Copolymerization of silyl vinyl ethers with olefins by (α-diimine) PdR+

Article abstract of DOI:10.1021/ja064398w

This paper reports that (α-diimine)PdMe⁺ catalyzes the copolymerization of olefins and silyl vinyl ethers. The reactions of (α-diimine)PdMe⁺ (α-diimine = (2,6-ⁱPr₂-C₆H₃)N=CMe-CMe=N(2,6-ⁱPr₂-C₆H₃)) with excess vinyl ethers CH₂=CHOR (1a-d: R = ^tBu (a), SiMe₃ (b), SiPh₃ (c), Ph (d)) in CH₂Cl₂ at 20 °C afford polymers for 1a (rapidly) and 1b (slowly) but not for 1c or 1d. The structures of poly(1a,b) indicate a cationic polymerization mechanism. The reaction of (α-diimine)PdMe⁺ with 1-2 equiv of 1a-d proceeds by sequential C=C π-complexation to form (α-diimine)PdMe(CH₂=CHOR)⁺ (2a-d), 1,2 insertion to form (α-diimine)Pd(CH₂CHMeOR)⁺ (3a-d), reversible isomerization to (α-diimine)Pd(CMe₂OR)⁺ (4a-d), β-OR elimination to generate (α-diimine)Pd(OR)(CH₂=CHMe)⁺ (not observed), and allylic C-H activation to yield (α-diimine)Pd(η³-C₃H₅)⁺ (5) and ROH. The reaction of (α-diimine)PdMe⁺ with 1-hexene/1b and 1-hexene/1c mixtures in CH₂Cl₂ at 20 °C affords copolymers containing up to 20 mol % silyl vinyl ether. The copolymers were purified to be free of any -[CH₂CHOSiR₃]_n- homopolymer. The copolymer structures are similar to that of homopoly(1-hexene) generated under the same conditions. The major comonomer units are CH₃CH(OSiR₃)CH₂-, CH₂(OSiR₃)CH₂- and -CH₂CH(OSiR₃)CH₂-. The 1-hexene/CH₂=CHOSiR₃ copolymers can be desilylated to give 1-hexene/CH₂=CHOH copolymers. The results of control experiments argue against cationic and radical mechanisms for the copolymerization, and an insertion/chain-walking mechanism is proposed. Copyright

Full text of DOI:10.1021/ja064398w

Published on Web 08/29/2006
Copolymerization of Silyl Vinyl Ethers with Olefins by (r-diimine)PdR+
Shuji Luo and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received June 21, 2006; E-mail: rfjordan@uchicago.edu
Scheme 1a
The development of methods for the incorporation of polar
CH2dCHX vinyl monomers in metal-catalyzed insertion polymer-
izations of olefins is a challenging goal.1 In a key advance,
Brookhart discovered that (R-diimine)PdR+ species catalyze the
copolymerization of alkyl-acrylates with olefins.2 Here we report
that (R-diimine)PdMe+ (R-diimine ) (2,6-iPr2-C6H3)NdCMeCMed
N(2,6-iPr2-C6H3)) copolymerizes silyl vinyl ethers with olefins to
OSiR3-substituted polyolefins that can be desilylated to yield OH-
substituted polyolefins.
a Pd ) (R-diimine)Pd.
Vinyl ethers (CH2dCHOR) are attractive potential comonomers
for insertion polymerization because their properties can be tuned
by variation of the OR group. However, (i) vinyl ethers are
Table 1. Reactivity of CH2dCHOR with (R-Diimine)PdMe+
CH2dCHOR
tBu
SiMe3
SiPh3
Ph
susceptible to cationic polymerization by electrophilic metal
catalysts,3 (ii) insertion barriers for LnMR′(CH2dCHOR) species
are predicted to be high due to the electron donation by the OR
group,4 and (iii) LnMCH2CH(OR)R′ species generated by insertion
may undergo â-OR elimination, which would terminate chain
growth. Nevertheless, Wolczanski found that (tBu3SiO)3TaH2 inserts
CH2dCHOR (R ) alkyl, Ph) to generate (tBu3SiO)3TaH(CH2CH2-
OR) and that subsequent â-OR elimination is slow when R is bulky
Keq vs ethylene (-60 °C)a
1.2(1) 0.17(1) <0.01
0.04(2)
t
1/2, conversion of 2 to 3/4 (0 °C)b > 1 h 15 min 8.9 min 7.7 min
3/4 ratio (20 °C)
73/27 0/100
0/100
2.2 h
not obsd
<3 min
t1/2, conversion of 3/4 to 5 (20 °C) 88 h
5.5 h
a Keq ) [2][CH2dCH2][PdMe(CH2dCH2)+]-1[1]-1 b t1/2 for insertion
.
