Synthesis and Properties of Silicones and Silicone-Modified Materials

Chapter 2

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 15, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch002

Controlled Synthesis of All Siloxane-Functionalized Architectures by Ring-Opening Polymerization J. Chojnowski, M. Cypryk, W. Fortuniak, K. Kaźmierski, K. Rózga-Wijas, and M. Ścibiorek Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Łódź, Poland

Ring-opening copolymerization of cyclotrisiloxanes was used for the synthesis of various well-defined all-siloxane block and gradient copolymers. These copolymers contain functional groups in organic radicals and are specifically functionalized at the chain end. They were also further exploited for synthesis of the star-branched and dendritic-branched structures. They were also attached to the surface of silica to obtain silica-siloxane hybrid materials.

There has been an increasing interest in using polysiloxanes as fragments of various macromolecular architectures, due to unusual combination of properties of these polymers. The polysiloxane chain has exceptionally high dynamic and staticflexibility,which is related to very low barriers of rotation around the Si-0 bond and of linearization of the Si-O-Si angle (i). Thus, the polysiloxane chain has a very high conformational freedom easily adopting various shapes. It adapts

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© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


13

itself readily to its surrounding and the functional groups attached to polysiloxanes are available for the interaction with neighboring molecules. Taking into account that substituents appear only at every second atom in the chain, the Si-0 bond is relatively long (1.63 Â) and the SiOSi angle unusually large (145°), the polysiloxane flexibility is not much restricted by substituents unless they are very bulky. The nature of the polysiloxane backbone is inorganic, which gives this polymer a high thermal stability and also, in some sense, an amphiphilic character. Its inorganic skeleton is formed of strongly polar Si-0 bonds, but it bears nonpolar organic groups. Due to this feature, the polysiloxane tends to go to the interface, adopting a conformation in which its polar skeleton sticks to more hydrophilic surface, while organic groups are directed towards more hydrophobic surface. In this way it decreases the interfacial surface tension. Hybrids of polysiloxanes with many organic polymers are very attractive materials. Particular attention has been paid to siloxane-organic block and graft copolymers (2-4). Polysiloxanes in combination with organic polymers are also exploited for the construction of more complex branched structures, such as star shape (5) and dendritic (cascade shape) copolymers (6,7) as well as for the formation of cross-linked materials (8). In contrast to very broad literature on siloxane-organic hybrids, relatively little attention has so far been devoted to the generation of all-siloxane macromolecular architectures, see for ex. refs (6,9,10). Complex structures based on all-siloxane inorganic skeleton may be attractive as new materials of a high thermal stability, good solubility, expected interesting morphology and surface properties. Polysiloxanes may be readily modified within organic groups, which can dramatically change their behavior. For example, introduction of hydrophilic groups may make the polymer water soluble (11-13). Combination of such a polymer in one hybrid with hydrophobic polydimethylsiloxane (PDMS) may lead to very interesting amphiphiles (11). On the other hand, combination of PDMS as soft segment with a stiff crystalline polysiloxane may give materials of interesting morphology leading to a high mechanical strength and improved thermal stability (10). The purpose of our study is to elaborate methods of the controlled synthesis of all-siloxane copolymers of various topologies, such as diblock, triblock, multiblock, star-branched and dendritic-branched structures. Our general approach is to synthesize various structures being combinations of the two types of segments, the one composed exclusively of hydrophobic, inert dimethylsiloxane units and the other containing siloxane units functionalized in organic groups. The functionalized units could appear as the sole units in the segment, but it is often preferable to "dilute" them with dimethylsiloxane units. Thus, the segment itself may be the siloxane-siloxane copolymer. The functions in organic groups are to give the polymer special physical or biological properties, such as hydrophilicity, biocidal properties,


14 spectral properties and others or provide the polymer with specific chemical reactivity, such as complex formation ability or catalytic properties.


