomer conversion of 20 % is reached after less than 1 h. Then conversion and 16

2 2

3000 -i 2500 H

2000 A

si

1500 4 1000 H 500 4 4 a) 0 0

10

20

30

40

50

60

70

conversion in % Figure 4. Polymerization of styrene (O), methyl acrylate (O), and methyl methacryalte (A) with HST as initiator. Polymerization conditions: [M] /[I]o = 40, Τ = 130°C. (a) number average molecular weight vs. time. 0


415 2,4-1

Δ

Δ Δ

2,2 2 1,8 -

^


5

» . -

1,4

0 0 θ θ

0

ο°ο

1

Ο

Ο

ο

ο 1,2 -

Ο

ο

ο

4 b)

ο

—ι—

ο

10

15

25

20

time in h Figure 4. (b) polydispersity index vs. time. molecular weight do not change any more significantly with time (Figure 4a). The polydispersity index for all samples is higher than 2 (Figure 4b). The H N M R analysis of the P M M A oligomers shows a high concentration of end groups (Figure 5): (i) the resonances for the unsaturated P M M A endgroups [H C=C(COOMe)-CH ] are centered at δ = 6.2 (1H), 5.5 (1H), 3.7 (3H) and 2.5 (2H) ppm; (ii) those of the HS endgroup[CH -CH(C6H )-] are centered at δ = 1.2 (5H), 3.0(1H) and 1.25 (3H). The assigned δ -values are in accordance with calculated values according to incremental tables. l

2

3

I I f I I M

ppm

ι ι ι

7

ι

2

5

I f I 1 ι ι ι ι ι

6

ι

ι ι ι ι ι ι

ι

ι ι

5

ι

I 1 I I I I I I I

4

j

I I I I I I I I I

3

j

I I I I I I I I I

2

)

ι ι ι ι ι ι ι ι ι

ι

I > ι ι ι ι ι ι ι

1

Figure 5. *H NMR spectrum of PMMA in CDCh; initiator: HST.


0

ι

I I I I

416 Side reactions observed in nitroxide mediated polymerization of methyl methacrylate, methyl acrylate, and styrene. What is our explanation of the lack of control observed in the polymerization of M M A ? We assume in agreement with the literature that the high concentration of unsaturated end groups as indicated in the H N M R spectrum can be explained by a formal elimination of T E M P O H - which may occur i n a concerted reaction or by abstraction of a hydrogen atom from α-position to the radical center by the stable T E M P O radical. Furthermore, the hydroxylamine may react with the radical species transferring a hydrogen atom to the radical center generating T E M P O and a saturated chain end. T E M P O i n excess acts as an inhibitor by shifting the eqmlibrium toward the dormand chains. A s a result, the rate of polymerization decreases to zero. The formation of excess T E M P O is observed by the typical red colour of this stable radical. To a substantially lower extent unsaturated endgroups are observed i n polystyrene and poly(methyl acrylate) . The strong tendency towards side reactions leading to unsaturated endgroups and T E M P O in excess found in the polymerization of M M A cannot only be ascribed to the statistical factor - five hydrogen atoms available to be abstracted from an active P M M A chain end i n comparison to two hydrogen atoms for an active PSt or P M A chain end. We believe that the combination reaction of the sterically demanding T E M P O with the active P M M A chain end - a tertiary radical - is sterically hindered in analogy to the recombination of two P M M A chain ends. Thus the addition of monomer is sterically strongly favored over the combination of an active chain with T E M P O . In analogy to the polymerization of M A more than one monomer unit may be added per activation/deactivation cycle . We believe that both elimination of T E M P O H and sterically hindered endcapping of P M M A chains by T E M P O are responsible for the lack of control in the polymerization of M M A . In order to obtain information on the stability and reactivity of chain ends i n the polymerization of styrene, methyl acrylate, and methyl methacrylate we studied the conversion of the alkoxyamine initiators HST, H M A T , and H M M A T as models for the active chain ends i n ethyl benzene (all polymer chains contain activated C - H bonds either by phenyl groups or by oc-carbonyl groups). The conversion of the alkoxyamines, the formation of T E M P O H and T E M P O , the formation of H S T and the formation of St, M A , and M M A were determined quantitatively by means of gas chromatography (GC) and / or H N M R spectroscopy. T E M P O and T E M P O H , however, did not give well resolved peaks by G C and therefore the sum of both was determined [TEMPO(H)]. The monomers St, M A , and M M A could not be determined quantitatively by G C since their retention time is close to that of ethyl benzene; however, the concentration of the monomers could be estimated from H N M R analysis. Additional peaks i n G C with high retention time and with low intensity were assigned to dimers of the primary radicals CHs(R})(R?)C or to oligomers of (R )(R )C=CH (Scheme 2). 1


l

21

16

!

