Bocchini, Sergio

Carniato, F.

Frache, Alberto

Monticelli, O.

Tonellotto, A.

English

PORTO Publications Open Repository TOrino

 
ON THE USE OF TI-POSS IN THE RING OPENING 
POLYMERIZATION OF L-LA 
 
 
O. Monticelli
1, A. Tonelotto
1, S. Bocchini
2, A. Frache
2, F. Carniato
3 
 
 
1Dipartimento di Chimica e Chimica Industriale, Università di Genova, via Dodecaneso 31, 16143 Genova, Italy 
(e-mail: orietta.monticelli@unige.it) 
2Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, sede di Alessandria, via T. 
Michel 5, 15121 Alessandria, Italy 
3Dipartimento di Scienze e Tecnologie Avanzate and Nano-SISTEMI Interdisciplinary Centre, Università del 
Piemonte Orientale ‘A. Avogadro’, via Bellini 25G, 15100 Alessandria, Italy 
 
Introduction 
 
Among the metal salts and coordination compounds studied for the ROP of lactide, also titanium-
based systems have been considered. Since the pioneer work of Kricheldorf et al.
[1] on the use of 
titanium alkoxides as initiators of the polymerization of lactones, several type of Ti complexes have 
been studied for the ring-opening polymerization of lactide to produce PLA. Indeed, some of the 
above systems were capable to give also stereoselective polymerization reactions.  
Among the siliceous systems, metal-containing silsesquioxane derivatives represent novel catalysts in 
the field of both homogeneous and heterogeneous catalysis
 (such as alkene polymerization, metathesis 
or epoxidation)
[2,3] These molecules are generally prepared from incomplete condensed structures, i.e. 
open-corner polyhedral oligomeric silsesquioxanes (POSS) molecules which can react with transition 
metal  compounds  to  form  metal-silsesquioxanes  (M-POSS).  Together  with  the  above  mentioned 
application,  M-POSS  turned  out  to  strongly  affect  thermo-oxidative  degradation  of  polymer 
nanocomposites.  
The present work has been focused on the use of a home-made Ti-based POSS (Figure 1), which is 
potentially capable of being active in the ring opening polymerization (ROP) of lactide. In particular, 
as shown in Figure 1, the titanium centers in these compounds retain one terminal alkoxide ligand, 
which is ideal for the ROP via the coordination-insertion mechanism established for known metal 
alkoxide initiators. 
 
 
 
 
 
 
 
 
 
 
 
Figure 1 - Heptaisobutyl titanium-POSS (Ti-POSS) 
Results and discussion 
 
The results of bulk ROP of L-lactide (LLA), carried out at 130°C with different polymerization times 
(tp) and [LLA]/[Ti] ratios, are shown in  Table 1.  
The linear correlation between the polymer molecular mass and monomer conversion, coupled with 
low PDIs, are indicative of a well controlled polymerization. Moreover, as the monomer-to-catalyst  
molar ratio [LLA]/[Ti] decreases from 300 to 75, the relative number-average molecular weight, Mn, 
decreases linearly while the PDI values of PLA remain unchanged under the conditions of the same 
Si
O
Si
O
Si
O
O
Si
O
Si
O
Si
O
Si
O
O
Si
O
O
O
R
R
R
R
R
R
R
R
CH
CH3
CH3
CH2 R=
OPri
Ti
 conversion (100%), implying the presence of a substantially controlled polymerization process. Thus, 
Ti-POSS turns out to have high reactivity in the controlled ROP of LA. 
 
Table 1 - Polymerization of LLA using Ti-POSS 
Entry  tp 
(h) 
[LLA]/[Ti]  conv. 
(%) 
Mn  Mw  Mw/Mn  Mv 
1  4  300  22  7840  9670  1.23  9000 
2  6  300  48  16950  21020  1.24  19000 
3  8  300  80  23500  29370  1.25  25000 
4  12  300  100  28600  37180  1.30  33000 
5  24  300  100  28200  38130  1.35  32000 
6  24  75  100  10300  12355  1.30  11700 
 
Moreover, 
1H-NMR measurements have demonstrated that POSS is chemically bound to PLA chains. 
On  the  basis  of  these  results,  it  is  possible  to  assume  that  the  polymerization  proceeds  via  a 
coordination-insertion mechanism. This process involves the insertion of lactide unit into Ti-O bond of 
a silsesquioxane molecule and the selective cleavage of the acyl-oxygen bond in lactide, which results 
in the formation of ester end group.  
The evaluation of the thermal properties has evidenced that the crystallinity of the systems, prepared 
by  using  Ti-POSS  as  initiator,  is  much  higher  than  that  of  commercial  polymers  with  similar 
molecular masses.  
 
Conclusion 
 
The  present  work  has  demonstrated  the  possibility  to  use  Ti-based  POSS  in  the  ROP  of  lactide, 
polymerization  which  occurs  through  a  coordination-insertion  mechanism.  Moreover,  the  direct 
insertion of the silsesquioxane molecule to the polymer backbone, which was assessed by 
1H-NMR 
measurements, has evidenced that the M-POSS promotes not only the polymerization of L-lactide, 
functioning as initiator, but allows also the attainment of a hybrid system.  
Another  relevant  aspect,  which  has  been  underlined  by  comparing  the  thermal  behavior  of  the 
synthesized  PLA  with  commercial  samples,  is  the  capability  of  POSS  to  enhance  the  polymer 
crystallinity.  This  finding  has  been  ascribed  to  the  local  polymer  chain  deformation  due  to  the 
tendency of POSS moiety, attached at one end of PLA chains, to aggregate. 
 
References 
 
1. H. R. Kricheldorf; M. Berl; N. Scharnagl Macromolecules 1988, 21, 286. 
2. J.M. Fraile; J.I. García; J.A. Mayoral; E. Vispe J. Catal. 2005, 233, 90. 
3. E. Kim; E. Shin; I.-K. Yoo; J. S. Chung J. Mol. Catal. A-Chem. 2009, 298, 36–39 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

On the use of Ti-POSS in the ring opening polymerization of L-LA

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