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Architectural Control in УLivingФ Free Radical Polymerizations Preparation of Star and Graft Polymers.

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stacking ofconsecutive layers is governed by a combination of probability rules
and well-defined nearest neighbor relationships. the crystal shows the observed
combination of diffuse and sharp scattering.
[I21 a) J. D. Dunitz i n X - r u j Anuli..ii.\ und the Siriii f i m of Orguiiic Moiecuk2s,
Cornell University Press. Ithaca. NY, 1979. pp 58-59: b) K DornbergerSchiff in Lr/ir,pung iiher OD-Sfrirklurm, Akademie, Berlin, 1966.
113) L. Pauling. T/IPNulure of h,
Clieni~d
Bond. 3rd ed., Cornell University Press,
Ithaca, NY, 1961. pp. 239%40.
(141 Alternatively. the total bond order in 3 is 9.75. less than the bond order in
benzene (10) but greater than that in hexasubstiluted benLenes (9.54) based on
the average bond lengths.
[IS] The mean values and their estimated standard deviations listed i n ref. [9]
are 1.554(21) 8, for cyclobutane (any substitution) and 1.510(14) A for
R2CH-C=C.
1161 The angle distortion effect seems to be nullified in benzenes with monocyclic
annelations, like I . perhaps due l o "bent bonds" in those systems, the angular
effect for bicyclics appears to be more effective. For an example of :I related
distortion in the norbornyne trimer, see: N . L. Frank. K . K. Baldridge, P.
GantLel, J. S. Siege1 7?~/ru/ie~lron
Lrfr. 1995. in press.
[17] Computational details: The molecular structure of 3 wits determined a t a variety of theoretical methods to determine self-consistency. Reported herc are the
split-valence 6-31G(D) [18] and triple-zeta valence TZV(D) [19] baaia sets.
employed at the restricted Hartree-Fock ( R H F ) self-consistent field (SCF)
level of theory. These basis set5 include a set of SIX d polarization functions o n
all hexvy atoms. These calculations were performed with the aid of the aiialycically ana1ytic;illy deterinined gradients and aearch algorithins contained i n
GAMESS [20]. Additional calculations at the MP2;6-31G(D) and Density
Functional Theory levels were performed to determine the effects ofdyiiainioal
correlation. Calculations using the former method. a post-RHF method that
incorporates correlation in terms of Moller-Plesset theory of order 2 (MP2)
[Zl].were performed using the GAUSSIAN 92 wile ofprogi-ams [ 2 2 ] .Calculations using Density Functional Theory (DFT) Methods. which inherently
incorporate effects of correlation in their development. were performed uith
the aid of the numerical methods within Dmol [23]. A double numeric;tl basis
set augmented by polarization functions, comparable in size to the 6-31G(D)
basis set of the traditional Harti-ee- Fock methods. was chosen for the D F T
calculations.
[18] a) P. C. Hariharan, J. A. Pople, Uicor. C/i;m. .4rru 1973. 38, 213: b) M. S.
Gordon. Cheiii. Phys. Lcfr. 1980. 76. 163.
[I91 a ) T H. Dunning. J. C/ic,tii.P l i n 1971, 55. 716. b) A . D. McLenn. G. S. Chandler. J. Choi~.P l i y . ~1980. 72, 5639; c) A. J. H W'ichters. J C/wiii. P/ii,.\.1970.
.52. 1033.
1201 M . W. Schmidt, K. K. Baldridge. J. A. Boatr. J. H. .lensen. S. Koseki. M . S.
Gordon, K. A. Nguyen. T. L. Windus. S. T, Elbert, QCPE Bid1 1990. 10. 52.
(211 J. A. Pople, J. S. Binkley. R. Seeger. /nr. J. Qiuiiiriuii C/ic,iii. Syinp. 1976. 10. 1
[22] Gaussian 92. Revision C : M. J. Frisch. G. W. Trucks. M. Head-Gordon,
P M . W. Gill. M. W. Wong. J. B. Foresiixiii. B. G . Johiiaon. H. B. Schlegel.
M . A. Robb, E. S. Replogle. R. Gomperts. J. L. Andres. K . Raghawichari. J. S.
Binkley. C. Gonzalez. R. L. Martin, D. 1. Fox. D. J. Defrees. J Baker. J. J. P.
Stewart. J. A. Pople. Gaussian Inc.. Pittsburgh. PA, 1992.
