Versatile grafting approaches to star-shaped POSS-containing hybrid polymers using RAFT polymerization and click chemistry

Article abstract of DOI:10.1039/c1cc13412d

An alkyne-bearing polyhedral oligomeric silsesquioxane (POSS) core was used to prepare POSS-containing polymer hybrids using 'grafting to' or 'grafting from' strategies in combination with reversible chain transfer and click chemistry.

Full text of DOI:10.1039/c1cc13412d

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COMMUNICATION
Versatile grafting approaches to star-shaped POSS-containing hybrid
polymers using RAFT polymerization and click chemistryw
Yun-Sheng Ye,a Wei-Chung Shen,a Chi-Yung Tseng,a John Rick,a Yao-Jheng Huang,b
Feng-Chih Changb and Bing-Joe Hwang*a
Received 9th June 2011, Accepted 16th August 2011
DOI: 10.1039/c1cc13412d
An alkyne-bearing polyhedral oligomeric silsesquioxane
(POSS) core was used to prepare POSS-containing polymer
hybrids using ‘grafting to’ or ‘grafting from’ strategies in
combination with reversible chain transfer and click chemistry.
‘graft from’ a core unit must be reacted under mild conditions.
Moreover, using ‘grafting from’ chemistry (or method for
preparing arms for ‘grafting to’ approach), controlled radical
polymerization (CRP) is able to yield well-defined polymers with
controlled molecular weights, low polydispersivity, functional
group tolerance that have high chain-end fidelity. Recently,
reversible addition–fragmentation chain transfer (RAFT) poly-
merization5 has attracted significant attention in polymer and
materials science due to its versatility and simplicity, where
additionally the polymer has a high degree of chain-end fidelity.6
Although the RAFT technique is compatible with the polymer-
ization of almost all conventional monomers, the covalent
attachment of the RAFT agents in the cage-structure is still
challenging thus restricting its utilization. Click chemistry is
potentially an ideal modular methodology for the introduction
of a wide variety of molecules. The copper(I)-catalyzed [2 + 3]
Huisgen cycloaddition proceeds with short reaction times, a wide
range of tolerated functionalities, high yields, and a tolerance
towards humidity and oxygen.7 Therefore, when RAFT polymer-
ization is combined with click chemistry techniques, it
becomes an even more powerful tool in materials science.8
In this contribution, both click chemistry and RAFT polymer-
ization were applied in the preparation of star-shaped
POSS-containing polymer hybrids using ‘grafting to’ and
‘grafting from’ approaches (Scheme 1). This study provides a
discussion of each method, including the effect of chain length
on click reaction, how the content of multifunctional RAFT
agents controls the polymer’s molecular weight (Mw) and the
grafting efficiency of each method. To the best of our
knowledge, nobody has reported examples featuring the combi-
nation of RAFT and click chemistry in POSS molecules.
Before proceeding with the ‘grafting to’ or ‘grafting from’
approaches, we prepared the core material (alkyne-functionalized
POSS; 8P-POSS), which was used to modify the azido-
terminated polymethyl methacrylate (PMMA-N3) and azido-
CTAs, using click chemistry and RAFT polymerization. In
the preparation of 8P-POSS (Scheme S1, ESIw), we chose
the highly reactive 3-propargylcarbonylpropionic chloride
(PCPCl) as the precursor, since it can effectively substitute
all hydroxyl groups of 8OH-POSS. Based on the results of
1H NMR, 13C NMR, gel permeation chromatography (GPC)
and elemental analysis (EA) (Table S1, ESIw), high purity
8P-POSS was prepared.
Organic–inorganic nanohybrid materials have attracted
considerable scientific and technological interest, due to the
combination of the properties of the organic and the inorganic
components conferred upon the resulting materials.1 These
nano-structured hybrid materials have a high suface area to
volume ratio. However, these characteristic properties induce
agglomeration and lead to the loss of the material’s attractive
properties. Surface modification with polymeric structures has
shown promise in improving the dispersion stability of the
nanoparticles in organic and aqueous media and in preventing
the segregation of inorganic fillers in polymer matrices.
Current methods for preparation of covalent polymer–silica
conjugates include ‘grafting to’ methods using Huisgen cyclo-
addition coupling,2 as well as ‘grafting from’ methods involving
initiator-functionalized inorganic particles.3
Polyhedral oligomeric silsesquioxanes (POSS)–polymer
hybrid materials have attracted a great deal of attention due
to the enhancement of the organic matrices’ properties, i.e.
increased thermal stability, reduced flammability and dielec-
tric constant—upon incorporation of POSS molecules.4 In
recent years, with the new living/controlled polymerization
techniques emerging in polymer science, many novel POSS-
containing hybrid polymers with well-defined topological
structures (tadpole-shaped, block, and star-shaped) have been
prepared.2c–e However, the design of a potential method for
achieving a well-defined topological structure while maintaining
control over the polymer’s molecular weight and polydispersity
in POSS is an extremely important challenge.
For the ‘grafting to’ or ‘grafting from’ approach to be
successful, in terms of yield, specificity and reversibility; the
coupling of arms, i.e. ‘grafts to’, or the functionalities used to
a Department of Chemical Engineering, National Taiwan University of
Science and Technology, Taipei, Taiwan.
E-mail: bjh@mail.ntust.edu.tw
b Institute of Applied Chemisty, National Chiao-Tung University,
Hsin-Chu, Taiwan
w Electronic supplementary information (ESI) avaiable. See DOI:
10.1039/c1cc13412d
c
10656 Chem. Commun., 2011, 47, 10656–10658
This journal is The Royal Society of Chemistry 2011

