RAFT Copolymerization and Its Application to the Synthesis of Novel

Chapter 35

Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch035

RAFT Copolymerization and Its Application to the Synthesis of Novel DispersantsIntercalants-Exfoliants for Polymer-Clay Nanocomposites Some Interesting Results from RAFT (Co)polymerization 1 2

1

2

13

Graeme Moad ' , Guoxin Li ' , Rudolf Pfaendner , Almar Postma ' , Ezio Rizzardo ' , San Thang , and Hendrik Wermter ' 1 2

1 2

1

1 3

!

2

CRC for Polymers and CSIRO Molecular and Health Technologies, Bag 10 Clayton South, Victoria, Australia Ciba Specialty Chemicals Inc., Lampertheim, Germany 3

This paper will discuss aspects of (co)polymerization of acrylates with reversible addition-fragmentation chain transfer (RAFT). An explanation for bimodal molecular weight distributions observed when making high molecular weight acrylic polymers with narrow molecular weight distributions (M./M V 12

y

2

Synthesis of unsymmetrical trithiocarbonates (e.g. 3, 4) can be carried out in a one-pot process as shown in Scheme 2. Yields are essentially quantitative (>95%) for substitution of primary and secondary alkyl halides but can be low for tertiary halides (5-40%). Tertiary trithiocarbonates (e.g. 2) (7) can be prepared in high yield by radical-induced decomposition of the appropriate bis(thioacyl) disulfide (8,9).

Scheme 2 C4H9SH

C^S^S^Ph

C H S*^S 4

9

Et NH 3

C H -S 4

9

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

Ο

517


R A F T polymerization of acrylates

The origin of humps. Many papers have appeared on RAFT polymerizations of acrylates (5,10-12). Bimodal or multimodal molecular weight distributions are sometimes observed in RAFT polymerization of acrylate esters (10,13) and certain other monomers. In some cases, (e.g. styrene polymerization (10) N,Ndimethyl acrylamide polymerization (14)) this has been rationalized in terms of by-products by radical-radical termination involving the propagating species. The evolution of the molecular weight distribution as a function of conversion for a RAFT polymerization with trithiocarbonate 3 is shown in Figure 1 (15). A high molecular weight shoulder with peak molecular weight approximately twice than that of the main peak is clearly evident for conversions >60%. The size of the shoulder is such that it cannot be wholly explained as a by-product from combination of propagating radicals. Moreover, it is clear from the UV-visible spectrum that the shoulder contains a trithiocarbonate chromophore. The product from radical-radical termination by combination would not contain a trithiocarbonate group thus it should not appear in a GPC trace when UV detection at 305 nm is employed.

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Molecular Weight (g mol " ) Molecular Weight (g mol " )

Figure 1. Normalized GPC traces at various reaction times/conversions for RAFT polymerization of η-butyl acrylate with trithiocarbonate 3 (a) UV detection at 220 nm and (b) UV detection at 305 nm. The RAFT agent 3 has two chromophores, the trithiocarbonate of the ZC(=S)S group with absorption at -305 nm and a phthalimido group on the 'R' group with absorption at -220 nm and a much weaker absorption at -292 nm. While polymer of the main peak has R:Z in the expected ~ 1:1: ratio, the high molecular weight shoulder has these groups in - 2:1 ratio (Figure 2). In a previous paper, we proposed that this shoulder might arise by copolymerization t

>



518

200 250 300 350 400 X(nm)

Figure 2. Normalized UV spectra ofpoly(n-butyl acrylate) with trithiocarbonate 3 (4hr, 77% com) in tetrahydrofuran (extractedfrom GP data). The main peak (—) and the high molecular weight shoulder (—) compared to a conventional poly(n-butyl acrylate) (·). Trithiocarbonate absorption maximum at 305 nm, phthalimide absorption maximum at 220 n poly(n-butyl acrylate) maximum at 200 nm.

Molecular Weight

λ (nm)

Figure 3. (a) Normalized GPC traces with UV detection at 292 nm (— j detection (—) for poly(n-butyl acrylate) with trithiocarbonate 3 (4hr, 77 conv.) after thermolysis at 220 °C under nitrogen (inset is a 70x vertical expansion) and (b) normalized UV traces in tetrahydrofuran solvent (extrac from GPC data) for: the main peak (—) and the high molecular weight sho (—) and the oligomerfraction(—).


519 of the macromonomer 5 which is formed by backbiting-p-scission as shown in Scheme 3. The present data are entirely consistent with this hypothesis and with the shoulder being due to polymer with structure 6.

Scheme 3 (B = C0 Bu, BA = w-butyl acrylate)


2

RAFT end-group removal. Some of the possibilities for removal of the thiocarbonylthio end group from RAFT-synthesized polymers are shown in Scheme 4 (2). They include hydrolysis or aminolysis (7,7) and radical-induced


520


reduction (7,7(5) or group exchange (77,77). We have recently shown that thermolysis is a convenient way of cleanly removing a trithiocarbonate group from the end or within a polymer chain (7,75). In the case of polystyrene the mechanism is believed to involve an elimination reaction which occurs selectively as shown in Scheme 5 (18). The thermolysis of xanthate terminated polymers has also been reported (19). For the case of both S-polystyrene and 5poly(/-butyl acrylate) Oisobutyl xanthate, the mechanism involves selective elimination to provide 2-butene and a polymer with a thiol end group.