[23] a ) B. Delley. J. C/i<mi.P/iy.\. 1990, Y3. 508. DMol is available cornmerically from
BIOSYM Technologies, San Diego. CA. b) B. Del1ey.J C / i ~ i i iP. h r ~1991. Y4.
7245.
[24] a) R. C. Haddon, K Ragha\achari. J. Am. C/ie/ii SO<..1982. 104. 3516-3518:
b) J. Am. C / i w i . S o t . 1985, 107, 289-297. c) 1'.Xie. H. F. Schaefer 111. G.
Lidng, J. P. Bowen. J. Am. (%on.Sor. 1994. 1 t 6 . 1442- 1449.
[25] a ) M. Nishio. M. Hirota, 7i~rr.ulic~Iron1989. 45. 7201 7245; b) F. COLLI.J. S.
Siegcl, Piiw Appl. (%em. 1995. 67.683.
Architectural Control in "Living"
Free Radical Polymerizations: Preparation
of Star and Graft Polymers**
Craig J. Hawker*
The ability to accurately control macromolecular architecture
is becoming an increasingly important theme in polymer science
with the interest being driven by the desire to prepare materials
with new and/or improved properties.['] One way to achieve
these goals is to introduce branches into the polymer backbone.
For example, graft and star systems have been shown to be
useful as rheology control agents, compatabilizers for polymer
blends, and emulsifiers.r21Traditionally, graft and star polymers
have been prepared by anionic[31and cationic polymeri~ation,[~l
or by group transfer technique^.'^' However these techniques
suffer from rigorous synthetic conditions, an inability to form
many random copolymer systems, and incompatibility with a
wide range of monomer units. One goal of synthetic polymer
chemistry is to devise a free radical approach to the preparation
of graft and star systems which has the same degree of macromolecular control as the above techniques but does not suffer
from their synthetic drawbacks. Previous attempts at using free
radical methods have not been entirely successful due to a lack
of macromolecular control and combination reactions. which
result in network formation.[61In this report we describe a novel
"living" free radical methodology which overcomes these problems and allows the synthesis of star and graft copolymers with
controlled molecular weights and low polydispersities under
mild conditions.
This new polymerization method is based on the use of novel
initiators containing a covalent adduct of styrene and 2,2,6,6tetramethylpiperidinyloxy (TEMPO) .['I Previously we have
demonstrated that the molecular weight and chain ends of
polystyrene can be accurately controlled by the use of 1 as an
initiator.[*]These results, coupled with the pioneering work by
Georges et aLL9Iand Rizzardo et aI.["] suggest that the polymerization proceeds with little or no termination and may be considered living in nature. It is this lack of termination reactions,
coupled with a high degree of macromolecular control, which
may allow the synthesis of well-defined star and graft polymer
systems using TEMPO-based initiators.
To investigate this question we examined the synthesis of a
trifunctional initiator, 2, which contains three initiating styreneTEMPO groups. It was hoped that each of the three styreneTEMPO groups of 2 would independently initiate a growing
polymer chain and that these individual polymer chains would
grow at approximately the same rate to give well-defined star
macromolecules. It was found that the benzyl ester group of the
starting material 1 could be hydrolyzed with potassium hydroxide to give the alcohol 3 in excellent yields. Reaction of 3 with
1.3,5-benzenetricarbonyl chloride (4) in the presence of 4-dimethylaminopyridine proceeded smoothly to give the desired
trifunctional initiator 2 in 71 '/o yield after purification
(Scheme 1 ) . Under the conditions developed previously,"~"
bulk polymerization of 200 equivalents of deuterated styrene
with 2 at 130 "C for 72 h was found to give the polystyrene 5 in
84%) yield with no detectable amounts of crosslinked or insoluble inaterial (Scheme2). Analysis of 5 revealed a molecular
[*] Dr. C. J. Hawker
IBM Research Center, Alinaden Research Center
650 Harry Road. San Jose. CA 95120-6099 (USA)
Telef:ix' Int. code +(408)927-3310
[**I
Financial support of this work was provided by Nalional Institute of Standards
;tnd Technology through ATP contract No. 70 NANB-3H-365.