of PdMe(CH2dCH2)+ at 0 °C is ca. 8 s.
on the lab time scale at 20 °C and react with MeCN to form (R-
diimine)Pd(CH2CHMeOR)(NCMe)+ (3‚NCMe) at -40 °C. NMR
and DFT results show that 3 and 4 are O-chelated. No evidence
for the 2,1 insertion product (R-diimine)Pd{CH(OR)CH2Me}+ or
its chain-walk isomers was observed for 1a-d. The 3/4 mixtures
react further at 20 °C to generate (R-diimine)Pd(η3-C3H5)+ (5) and
ROH, presumably by â-OR elimination of 3 to form (R-diimine)-
Pd(OR)(CH2dCHMe)+ (not observed) and allylic C-H activation.9
The viability of the allylic activation was established by the model
t
(e.g.. Bu).5 Reasoning that problems i-iii could be avoided by
suitable tuning of the OR group, we investigated the reactions of
t
CH2dCHOR (1a-d: R ) Bu (a), SiMe3 (b), SiPh3 (c), Ph (d))
with (R-diimine)PdMe+ (as [(R-diimine)PdMe(Et2O)][SbF6] or
generated in situ from (R-diimine)PdMeCl and [Li(Et2O)2.8]-
[B(C6F5)4]).6
The reaction of (R-diimine)PdMe+ with excess 1a (10-50 equiv,
CH2Cl2, 20 °C) results in rapid quantitative polymerization of 1a
and rapid Pd0 formation. The -[CH2CHOtBu]n- polymer contains
aldehyde and acetal end groups and internal -CHdCH- units.
Very similar polymers are generated by the reaction of 1a with
[Ph3C][B(C6F5)4] or [Li(Et2O)2.8][B(C6F5)4]. These results are
consistent with a cationic polymerization mechanism.7 Addition of
2,6-tBu2-pyridine does not significantly affect the polymerization
of 1a by (R-diimine)PdMe+, which suggests that (R-diimine)PdR+
species may play a direct role in initiation. The cationic polymer-
ization and Pd0 formation preclude (R-diimine)PdMe+-catalyzed
copolymerization of 1a with olefins; for example, the reaction with
1a/ethylene results in 1a homopolymerization. In contrast, 1b is
only slowly cationically polymerized by (R-diimine)PdMe+, and
neither 1c nor 1d are polymerized by this catalyst.8 Thus, cationic
polymerization can be minimized or avoided by using silyl or aryl
vinyl ethers.
The reactions of (R-diimine)PdMe+ with 1-2 equiv of 1a-d
were investigated to probe for insertion reactivity under conditions
where the vinyl ether concentration is low and cationic polymer-
ization is slow. As shown in Scheme 1, (R-diimine)PdMe+ reacts
with 1a-d by CdC π-complexation to form (R-diimine)PdMe-
(CH2dCHOR)+ (2a-d), followed by 1,2 insertion to produce (R-
diimine)Pd(CH2CHMeOR)+ (3a-d) and reversible isomerization
to (R-diimine)Pd(CMe2OR)+ (4a-d) by chain-walking (i.e., â-H
elimination and reinsertion). Complexes 3 and 4 interconvert rapidly
n+
reaction of [(tmeda)Pd(OPh)]n with propylene to yield (tmeda)-
Pd(η3-C3H5)+ and HOPh quantitatively.
The binding strength of 1a-d to (R-diimine)PdMe+ was assessed
by competitive binding experiments with ethylene. The Keq data
(Table 1) show that 1a binds with similar strength as ethylene but
1b-d bind more weakly. The kinetics of key steps in Scheme 1
were measured by NMR, and t1/2 data are listed in Table 1. 2a-d
insert more slowly than does (R-diimine)PdMe(ethylene)+. The
trends in binding strength and insertion rates reflect a balance of
steric effects (bulky OR groups inhibit binding and insertion) and
electronic effects (strong donor OR groups enhance binding but
inhibit insertion).4 The conversion of 3/4 to 5 is slow, except in
the case of phenyl vinyl ether, for which 3d/4d react faster than
they are formed from 2d and hence were not directly observed.
The cationic polymerization trends and the results in Table 1
suggested that 1c and possibly 1b would be viable comonomers
for olefin polymerization with (R-diimine)PdMe+. As 1b,c are much
less reactive than ethylene, reactions with the less reactive olefin
1-hexene were explored.