Scheme 4 1or 2° amines, e

Χ YΧ YΧ Y

n Χ YΧ Y Χ Y Π

CN

Scheme 5 HS ^S"~C4Hg N

u 3 ^

S

~

C

4

H

9 +

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S

C4H9SH

\ x

PhJ Ph Ph

+ S=C=S

n

In contrast, with poly(w-butyl acrylate) (PBA) end group loss is thought to involve initial C-S bond homolysis followed by backbiting-p-scission as shown in Scheme 6 (75). Examination of the product post-thermolysis by GPC with UV detection shows that the trithiocarbonate group is gone and that the high molecular weight shoulder contains twice the number of phthalimido groups as the main peak (main peak has structure 7 while the shoulder is derived from 6). The oligomer fraction (derived from 8) contains few phthalimido residues so appears of reduced intensity in the trace with U V vs RI detection (Figure 3). This suggests thatfragmentationfollowing consecutive backbiting events occurs specifically to give a low molecular weight radical and a high molecular weight macromonomer. Such specificity is also observed when backbiting-p-scission occurs during polymerization as has been attributed to steric factors.



521

Scheme 6 (B=C0 Bu) 2

7

8


522


R A F T copolymerization

One of the major advantages of radical polymerization over most other forms of polymerization is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionality. RAFT polymerization retains these advantages. Although, there are reports on differences in reactivity ratios in living (2025) vs conventional radical copolymerization, most reports suggest that reactivity ratios are identical in living and conventional radical polymerization. In comparing observed reactivity ratios in living vs conventional polymerization systems it is important to take into account the effect of molecular weight on copolymer composition (24-27). In a conventional polymerization the molecular weight is typically high even at low conversion. In a living polymerization molecular weights are low at low conversion and increase with conversion. When molecular weights are very low, the initiator process influences the overall copolymer composition. The effects of specificity in the initiation and termination process on copolymer composition are known. Copolymers produced by living polymerization processes differ from those produced by conventional polymerization in one important aspect. Living polymerization processes produce gradient or tapered copolymers. Disparity in reactivity ratios causes the composition of the monomer feed to drift with conversion. As a consequence, in conventional radical copolymerization the copolymer macromolecules formed at the beginning of the experiment will be different from those formed at higher monomer conversion; the high conversion product is a polymer blend. In a living polymerization process compositional drift is captured within each chain. The copolymer will have a blocky character and the degree of blockiness will depend on the disparity of the reactivity ratios. If reactivity ratios are particularly disparate then it is possible to form a block copolymer from a batch polymerization. Thus the copolymerization of M A H with styrene by NMP (28) or RAFT (29,30) with excess styrene provides P(MAH-alt-styrene)-block-PS. There is a similar outcome in other copolymerizations which show a strong alternating tendency such as styrene with maleimides or acrylonitrile (AN) (2). The copolymerization of i-butyl acrylate (tBA) with vinyl acetate (VAc) by RAFT provides P(tBA-tfa/-VAc)-block-VAc

(2,31). A related issue is that differences in the activation and deactivation processes for the propagating species with different monomers at the chain ends may also affect copolymer composition, particularly when conversion is very low or very high or the rates of activation are very disparate. RAFT polymerization of M M A with benzyl dithiobenzoate provides very poor control (73) yet copolymerizations of styrene with M M A succeed while there is styrene in the monomer feed.


523

Exfoliants/Intercalants/Dispersants for Polymer-Clay Nanocomposites A series of copolymers all based on long chain acrylates or methacrylates but of differing composition, molecular weight and architecture (homopolymers, statistical copolymers, block copolymers and gradient copolymers) were prepared using RAFT agents 1, 3 or 9. All allowed the preparation of high conversion poly(octadecyl acrylate) (PODA) with a narrow molecular weight distribution (M /M generally 95% monomer conversion with RAFT agent 1 and AIBN initiator (10:1 mole ratio) at 60 °C in toluene unless indicated otherwise. Monomer conversions were >95% after 60 h Average no of monomer units per copolymer chain from NMR analysis (DPI = ODA orDA; DP2 = polar comonomer). Λ£ from NMR based on end group determination. M and MJM from GPC (tetrahydrofuran 1 mL min" , 22 °C) in polystyrene equivalents. Polymerization in tetrahydrofuran solvent at 60 °C. Polymerization in toluene:2-butanone (1:1) solvent at 80 °C. Polymerization at 80 °C. Polymer insoluble in tetrahydrofuran. b