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arms of the star polystyrene, 5 was hydrolyzed with potassium
hydroxide (Scheme 2). Significantly. the hydrolyzed product 6
was found to have a molecular weight M , of 7000 (PD = 1.12)
which agrees closely with the theoretical value for one arm of the
star polymer ( M , = 7000). These results support the formation
of a well-defined three-arm star macromolecule from 2, the
molecular weight of which can be accurately controlled by the
initial feed ratio.
Further evidence for the proposed structures wiis obtained by
comparing the ' H NMR spectra of the trifunctional initiator 2
with that of the deuterated star polystyrene 5 m d its hydrolyzed
product 6. As c'nn be seen in Figure 1. the signals of the protons
2
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-6
5
I
1
Fig. 1. a ) ' H N M R spectrum (300 MHL. CDCl,) of thc trililnctlcinal initiator 2;
b) 'HN M R spectrum (300 MHL. CDCI,) of the three-arm dcuteratrd polystyrene
star 5 : c) ' H NMR spectrum (300 MHz. CDCI,) of the h y d i ( > l y s product 6 .
KOH
6
Schcmc 2
weight M,, of' 16500 amu and a polydispersity (PD) of 1.20.["]
From the feed ratio, and assuming a three-arm star polymer is
formed. the theoretical molecular weight M , of 5 should be
21 000 with each arm having a number average molecular weight
of 7000. This discrepancy in molecular weights is, however, fully
consistcnt with the proposed structure since the hydrodynamic
volume o f ;I star polymer is less than that of a comparable linear
polymer. To determine the molecular weights of the individual
Ha, H,, and H,, which stem from the styrene unit in 2. appear
as three A B quartets at 6 = 4.30-5.10. and the protons of the
symmetrically substituted core unit appear ;IS ii singlet at
6 = 8.20. On reaction with [DJstyrene the resonances for Ha,
H,. and H, undergo upfield shifts which correlate with the insertion of deuterated styrene units between the TEMPO and
styrenic units of 2, while the resonance for the core unit remains
as a singlet at 6 = 8.16. On hydrolysis the resonance for the core
unit at about 6 = 8.20 disappears, and the resonances for Ha,
H,, and H, undergo shifts which are consistent with the generation of a hydroxy end group. In fact the spectrum of 6 is identical to that obtained when the alcohol 3 is used to initiate the
polymerization of deuterated styrene.18] In all three spectra the
resonances for the TEMPO group are observed at 6 = 0.0- 1.5
which demonstrates the stability of the TEMPO linkage to hydrolysis conditions.
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Repetition of the above polymerization/hydrolysis reactions
with up to I000 equivalents of styrene was found to give similar
results and illustrates the usefulness of this technique for the
preparation of high molecular weight materials. For example.
reaction of 2 with 700 equivalents of styrene was found to give
a three-arm star polystyrene in 8 8 % yield which had a M , of
53 000 and a P D of 1.I 9. While this value is again less than the
theoretical number average molecular weight of 73 000, hydrolysis to give the individual arms resulted in much closer agreement ( M , = 22000; P D =1.09) with the theoretical number
average molecular weight of 24000. This high degree of macromolecular control coupled with a lack of crosslinked material
supports a “living” nature for this novel polymerization process
with no termination due to combination or disproportionation.
The versatility and usefulness of this ”living” free radical chemistry is also demonstrated by the ability to use monomers such
as p-chloromethylstyrene as well as acrylates and methacrylates.
For example. previously unreported star polymers containing
narrow dispersity arms which are random copolymers of styrene
and butyl acrylate can be prepared from 2.
This lack of termination steps also opens up the possibility of
using this polymerization process to prepare graft systems. In
an effort to also investigate the compatibility of the styreneTEMPO group with standard free radical procedures the synthetic strategy for preparation of the graft systems involved
initial synthesis of a monomer unit incorporating the styreneTEMPO group, copolymerization with styrene under normal
free radical conditions. followed by a “living” free radical polymerization with a second feed of styrene. The synthesis of the
polymeric initiator is outlined in Scheme 3. Reaction of 2 with
p-chloromethylstyrene in the presence of sodium hydride was
found to give the desired monomeric derivative 7 in 71 YOyield
after purification. Copolymerization of 7 with styrene was conducted under normal conditions using azobisisobutyronitrile
(AIBN) as an initiator in refluxing tetrahydrofuran. Significantly the polymerization proceeded smoothly to give the desired
copolymer 8 ( M n = 12000; P D = 1.80) in 729‘0 yield after purification. Analysis of the copolymer by ‘H and I3C N M R spectroscopy showed the expected resonances for the styreneTEMPO group, and comparison with the aromatic styrenic resonances for the backbone polymer permitted the ratio of
monomer units to be determined. The experimentally deter-
3
8
Scheme 3
@
mined value of 1 : 19 agrees with the feed ratio and shows that
the styrene-TEMPO group has little effect on reactivity ratios.