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Fig. 1 GPC traces of grafted PMMA-g/t-POSS.
Scheme 1 Two types of grafting reactions used in the synthesis of
weight has an impact on the grafted ratio of the
PMMA–POSS hybrids. Obviously, as the polymer’s molecular
weight is increased, the graft density decreases (95.5% to
28.4%). This result can be explained by the decreased
reactivity of the chain-end azide functionality that becomes
less accessible as the polymer adopts a more random coil
structure at this higher molecular weight.9
star-shaped POSS-containing polymer hybrids.
For ‘grafting to’ approach, a series of well-defined PMMA-N3
were prepared by the RAFT polymerization of methyl metha-
crylate (MMA) using a small amount of AIBN as the radical
source ([AIBN]/[charge transfer agents (CTAs)] o 0.2) and
azido-CTAs as the chain transfer agent. All polymerization of
MMA, mediated with azido-CTAs, resulted in a linear
increase in average molecular weight (Mn) with conversion
(Fig. S10, ESIw): the correlation between theoretical and
experimental values was excellent throughout the conversion
ranges (Table S2, ESIw). The polymerizations proceeded with
good control, as evidenced by the narrow and unimodal mole-
cular weight distribution [polydispersity index (PDI) o 1.25].
Before conducting the ‘grafting to’ approach, we investigated
the efficiency and fidelity of copper-catalyzed azide–alkyne
coupling between azido-functionalized polymer end groups
with acetylene species. We considered 3-propargyl carbonyl
propionic acid (PCPA) as a model for preparing the star
polymer. After the click reaction (Fig. S11, ESIw), the resulting
polymer showed the appearance of the new methylene proton
adjacent to the triazole ring at d = 4.62 ppm (–CH2–CH2–
triazole–), 5.30 ppm (–triazole–CH2–COO–) and 7.78 ppm
(–N–CHQC–). The end group click reactions were efficient,
the PMMA-N3 (50) reacted with PCPA to yield carbonyl-
terminated polymer with nearly 98% end groups functionalization.
Finally, the ‘grafting to’ approach via click chemistry
between the PMMA-N3 and 8P-POSS afforded the star shaped
POSS-containing hybrid PMMA. The synthetic steps were
characterized by GPC and are shown in Fig. 1. Because of
the clear separation between the ungrafted PMMA peak and
the PMMA–POSS hybrid, we were able to quantify the
contents of ungrafted PMMA in the as-prepared PMMA-g/
t-POSS hybrid. On the GPC traces, the peaks at shorter and
longer retention times in each GPC trace corresponded to the
elution of the larger PMMA-g/t-POSS hybrid and smaller
ungrafted PMMA-N3, respectively. The amount of grafted
PMMA was determined quantitatively by comparing the
grafted PMMA peak area ratio after using Gaussian fitting
and number-averaging the molecular weight of PMMA–POSS.
Comparing the different chain length of the PMMA-N3 grafting
to POSS provides an indication that the polymer’s molecular
There are two types of multifunctional RAFT agents, i.e.
those used for the Z-group and the R-group approaches, to the
synthesis of the star polymer (see ESIw). In this study, we
selected the R-group as a part of the core, due to the growing
linear chains experiencing a higher probability of irreversible
terminations occurring and forming dead linear polymers free
of any thiocarbonylthio groups in the Z-group approach.10 To
covalently attach the RAFT agents onto the POSS surface, the
azide–alkyne coupling reaction was employed (Scheme S1,
ESIw). The 1H NMR spectra (Fig. S9, ESIw) show that the
signals at d = 4.71 (–COO–CH2–CRCH) and 3.41 ppm
(CH2–CH2–N3) assigned to the methylene protons originating
from 8P-POSS and azido-CTAs were shifted to 5.25
(–COO–CH2–triazole–) and 4.49 ppm (–triazole–CH2–CH2–),
respectively. Moreover, the peak at 7.93 ppm corresponding to
the triazole ring appeared, while no residual propargyl groups
(d = 2.51 ppm) were detected in 8CTAs-POSS. FTIR
(Fig. S12, ESIw), EA and GPC results also confirmed that
8CTAs-POSS was successfully prepared (Table S1, ESIw).
For the RAFT polymerization to the silica based cage
structure, the high surface density anchored CTAs commonly
exhibited a lower polymerization resulting in a low surface
density and large amounts of cross-terminated polymer, due to
the ‘localized high RAFT agent concentration’ effect.3c,d,11 In
this study, the 8CTAs-POSS used for MMA radical polymeri-
zation had a high surface density (182 mmol gÀ1) as compared
with previous studies.3c,12 Interestingly, in comparison with
the GPC traces of linear polymers prepared with azido-CTAs
([M]0/[CTAs] = 150), the GPC traces of star polymer pre-
pared with the same [M]0/[CTAs] ratio were shifted to higher
molecular weights (Fig. S13, ESIw). Dependence of Mn and
PDI on conversion for MMA polymerization (Fig. S14, ESIw)
shows that the Mn increased in a linear fashion with monomer
conversions, and no appreciable differences of the Mn were
observed as compared with linear polymerization. However,
the PDI of PMMA prepared with 8CTAs-POSS gradually
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Chem. Commun., 2011, 47, 10656–10658 10657