The reaction of (R-diimine)PdMe+ with 1-hexene/1c mixtures
(CH2Cl2, 20 °C) produces copolymers containing up to 20 mol %
silyl vinyl ether (Scheme 2). The copolymers were isolated and
purified by washing with acetone and extracting into hexane. The
yields are similar and the molecular weights (Mn ) 18,000, PDI )
9
12072
J. AM. CHEM. SOC. 2006, 128, 12072-12073
10.1021/ja064398w CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Scheme 3a
1.7) are lower compared to results for hexene homopolymerization
under identical conditions. The copolymers contain 90-100
branches/1000 C (60% Me, 20% Bu > long > Et > Pr), which is
similar to what is observed in control hexene homopolymeriza-
tions.10 NMR data establish that the major comonomer units in the
copolymers are CH3CH(OSiPh3)CH2- (I, 85%) and CH2(OSiPh3)-
CH2- (II, 6%).
The following lines of evidence establish that the poly(hexene-
co-CH2dCHOSiPh3) is a copolymer and does not contain -[CH2-
CHOSiPh3]n- homopolymer. (i) As noted above, 1c is not
polymerized by (R-diimine)PdMe+ under these conditions. (ii) If
-[CH2CHOSiPh3]n- had formed, it would be removed by the
workup procedure, since it is soluble in acetone but not hexane.
(iii) NMR spectra of the copolymer do not contain the characteristic
broad resonances of -[CH2CHOSiPh3]n-. (iv) HMBC NMR data
establish that the -OSiPh3 units of the copolymer are covalently
linked to the polyhexene chain.
a P ) growing chain.
OSiPh3 group in (R-diimine)Pd(CH2dCHOSiPh3)P+ species may
inhibit this process, so that 2,1 insertion becomes competitive in
1c copolymerization. The “in-chain” placement III may be dis-
favored for 1c because the (R-diimine)PdCH2CH(OSiPh3)P+ and
(R-diimine)PdCH(OSiPh3)CH2P+ species generated by insertion of
1c are too crowded to readily insert olefins.
In summary, silyl vinyl ethers are readily incorporated in hexene
insertion polymerization catalyzed by (R-diimine)PdMe+ and the
mode of incorporation is influenced by the structure of the OSiR3
group. The copolymers can be desilylated to produce hexene/CH2d
CHOH copolymers. Monomer 1c has also been copolymerized with
other olefins ranging from ethylene to 1-octadecene. We anticipate
that silyl vinyl ethers will be suitable comonomers for other olefin
polymerization catalysts, and our studies in this direction will be
reported in due course.
The 1-hexene/1c copolymer was desilylated by reaction with HCl
(CHCl3, 2 d; 94% conversion of OSiPh3 groups to OH groups).
The NMR spectra of the desilylated copolymer were unchanged
after elution through SiO2 with hexanes, which would remove any
-[CH2CHOH]n- (if present). This result confirms that the hexene/
1c copolymer does not contain -[CH2CHOSiPh3]n-.
Acknowledgment. This work was supported by the U.S.
Department of Energy (Grant DE-FG-02-00ER15036).
The reaction of 1-hexene/1b mixtures with (R-diimine)PdMe+
generates mixtures of -[CH2CHOSiMe3]n- homopolymer and
1-hexene/CH2dCHOSiMe3 copolymers containing up to 11 mol
% comonomer. The consumption of hexene is significantly
decreased compared to control hexene homopolymerizations under
identical conditions due to the Pd0 formation associated with the
cationic polymerization of 1b. The -[CH2CHOSiMe3]n- can be
removed from the hexene/1b copolymer by eluting the polymer/
copolymer mixture through SiO2 with hexanes. The major comono-
mer units are CH3CH(OSiMe3)CH2- (I, 60%) and -CH2CH-
(OSiMe3)CH2- (III, 30%).
The reaction of 1-hexene/1c mixtures with [H(Et2O)2][B(C6F5)4]
under the conditions of Scheme 2 results in cationic polymerization
of 1c with no hexene incorporation. The reaction of 1-hexene/1c
mixtures with AIBN in C6D5Cl at 60 °C for 20 h, in the presence
or absence of Li salts, does not produce copolymer. In contrast,
(R-diimine)PdMe+ copolymerizes 1-hexene/1c under these condi-
tions. These results argue against cationic and radical mechanisms
in Scheme 2.
We propose that the copolymerization in Scheme 2 proceeds by
a normal insertion/chain-walking mechanism.2 As shown in Scheme
3, comonomer units I and II are generated by 1,2 or 2,1 insertion
followed by chain-walking and hexene insertion, and III is
generated by 1,2 or 2,1 insertion followed directly by hexene
insertion. Although only 1,2 insertion was observed in the sto-
ichiometric reactions of 1a-d with (R-diimine)PdMe+, steric
crowding between the migrating polymeryl (P) group and the
Supporting Information Available: Experimental procedures and
characterization data for all compounds and polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
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