Bulk polymerization of a mixture of the copolymer 8 and 200
equivalents of styrene at 130 ”C for 72 h resulted in polymerization of the added styrene to give the proposed graft system 9.
Comparison of the gel permeation chromatographic (GPC)
traces for the starting polymer 8 and the graft system 9 clearly
shows an increase in molecular weight for 9 and the absence
of unreacted starting polymer
(Fig. 2). In this case the nature of the grafted polymer
chain 10 could not be probed
by hydrolysis due to the stability of the ether linkage. and
cleavage of this group was
therefore accomplished by
treatment of 9 with an excess of trimethylsilyl iodide
(Scheme4). This results in a
dramatic change in the GPC
trace for the isolated product,
no peak for the starting graft
system is observed, and the
polydispersity of the sample is
lowered from 2.01 to 1.26. Interestingly. the number average molecular weight for 10
was determined to be 23 000,
which agrees closely with the
theoretical M , for the grafted
chains of 21 000. These results
demonstrate that a graft sysVImL
tem is produced and that the
Fig. 2. a ) Gcl permeation chroTEMPO groups attached to
matogram of the starting copolymer
the polystyrene backbone of 8
8 : b) gel permeation chromatograph
are capable of initiating the
of the graft polymer 9 : c) gel permepolymerization of styrene to
ation chromatogram of the cleaved
graft polymer 10.
give grafts of controlled
molecular weight and low
polydispersity. The small
shoulder at lower molecular weight in the G P C trace for
10 correlates with the molecular weight of the starting polymer 8. and is due to the cleaved backbone polymer, whose
signal underlies the main peak for the grafted chain. This
ability to conduct one free radical polymerization and then
by simply increasing the temperature and adding new
monomer, to conduct a second “living” free radical polymerization is intriguing and may open up new paths to unusual
macromolecular architectures. As for the star polymers unusual graft copolymers can be readily prepared by this
“living” free radical technique from monomers which cannot
be polymerized by better known anionic or cationic techniques.
In conclusion we have demonstrated that “living” free radical
polymerizations based on TEMPO derivatives allow for the
accurate control of macromolecular architecture. Star and graft
copolymers can be prepared from the appropriate multifunctional initiators with no observation of crosslinking or termination by combination even under melt conditions. The molecular
weights of the arms, or grafts, can be controlled by varying the
equivalents of monomer added while maintaining very low
polydispersities. We believe that this novel polymerization process offers the architectural control previously obtainable only
under synthetically more rigorous anionic or cationic conditions.
-
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layer extracted nith dichloroincthane ( 2 x 50 i n L ) . The cmibined orpanic layers
\\ere dried, ebaporated to dryness and purified b) llash chromatography eluting
\ b i t t i 1 . 1 hexane,dichloromethane increasing to dichloromethane This gave the
styrene dci-ivative 7 as ii pale yellou oil. Yield 71 '%. 'HN M R (C'DCI,). h = 0.63,
1.01 ( e : i c h b r a . 6 H . C H , ) . 1.15 l.S5(in. I 2 H . 3 x C H 2 a i i d 2 .:C'H,).i65(ABq.
./=~HI.~H.CHH).~~~(~B~.J=~
( \ .H
7 H~. C.' /~
f , )H. 4.. X~4 (Hd / / ) . ~
o f d . J = 2 and 6 H r . 1 H. CIIH). 5 20 (d o f d . J = 2 iind 7 HI. I H. =C'IIH).5.71
( d 0 f d . J = 2 a n d 6 HI. 1 H.= C H t I J . 6 . 6 6 ( d o f d . J = h a n d 7 111. I H = C H ) . 7 OX
( d . 2 H . A r l t ) . 7 . 2 5 ~7.52 (m. 7 H . A r l l ) . " C N M R (CDCI.): 0=17.17. 20.56.
33.87. 404X. 50.36. 72.74, 72.83. 85.41. 113.52. 12604. 12723. 127.51. 127x2.