c

η

d

1

B

B

e f

g h


526 Polypropylene (PP) nanocomposites were prepared by melt mixing unmodified sodium montmorillonite (Cloisite Na*), dispersant and PP to form a masterbatch containing up to 70% of the clay which was then processed with further PP to form a 5 wt% clay composite. The melt mixing steps were carried out with a twin screw extruder. The nanocomposite samples were then injection molded into standard dumbbells. The experimental protocol has been described elsewhere (32,44). The visual appearance of nanocomposites containing 5 wt-% Cloisite Na and 1% acrylic copolymer is similar to those of 'benchmark' systems prepared with an organoclay (Cloisite 20A) with 7-15 wt-% PP-grafi-MAH as compatibilizer. The state of clay dispersion for the nanocomposites was assessed by wide angle X-ray scattering and by scanning (SEM) and transmission electron microscopy (TEM). There was excellent dispersion and partial exfoliation as indicated by Figure 4 (SEM) and Figure 5 (TEM). The nanocomposites based on unmodified clay (5 wt% Cloisite Na*) and our dispersants (1 wt %) showed similar improvements in modulus, of 20-30%, and were significantly more ductile than the reference nanocomposite based on Cloisite 20A) with PP-grafi-MAH (Table 3). Our copolymer additives have the effect of simultaneously improving both the modulus and the elongation at break of the Cloisite Na composite. The improvement in properties for the Cloisite Na based nanocomposites is in line with the extent of exfoliation as observed by microscopy. Improvements in thermal and thermooxidative stability follow the same trend. Differences in crystallinity may in part account for the greater ductility and slightly lower modulus of the systems based on Cloisite Na vs reference organoclay based systems.


+

+

+

+

Table 3. Summary of Tensile Properties for Polypropylene Samples Material

Elongation at break

PP Cloisite Na" 1

" + copolymer " Cloisite 20A+PB

C

actual%

rel. P P %

(-1000)

Elastic modulus rel. PP

100

actual (MPa) 1500

20-40

2-4

-1700

1.2

300

30

2000

1.3

40

4

2200

1.4

a

1

5 wt-%CloisteNa. 5 wt-% Cloiste Na and 1 wt-% poly(ODA-gra/-MAH) (23). Reference nanocomposite - 5 wt-% Cloiste 20A and 7.5 wt-% PP-grafi-MAH (Polybond 3200). b

c



527 Thermogravimetric analysis (TGA) analysis (Figure 6) showed that the thermooxidative stability of nanocomposites prepared with 5 wt-% unmodified clay and 1 wt-% copolymer is substantially improved (-40 °C) with respect to PP or PP with the clay alone. The enhancement of thermooxidative stability of these materials is not as great as that of a reference organoclay-based nanocomposite prepared with 5 wt-% Cloiste 20A and 7.5 wt-% PP-graft-MAH (Table 3). It is of interest that addition of 7.5 wt-% PP-graft-MAH by itself (no clay) provides an improvement in thermooxidative stability. The thermal stability under nitrogen was also improved over PP for the nanocomposite samples with copolymer additives. The addition of Cloisite Na by itself has no substantial effect on the stability of PP. The reference nanocomposite based on Cloisite 20A had comparatively poor thermal stability with a more sudden weight loss profile (Figure 6).

Figure 4. Low (upper) and high (lower) magnification ESEM images of cryofracture surface for PP composites with 5 wt% Cloisite Na+ alone (right) and with 1 wt% poly(ODA-grad-MAH) (23) additive (left).



528

Figure 5. TEM image of cryofracture surface for PP composite with 5 wt% Cloisite Na+ and 1% poly(ODA-grad-MAH) (23) additive (reproduced with permissionfromreference (32). Copyright 2006 Wiley-VCH). 100

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300

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350

400

Figure 6. TGA mass loss curves for heating rate 10 °C min in air and under nitrogen for (a) polypropylene (PP, MoplenHP400N processed with 0.2 wt-% B225 stabilizer) (—); (b) PP with 5 wt-% Cloisite Na ("); (c) PP with 5 wt-% Cloisite Na and 1 wt-% poly(ODA-grad-MAH) (23) (——); (d) PP with 7.5 wt-% PP-graft-MAH ( ); (e) PP with 5wt-% Cloiste 20A and 7.5 wt% PP-graft-MAH (—-).

450

5 0 0

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529 The mechanism of stabilization by nanoparticulate fillers has been attributed in part (45) to the formation of a barrier which impedes both ingress of oxygen and evolution of volatiles. Consistent with this hypothesis we see an improvement in thermal stability according to the extent of exfoliation of the clay. It was also found that higher processing temperatures could be used both for producing the nanocomposite and in injection molding with no adverse affect on PP properties. While nanocomposites were typically produced with extrusion temperatures of 200 °C and a throughput of 10 kg h" , several experiments showed that processing temperatures of 260 °C and throughput of 20 kg h" could be used without detriment to the MFI, tensile properties, color or state of clay dispersion and without formation of odorous by-products. 1


1

Conclusions

High molecular weight acrylic polymers with Λ7/Λ7

RAFT Copolymerization and Its Application to the Synthesis of Novel

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