]

9

1

2

2



417

1

2

Scheme 2: Decomposition of the alkoxyamine initiators HST (R = H, R = C^i ) HMAT (R = H, R = COOCH ), and HMMAT (R = CH , R = COOCH ) in ethylbenzene. 5

1

2

1

3

f

2

3

s

Compared with H M A T and H M M A T for HST the lowest decrease i n concentration is observed (Figure 6a). After 18 h only 40 % are consumed and consequently 40 % of TEMPO(H) is formed. A good agreement between G C - and N M R data is observed. In the interpretation of these values the formation of HST by reaction of the phenylethyl radical - produced by hydrogen atom transfer from ethylbenzene - with T E M P O has to be taken into consideration. The concentration of styrene increases to a value of about 60 % after 15 h and decreases then slightly. For H M A T after 18 h 83 % are consumed (Figure 6b). In the same time the concentration of TEMPO(H) and H S T increases to final values of 66 % and 13 %, respectively. The concentration of M A reaches a maximum value of 30 % after 10 h and decreases slightly afterwards. For H M M A T a fast conversion is observed due to the fact that recombination of the radicals formed by homolysis is a sterically hindered process (Figure 6c). After 6 h 90 % of the alkoxyamine is consumed. The formation of TEMPO(H) and the consumption of H M M A T occurred with the same rate. The concentration of H S T increases slowly to a final value of 25 % and the concentration of M M A decreases slightly after a maximum value of nearly 40 % was reached after 4 h. The decrease of the monomer concentration after a certain reaction time can be explained by oligomerization.


418 1 9 0,9

a)

0,8 • • [HST] (GC)

0,7 •

Ο

ο — 0,5 • g

[HST] NMR

# [S] NMR * [TEMPO(H)] (GC)

0,4 0,3


0,2 0,1 0 **10

20

15

time in h

b) • [HMAT] (GC)

Ο

Ο [HMAT] (NMR) Φ

[MA] (NMR)

Φ [TEMPO(H)] (GC) • [HST] (GC)

10

20

15

time in h 1 * 0,9 -

c)

0,8 0,7 -

â [HMMAT] (GC)

0,6 -

Δ [HMMAT] (NMR)

0,5

A [MMA] (NMR) A [TEMPO(H)] (GC)

0,4 •

0,3

4

• [HST] (GC)

0,2 0,1 0 0

10

15

20

time in h Figure 6. Conversion of alkoxyamine-initiators HST ( Φ), HMAT ( Φ), and HMMAT (A) in ethyl benzene. Reaction conditions: [EtB] /[I]o = 40, Τ = 130°C. 0


419 Three reactions were taken into consideration for the explanation of the alkoxyamine consumption and the formation of HST (Scheme 2): (i) elimination of T E M P O H and formation of styrene (R = H , R = CMs), methyl acrylate (R = H , R = C O O C H ) , or methyl methacrylate (R = C H , R = COOCH3), which depends on the equilibrium constant between active and dormant species and the probability of Η-abstraction, (ii) Η-transfer from T E M P O H or ethyl benzene to the active species followed by combination of the resulting radicals with formation of HST, and (iii) Η-transfer from ethyl benzene to T E M P O and consecutive combination of the resulting phenylethyl radical with T E M P O . Recombination or disproportionation of the carbon centered radicals produces an excess of T E M P O radicals. The formation of T E M P O H was confirmed by the reaction of this hydroxylamine with diphenylchlorophosphate and subsequent P N M R analysis . The chemical shift of the resulting ester was found to be δ- -6.5 ppm and identical with an authentic sample. In order to verify the influence of T E M P O H on the polymerization course we performed two parallel experiments in which styrene was polymerized thermally i n the presence of 0.27 mol/L T E M P O H and in the absence of any additive (Table I.). In the presence of T E M P O H the monomer conversion is 31 % and the number average molecular weight as determined by means of G P C is 2450. The radicals formed by self initiation of styrene at first act as an acceptor for the hydrogen atoms generating T E M P O . Later such radicals are trapped by T E M P O generating alkoxyamines which further initiate the polymerization. *H N M R analysis of the obtained oligomers confirms the existence of alkoxyamine endgroups . 1

1

2

2

1

2


3

3

3 1

21

21

Table L Thermal polymerization of styrene in bulk at 130 °C for 20 h 1-Hydroxy-2,2,6,6-tetra-