127.XX. 136.62. I3X 14. iind 141 79. m i i s speclruni ( E I J !if :103
Copo1ymerir;ition of7 w,itli styiene A solution of thc atyrene- I'bMPO monomer 7
(450 nig. 1.15 mmolj. styrene (2.40 g. 23.0 nimol. 20 cquir). .ind AIBN (40 mg,
0.23 minol) in dry tetrahydrofuran (20 inLj \ + a s heated at reflux under argon
for 24 h. The rcaction inixture wiis evaporated to dryncss. rcdissolved i n
dichloromcthane (10 inL) and precipitated i n t o inethanol ( 5 0 0 m L j folloued by
h e x m e (500 m L j . The copolymcr 8 w a s isolated iis :I white powder Yield 72"'o:
,M,,
= 12000 and P D = 1.80: ' H N M R (CDCI,). ,i= 0.6.'. 0 9 0 1.70. 3 65. 3.95.
4.30. 4.82.6.40-7.25 ( h r m ) : '-'C N M R (CDC'I,)-n=17.25. i9.0 43 5. 125.3 (br).
127.5 ( h r j . 128.30. 144.7- 145 Y (;I number ofreson~iinces\\ere tiio biii:ill t o obserrej.
Pi-cparation of graft polqatyrme 9: 4 aolution of the polymerii initiat(ir 8 (200 nig.
0.085 minol. 1.0equiv) i n styrene (1.82 g. 17.5 iniiiol. 2 O O c q i i i ~ )\\'IS heated at
130 C \vith stirring under argon for 72 h During this timc tlic viscosity of thc
\elution wiis observed t o gradually increase a n d the clear rciiction mixturc ebentu;ill>wlidilied The reaction mixture H B S then dissolved i n dichl~iroinethonc( 2 5 mL)
and prccipit'ited i n t o liexane ( I L ) lotloNed by re-precipitation i n t o mcth,inol(I L j .
The gralc polystyrene 9 mas i ~ o l a t e das ii bvhite sdid after dr)ing. Yield 80
)M,, = 8 6 0 0 0 a n d P D = 2 . 0 1 . ' H N M R ( C D C I , j : 6 = 0 . ~ 0 ~
1 :I)lhi-mj.6.4Ob72 ;
"C N M R (CDCI,): 6 = 39.0-44.5. 125.0 ( h r ) . 127 5 ( h r j . I13 5 146.0.
Recei\cd. Jiinuary 13. 1995
Revised version: March 17. I995 [Z 7634 IE]
Gerinan version: A i f p i i . ( % c i i f . lY95. 107. 1623 1627
Keywords: graft polymers . polymers . radical polymerization
star polymers
Scheinc -I
~ . \ ~ ~ ~ ~ l ~ ;P
l r
~ o~ c~~~dt"7, t ~ i l
2 . To :I w l u t i o i i o f t h e alcohol 3 (1.0 g. 3.6 mmolj and 4 (290 mg, 1 1 mmol) i n dry
teti-aliy~lrolui-,iii(20 mL) u'as added dropwise a solution of 4-dimethylaininopyridine ( 5 0 nig. 0 4 ininol) kind pyridine (300 mg, 3.8 mmolj in tetruhydrofuraii
(2.0 nil.) The rctiction mixture was stirred at room temperature under argon for
16 h ; i d then cbaporated t o dryness. The residue was partitioned between
dich1or~iineth:inc(.'(I mL) and water ( 5 0 m L j . and the aqueous layer extracted u i t h
dic1ilor~imctli;inc( 2 x 50 m L ) . T h e coinbiiied organic layers were dried, evaporaled
t o dryness. .ind purified by flash chromatography eluting with 1 . 2 hexane:
dich1oroiiieth;inc increasing t o dichloromethane. This gave the trifiinctional initiator 2 iis :I p;ik ycllou oil. Yield 71 04: ' H N M R (CDCI,): J = 0.79. 1.03. 1.25. 1.62
(cach hi-,. 12H. C'H ,I. 1.34 1.58 (m. 6 H . CH,). 4.60 (ABq. J = 6 Hr. 1 H. C H H ) .
4.91 (AHq, .I = 0 H/. IH . C H t l ) . 5.14 (t. J = 3 Hz. 1 H. C H ) . 7.25 7.52 (m.5H.
.Art<),X 20 (\. 3 11. Ai-Hj. " C N M R (CDCI,): 4 = 17 17. 20.38. 34.12.40 47. 60.15.