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did not occur. Thus the star polymer’s growth might be
accompanied by a parallel polymerization of single chains.
For MMA polymerization, 8CTAs-POSS is an excellent chain
transfer agent, as indicated by the narrow polydispersities
observed in contrast to that found with linear polymerization.
In summary, star-shaped POSS-containing PMMA was
successfully prepared by the combination of RAFT and
click chemistry. These powerful tools gave control over the
polymer’s: length, architecture and graft density in both the
‘grafting to’ and ‘grafting from’ approaches; however, graft
density decreased due to the effect of steric hindrance
predominating on the surface of POSS. The properties and
morphologies of these hybrids are under investigation. Here,
we provide
a versatile strategy to prepare star-shaped
Fig. 2 GPC traces of grafted PMMA-g/f-POSS.
POSS-containing polymer hybrids, together with valuable
information showing the advantages of different grafting
reactions applied to surface modification applications.
increased at higher MMA conversions (458%). This behavior
can be explained by the star polymer having different hydro-
dynamic characteristics from the linear polymer. In fact, the
HF-etched sample showed only one symmetrical peak and the
PDI were as narrow as 1.15 in the final product. This result
strongly suggested that the 8CTAs-POSS was an even more
effective chain transfer agent than the free CTAs in solution
due to the much higher fragmentation rate of the anchored
intermediate radical compared with the free interemediate
radicals and also a highly effective chain transfer reaction. In
addition, the GPC trace of the final product shows no obvious
shoulder on the high molecular weight side of the peak, which
means interparticle polymeric radical coupling due to high
surface densities of CTAs on POSS particle did not occur even
at high conversions (480%). Although the exact reason for
this phenomenon is not clear, we speculate that a combination
of the unique confined geometry and the higher polymeriza-
tion rate of the anchored interemediate macro-RAFT radical
on the POSS surface may contribute to the high opportunity
for star and linear chain radical termination.
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10658 Chem. Commun., 2011, 47, 10656–10658
This journal is The Royal Society of Chemistry 2011