Conversion

methylpiperidine in mol/L

in%

0.27

31

2450

2.02

0.0

86

90600

2.08

M

n

M y . / / M n

Copolymerization of methyl acrylate and methyl methacrylate with styrene As mentioned before, we believe that the reason for the lack of control i n the polymerization of methyl methacrylate is due to the sterically hindered combination reaction of T E M P O and active chains that leads to an excess of T E M P O . Based on these considerations we have studied the effect of styrene as a comonomer i n the polymerization of methyl methacrylate and methyl acrylate. The copolymerization of M M A with St serves two purposes: (i) a pathway to an easier combination of T E M P O with the active chain is given, i f the active chain end is styrenic and (ii) the additional radicals produced by thermal self initiation of styrene consume excess


420


T E M P O eventually produced i n side reactions, thus keeping the polymerization running. The copolymerizations were performed i n bulk i n sealed glass ampules at 130 °C (some experiments at 110 °C) for 16 h. The molar fraction of styrene in the feed was varied between 0.1 and 0.9 and the molar ratio of monomers to the alkoxyamine intitator applied was 40. The conversion of the obtained samples was determined gravimetrically after evaporation of residual monomer i n high vacuum and the molecular weights were determined by means of G P C using polystyrene standards. Table Π. Copolymerization of styrene and methyl acrylate

M

XP

No.

w

fst

Fst

M »,th.

n.exp.

wt. %

Μ

1

40

0

0

1480

1300

1,52

2

57

0,1

0,17

2180

1930

1,35

3

67

0,2

0,28

2620

2340

1,32

4

71

0,3

0,39

2830

2670

1,29

5

75

0,4

0,47

3030

2800

1,29

6

77

0,5

0,53

3150

2980

1,30

7

85

0,6

0,62

3530

3010

1,24

8

84

0,7

0,71

3540

3040

1,21

9

82

0,8

0,80

3510

3040

1,20

10

79

0,9

0,89

3430

2960

1,18

11

58

1

1

2570

2290

1,27

η

For the copolymerization of M A with St (Table II) with increasing molar fraction of styrene i n the feed (f ) from 0.1 to 0.9 the conversion obtained after 16 h increases from 57 % to a maximum of 85 % at f = 0.6 and decreases again to 79 % at f = 0.9. However, all conversions are higher than the conversions obtained in the homopolymerization of each M A and St. The polymerizaion rate is dependent on the radical concentration which is controlled by the equilibrium constant of the alkoxyamine homolysis, the rate of formation of additional radicals by thermal self initiation and further by the rate constants for the monomer addition ( k , k n , k k i). As the rate of the formation of additional radicals is controlled by [St] and st

st

st

î2

2 2

32 3

2


421 should increase with increasing concentration of styrene, we believe that the observed polymerization rates for the copolymerization of St and M A are best explained by the values of the homo- and cross propagation rate constants. For the system M A / S t at 130 °C with H S T as initiator we determined the copolymerization parameters by the method of Fineman and Ross to be r = 0.9 and Γ Μ Α = 0,2. The copolymer composition was determined by means of Ή N M R spectroscopy for conversions < 10 %. Although the accuracy of the applied method is limited, these values are close to the values found in the literature for the free radical polymerization at 60 °C: r = 0.7 ± 0.1 and Γ Μ Α = 0.2 ± 0.1 . This was expected by several authors before . Recently, for another living radical system, A T R P , a good agreement of the copolymerization parameters of M M A and nBuA with those of the free radical copolymerization was found . The molecular weight shows the same dependence on the molar fraction of styrene as the conversion whereas the polydispersity index decreases from 1.35 to 1.18 with increasing molar fraction of styrene from 0.1 to 0.9. 24

st

25

st


40,53,26

27

Table ΙΠ. Copolymerization of styrene and methyl methacrylate

M

Xp No.