67 30. X i XY. 127 74. 128.18. 128.85. 130.92. 131.55. 13453. 140.33. kind 164.54:
miis\ spectruin ( 1 . 1 ) iu;: 9x7: anal calcd. for C,,,H,,N,O,,: C 72 9. H 8.26. N 4.25:
liiund: ( 7 3 2. I 1 7.99. N 4 43.
Preparation 01' three-arm polystyrene 5 from 2 . A solution of the trifunctionul
TEMPO initiatoi. 2 (150 mg. 0.152 inmolj i n [DJstyrene (3.40 g, 30.4 minol.
200 e q i i i b ) h i i s hcated a t 130 C uith stirring under argon for 72 h During this time
the \ i \ c o \ i t ~ of i l i e \elution \wis observed to gradu;illy iiicreii~eand the cleai- reiict i o i i mixture e\cntually solidified. The reaction mixture was rhcn dissolved in
dichl~~r~~inctli.iiic
(25 mL) a n d precipitated into hexane (1 L) Vollowed by re-prccipitation into i)ie~Iimol( 1 L ) . The three-arm polystyrene 5 was isolated a b a white
xilid alter dr)in:. Yield 84
M,, =16500 and P D = 1 . 2 0 : ' H NMR (CDCI,):
d = O40(hi-dJ.O.~lO 1 . 7 0 ( b
2.65nnd2.90(hrd.H,j.4.3S(n~.2H.H,~andH,j.
0.40 7.15 ( h r i n i . X 40 ( s , core H's): "C N M R (CDCI,): 0 =17.10. 20.90, 39.443.X. 125.0 (bri 127.14 ( h r ) . 12X.22. 129.42. 132.73, 144.8-145.X ( a number of
i-eum:iiice~U C J C 1 0 0 small to observe).
7 : TO ;I \ ( l l t i t i c i i i < ) t t l l e ;ilcohol 3 (1.0- 3.6 mmol) i n dry tetrahydrofur;in ( 2 0 mL)
wii\ addcd \odium Iiydride (200 mg. .0 inniol) and the mixture stirred at rooin
teiiiperaturc f o i - 10 min. A \olution otp-vinylhentyl chloride (1.52 g. 10.0 minol,
3.0CCILII\ j \+;I\ aclded a n d the inisture stirred at room tempcratiire for 1 h. then
heated .it irellii\ ror I6 ti. The reaction mixture was then evapor'ited to drynesa iind
p:irtitioncd hetuscii dichlorometh;iiir (50 mLJand water (50 mL) and the aqueous
.
[I]J M J. k'r&chet. Sfirrrcc, 1994. 263. 1710.
[?I P. F. Rempp, P. J. Lutz i n C'oiiipr.ehc~i.\i~eP i ~ l j n i r rChoffi,ri'l~.h l . 7 ( E d . : S. V.
Aggarwal). Pergamon. Oxford. 1988, Chapter 12.
[3] P. J Lutr. G. Beinert, P. F. Rempp. Muuoiiiol. <'li(wf.1982. 1x3. 2787.
[4] H . A. Nguyen. J. P. Kennedy. P o l v n . Bull. 1983. 10. 74
[ S ] 1). Y. Sogah. W. R. Hertler. 0 . W Webster. G . M. Cohen. Miu rr,niohculc,.s
1987. 20. 1473.
[6] G. C. E'istmond. L. U: Harvey. Polrnfrr. 1985. 17. 275.
[7] C. J. Hawker. J .An!. C'l7cw1 So(. 1994. 116. I1 185
[XI C. J. Hawker. J. L. Hedrick. Mu~riJi~olr,~,rler
1995. 28. 2W:.
['I] M. K. Georges. R . P. N . Veregin. P. M. Katmaier. C i K. Hamer. Mocroiiuil(%ilc,\ 1993. 26. 2987. Trcwl.~t'olwi. .Ti? r 7 i ? i ~ i u i d r i i f i i l i i d k i ) 1994. 2. 66.
[lo] E. Rirzordo. Chcrii. .Au.sr. 1987.54. 32: D. H . Solomon. f Rizrurdo. P. Ciicioli
CSIRO. US-A 4581 429. 1985.
[ I I] Number average molecular heights were determined c\periinentally by gel
permeation chromatography using commercially availnhlc narrow molecular
polystyrene sample a s standards.
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