Polyhomologation based on in situ generated boron-thexyl-silaboracyclic initiating sites: A novel strategy towards the synthesis of polyethylene-based complex architectures

Article abstract of DOI:10.1039/c5cc01579k

A novel strategy, based on the in situ generated boron-thexyl-silaboracyclic initiating sites for the polyhomologation of dimethylsulfoxonium methylide, has been developed for the synthesis of complex polyethylene-based architectures. As examples, the synthesis of a 4-arm polyethylene star, three (polystyrene)(polyethylene)₂ 3-miktoarm stars and a PE-branched double graft copolymer is given.

Full text of DOI:10.1039/c5cc01579k

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Polyhomologation based on in situ generated
boron-thexyl-silaboracyclic initiating sites: a novel
strategy towards the synthesis of polyethylene-
based complex architectures†
Cite this:DOI: 10.1039/c5cc01579k
Received 21st February 2015,
Accepted 10th April 2015
Zhen Zhang,ab Hefeng Zhang,ab Yves Gnanoua and Nikos Hadjichristidis*ab
DOI: 10.1039/c5cc01579k
www.rsc.org/chemcomm
A novel strategy, based on the in situ generated boron-thexyl- followed by (co)polymerization to afford graft copolymers or
silaboracyclic initiating sites for the polyhomologation of dimethyl- molecular brushes;8 (3) stitching reactions to freeze the resultant
sulfoxonium methylide, has been developed for the synthesis of polyhomologation star structure by transforming the sensitive
complex polyethylene-based architectures. As examples, the synthesis to oxidation/hydrolysis boron-junction to stable carbon;9 and (4)
of a 4-arm polyethylene star, three (polystyrene)(polyethylene)2 design of novel borane initiators.10
3-miktoarm stars and a PE-branched double graft copolymer is given.
For example, ROP of e-caprolactone initiated directly from
the PE-OH macroinitiator with stannous octanoate (Sn(Oct)2) as
Polyethylene (PE), the largest volume commodity polymer in the a catalyst affords polyethylene-b-poly(e-caprolactone). By trans-
world, is indispensable in many applications ranging from packaging forming the terminal –OH of the diblock copolymer to the ATRP
to precision-processed materials.1 The low cost, excellent physical initiating site polyethylene-b-poly(e-caprolactone)-b-poly(acrylic
properties, easy processability and recyclability of PE have led to its acid) triblock copolymers have been synthesized.5a Well-defined
commercial success.2 However, the poor compatibility of PE with PE brushes have been synthesized by ring opening metathesis
other polymers limits its further applications. To overcome this polymerization (ROMP) of norbornyl PE-macromonomers pre-
deficiency, the development of PE-based (co)polymeric materials pared by reaction of PE-OH with 5-norbornene-2-carboxylic acid.8
with novel architectures is needed.3
Stitching reactions have been used to synthesize PE-based linear,
Recently, Shea developed a living polymerization procedure macrocarbocyclic and 3-arm star structures.9 Utilizing novel borane
leading to linear well-defined hydroxyl-terminated polymethylene initiators is another efficient way to obtain 3-arm star and multi-
(polyethylene).4 The general mechanism involves the formation of a block copolymers. For example, polyhomologation of ylides with
complex between an ylide (monomer) and an organoborane (initiator) 1-boraadamantane led to a 3-arm PE10 and hydroboration of an allyl-
followed by migration/insertion of –CH2– into the initiator. As a terminated polystyrene oligomer with BH3 gave an organoborane
consequence, the methylene groups are randomly inserted one by macroinitiator which by polyhomologation led to poly(ethylene-b-
one (C1 polymerization) into the three arms of the initiator leading styrene) (PE-b-PS) block copolymers.11 Our group synthesized 3-arm-
to a 3-arm polymethylene star. By oxidizing/hydrolysing the resulted polybutadiene, polystyrene and (polystyrene-b-polydiene)borane
star, an OH-terminated polymethylene (polyethylene) is obtained.
stars by reaction of the corresponding living macroanions with
There are four methods for the synthesis of PE-based polymers BF3ꢀEtO2, which served as macroinitiators for the synthesis of
with different architectures via polyhomologation: (1) direct use of OH-terminated di- and triblock co- and terpolymers.12
the terminal –OH of PE-OH for ring-opening polymerization (ROP),5
In this communication, we propose a novel general strategy
or post-polymerization modification of the –OH appropriate for based on the in situ formation of B-thexyl-silaboracyclic structure,
atom transfer radical (ATRP),6 or reversible addition-fragmentation having two silicon-connected initiating sites and one blocked, for
chain-transfer polymerization (RAFT)7 initiating sites; (2) reaction the synthesis of PE-based complex macromolecular architectures.
of PE-OH with functionalized monomers to give macromonomers
To check the principle of this strategy, B-thexyl-methylphenyl-
silaborocycle 2 (mixture of 2a and 2b isomers), a small equivalent
molecule, was prepared by hydroboration of methylphenyldivinyl-
silane 1 with thexylborane13 and used in situ to promote the
polyhomologation of dimethylsulfoxonium methylide (Scheme 1).
Details of the synthesis and characterization are given in the
ESI.† After hydrolysis of the polyhomologation product, only a,
o-dihydroxy polymethylene was produced as evidenced by the