w

-

fst

M

F

st

„, . th

Μ „

wt. %

>KCp

. Mn

1

22

0

0

940

920

2,42

2

31

0,1

0,14

1330

1230

1,87

3

38

0,2

0,25

1640

1510

1,72

4

43

0,3

0,35

1860

1610

1,66

5

49

0,4

0,46

2130

1780

1,64

6

52

0,5

0,52

2260

1960

1,58

7

48

0,6

0,63

2100

1860

1,57

8

58

0,7

0,74

2540

2160

1,45

9

61

0,8

0,86

2690

2270

1,41

10

63

0,9

0,89

2780

2320

1,36

11

66

1

1

2920

2460

1,30



422 For the system M M A / S t (Table III) with increasing molar fraction of styrene in the feed an increase of the monomer conversion within 16 h from 31 % to 63 % is observed. In the same time the number average molecular weight of the copolymers increases from 1230 to 2320 and the polydispersity index decreases from 1.87 to 1.36. The conversion stays below the conversion obtained in the homopolymerization of styrene. Here again the rate of polymerization is determined by the homo- and cross propagation rate constants and by the concentration of styrene. Furthermore, with increasing styrene concentration side reactions that lead to T E M P O H and subsequently to T E M P O decrease. (This is observed i n the concentration of unsaturated end groups as determined by means of Ή N M R spectroscopy.) In order to examine i f these copolymerizations are controlled polymerizations we have performed a kinetical study with a molar fraction of styrene i n the feed of fst = 0.5. In Figure 7 we observe for the polymerizations performed at 130 °C a rapid increase of the conversion within 4 h for St/MA to 60 % and S t / M M A to 40 % and thereafter a slower increase. A t lower temperatures (110 °C) the rate of polymerization is slower as expected. The plot of the polydispersity index vs. time (Figure 8) reveals always a higher value of M / M for the system M M A / S t than for the system M A / S t and, in addition, for the system M M A / S t a slight increase of the M / M value with time. A t lower temperatures the M / M values are lower. This is an indication of the fact that the rate constant for side reactions which result in an increase of the polydispersity index through the formation of dead chains are reduced at lower temperatures. w

w

n

n

w

n

80

70

Ο Ο

60 4

50

m

a

4

Δ

Ο

40 4

S

30 4 Δ Δ Ο

20 4

10 4

12

8

—ι 16

time in h Figure 7. Plot of conversion vs. time for the copolymerization of equimolar amounts of MMA or MA with St with the alkoxyamin initiator HST. Polymerization conditions: (A) [MMA +StJ / fljo = 40, Τ = 110 °C; (A) [MMA +StJ /[IJo = 40,T= 130 °C; (O) [MA +StJ /[IJ = 40, Τ = 130 °C. 0

0

0

0


423 1,81,7Δ

1,6a

l>5- Δ Λ


1,3

Δ

Δ

Ο

ο

1,2

Α

Δ Α

Δ

Ο

ο

Ο

Ο

ι,Η ι

-1

0

16

12

8 time in h

Figure 8. Plot of polydispersity index (M-JMJ vs. time for the copolymerization of equimolar amounts of MMA or MA with St with the alkoxyamin initiator HST. Polymerization conditions: (A) [MMA +St] / [I]o = 40, Τ = 110 °C; (A) [MMA +St] /[I] = 40,T= 130 °C; (O) [MA +St] / [I]ο = 40, Τ = 130 °C. 0

0

0

0

3500 3000 0

2500 • 2000 • 1500 1000

'y

500 0

J

0

10

,

20

,

30

,

40

—ι

50

70

60

80

conversion in % Figure 9. Plot of number average molecular weight M vs. conversion for the copolymerization of equimolar amounts of MMA or MA with St with the alkoxyamin initiator HST. Polymerization conditions: (A) [MMA +St] /[I] = 40, Τ = 110 °C; (A) [MMA +StJ /fIJo = 40,T= 130 °C; (O) [MA +St] /fI] = 40, Τ = 130 °C. n

0

0

0

0

0


424 The plot of M v s . conversion for the copolymerization of M M A with St (Figure 9) performed at 130 °C reveals a linear dependence, however, the values are not i n good agreement with the calculated values indicating that more T E M P O is produced by side reactions than radicals are produced by thermal self initiation. Upon decreasing the temperature to 110 °C the plot of M vs. conversion results i n a linear dependence close to the calculated one. Obviously, the side reactions leading to dead chain ends and T E M P O in excess show a strong temperature dependence. For the M A / S t system a linear plot of M vs. conversion is observed even at 130 °C. n

n


n

Conclusions The homopolymerization of styrene and methyl acrylate and the copolymerization of methyl acrylate with styrene mediated by T E M P O can be performed in a controlled way up to conversions of 50 % to 90 % depending on the monomer feed. Homopolymerization of methyl methacrylate under the same conditions is an uncontrolled process due to side reactions. The reaction control in the copolymerization of methyl methacrylate with styrene depends on the monomer composition in the feed. A t molar fractions of M M A < 0.5 a controlled copolymerization is obtained. Parameters like polymerization temperature and conversion are crucial for the reaction control.

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