a King Abdullah University of Science and Technology (KAUST), Physical Sciences
and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory,
Thuwal 23955, Saudi Arabia. E-mail: nikolaos.hadjichristidis@kaust.edu.sa
b King Abdullah University of Science and Technology (KAUST), Physical Sciences
and Engineering Division, Thuwal 23955, Saudi Arabia
† Electronic supplementary information (ESI) available: Experimental details as well as
supporting data including NMR and GPC profiles. See DOI: 10.1039/c5cc01579k
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Scheme 3 Synthesis of PE-based 3-miktoarm star polymers by combination
of ATRP and polyhomologation.
Scheme 1 Synthesis of a, o-dihydroxyl PE and its GPC chromatogram.
appearance of only one peak in the GPC chromatogram and the
absence of the thexyl group in the NMR spectrum, meaning
that the B-thexyl site is inactive to polyhomologation and all
other B-initiating sites (in 2a and 2b isomers) are equivalent, in
agreement with Shea’s work using B-p-methoxyl-phenylethyl-9-
BBN as an initiator.9 This is easy to explain since after the first
insertion of a –CH2– on the boron-tertiary carbon (B-CH) all
sites (B-CH2) become equivalent.
Subsequently, this strategy was employed for the macro-
molecular design and synthesis of a 4-arm polyethylene star, three
(polystyrene)(polyethylene)2 3-miktoarm stars and a PE-branched
double graft copolymer.
The general reaction scheme for the synthesis of a 4-arm star is
given in Scheme 2. The hydroboration of 3,8-dimethyl-3,8-divinyl-4,7-
dioxa-3,8-disiladeca-1,9-diene 4 with thexylborane gives bis-B-thexyl-
silaboracycles 5. After treating 5 with excess ylide and subsequent
oxidation–hydrolysis reaction using TAOꢀ2H2O, 4-arm PE star 6
(Mn,NMR = 3.0 ꢁ 103, PDIGPC = 1.63) was obtained (Fig. S2, ESI†).
Details are given in the ESI.†
The synthesis of PE-based 3-miktoarm stars was achieved
according to Scheme 3 by combining polyhomologation with
ATRP through an heterofunctional initiator, 3-(methyldivinylsilyl)-
propyl-2-bromo-2-methylpropanoate 7 (the synthesis procedure is
given in the ESI,† Scheme S4). The 2-bromo-2-methylpropanoate
group was used to initiate the ATRP of styrene (CuBr/PMDETA
(N,N,N0,N0,N00,N00-pentamethyldiethylene-triamine)) and methyl-
divinylsilyl after transformation to B-thexyl-silaboracycles to
initiate polyhomologation. Three samples of divinyl-terminated
polystyrene with different molecular weights and low polydisper-
sities have been designed and synthesized (8a: Mn,GPC = 16.0 ꢁ 103,
PDIGPC = 1.22; 8b: Mn,GPC = 11.0 ꢁ 103, PDIGPC = 1.18, Tg = 85 1C; 8c:
Mn,GPC = 5.2 ꢁ 103, PDIGPC = 1.18) (Fig. 1 and Fig. S3 and S4, ESI†).
The terminal methyldivinylsilyl group was readily transformed
to B-thexyl-silaboracycles by hydroboration with thexylborane
resulting in the corresponding macroinitiators for polyhomo-
logation (9a–c). High hydroboration efficiencies were indicated
by quantitative consumption of the terminal vinyl groups (d = 5.6 to
1
6.2 ppm) revealed by H NMR results. As an example, the NMR
spectra of 8a (before hydroboration) and 9a (after hydroboration)
are shown in Fig. 2 (spectra a and b). The resultant boron-
macroinitiators were used in situ for the polyhomologation of
dimethylsulfoxonium methylide leading to the corresponding
PE-based 3-miktoarm stars.
The success of this strategy was confirmed by both GPC and
NMR results (Fig. 1 and 2). After polyhomologation, the peak
shifted to a higher molecular weight range keeping the narrow
distribution profiles (10a: Mw,GPC-LS = 506 ꢁ 103, PDIGPC-LS
=
1.32; 10b: Mw,GPC-LS = 367 ꢁ 103, PDIGPC-LS = 1.56, Tm = 116 1C;
Fig. 1 GPC chromatograms of PS-MDVSi (8a) and the corresponding
3-miktoarm star (10a).
Scheme 2 Synthesis of 4-arm PE star and its GPC chromatogram.
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copolymer PS-co-PDVSiOMA-g-(PE-OH)2 13 (Mw,GPC-LS = 218 ꢁ
103, PDIGPC-LS = 1.35) (see ESI,† Fig. S5). High hydroboration
efficiency was indicated by the quantitative disappearance of
the signal at 5.7–6.2 ppm in 1H NMR spectra for vinyl protons.
The PS backbone is identifiable in NMR spectra (toluene-d8,
25 1C) and the PE fingerprint is much more evident by increas-
ing the temperature up to 80 1C (see ESI,† Fig. S6).
In conclusion, a novel strategy using the in situ synthesized
B-thexyl-silaboracyclic moieties with two silicon-connected initiating
and one blocked sites for polyhomologation was successfully
developed. This general strategy opens a new horizon for the
synthesis of PE-based complex macromolecular architectures.
Only a few examples are given in this communication, i.e. a 4-arm
polyethylene star, three PS-PE2 3-miktoarm stars and a PE-branched
double graft copolymer. Combination of this general strategy with
other living and living/controlled polymerization techniques will
lead to novel architectures such as multi-arm stars (8, 12, 16-arm),
H-shaped, molecular brush copolymers, etc.
Fig. 2 1H NMR spectra of (a) PS-MDVSi (8a) (chloroform-d, 25 1C); (b) PS-
Silaboracycle (9a) (chloroform-d, 25 1C); (c) PS-PE2 (10a) (toluene-d8,
80 1C).
Notes and references
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Scheme 4 Synthesis of PE-branched double graft copolymer by combi-
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10c: Mw,GPC-LS = 423 ꢁ 103, PDIGPC-LS = 1.35) (Fig. 1 and Fig. S3
and S4, ESI†). Also, the fingerprints of the PS and PE blocks
1
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