Generation of Functionalized Polysiloxane Segments Anionic ring opening polymerization and copolymerization of cyclotrisiloxanes is the method giving the best possibility of the control of the structure of the polysiloxane product. It is well known that a significant reduction of back biting and chain transfer, leading to a narrow molecular weight distribu tion, may be achieved in these processes (14). High precision of functionalization of chain end is also possible, which is crucial for the controlled generation of macromolecules of more complex topologies. The controlled functionalization of side groups may also be achieved by the polymerization and copolymerization of cyclotrisiloxanes containing functions in organic substituents. Two types of functionalized cyclotrisiloxanes were used, these having functional groups in all siloxane units and those having two dimethylsiloxane groupings and one functionalized siloxane unit. The representative of the second type is 2imidazolopropylpentamethylcyclotrisiloxane (equation 1). Its polymerization leads to polysiloxane with imidazole functions (15a).

The polymerization of a monomer with the target functional group is often inconvenient or impossible. Acidic or electrophilic groups do not tolerate anionic propagation centers. Some functional monomers are difficult to purify. For example, the removal of traces of a protic contamination, such as water, alcohol, acid from the imidazole-substituted cyclotrisiloxane is troublesome (15a). Interaction of propagation center with the functional group may promote chain transfer or back biting (16). For these reasons, the preferred synthetic strategy is to use monomers bearing precursor groups which are transformed into target functions in polymer. The most common precursors are: — Si-CH=CH , = Si-Η and ~ Si(CH ) Cl. The anionic ROP of (ViMeSiO) ,1 (17), ViMeSiO(Me SiO) ,2(17) and Vi SiO(Me SiO) , 3 (18) on lithium silanolate centers was studied. The polymerization proceeds with full preservation of vinyl groups and leads to a narrow molecular weight distribution. Recently, Paulasaari and Weber (19) 2

2

2

3

3

2

2

2

2


15

succeeded in synthesis of HMeSiO(Me SiO) , 4, and showed that this monomer may be chemoselectiveiy and regioselectively polymerized on the lithium silanolate center in THF at -70°C. Although the chloropropyl bonded to silicon was not expected to tolerate the silanolate propagation center (20), both monomers [Cl(CH ) MeSiO] ,5, and Cl(CH ) MeSiO(Me SiO) ,6, polymerized smoothly on the lithium silanolate centers in THF at ambient temperature with full control of the polymer product (21). Polymerization of the monomers with mixed units, such as 2,3,4 and 6, leads to the copolymer of functional siloxane with dimethylsiloxane. Since the propagation is not accompanied by any chain cleavage reaction and the monomer enters the chain undivided, the distribution of functional units in macromolecule is uniform. Each monomer unit contains one functional group. The order of units may be regular if the propagation occurs regioselectively, i.e., the ring is opened exclusively at one site. This is the case of the polymerization of 4 at low temperature (22). Usually, the regioselectivity is limited as various structural factors may be important in choosing of the place ofringopening. There are three nonequivalent sites of the ring opening noted by a, b and c in equation 2. 2


2

3

3

2

3

2

2

2

X I

I

I I X

N/v^D^DllsokiO" Mt (2) +

I

I

I I X

^k)k)èioèo"Mt I

I

+

I I

29

Basing on Si NMR data, the sequential analysis was made at the pentad level using 1 order Markov chain statistics for the polymerization of [Ph SiO(Me SiO) ], 7, (23) and 6 (22). Since the method does not differentiate between the opening at a and b, additional experiment of initiation was performed allowing to determine the order in the first monomer unit. These combined methods gave relative rates of ring opening at a, b and c (Table I). The site of the ring opening depends on both the substituents and the active propagation center. Substituent X is electron-withdrawing as compared with methyl, thus, substituted silicon is the most electrophilic center to which the attack of the silanolate anion is likely to be directed, leading to opening at a. This is the case of the polymerization of vinyl substituted monomer 2 on the lithium silanolate center in THF, where the opening at a prevails, although the regioselectivity is low. However, it significantly increases when the reaction is performed at lower temperature (Table I). Another course of the polymerization st

2

2

2


16


in the same system (=SiOLi/THF) was observed for monomer 7. The ring is preferentially opened at c, which may be explained either by a smaller steric effect or by formation of an intermediate complex between monomer and counter-ion (23). On the other hand, potassium silanolate complexed by crown ether opens the monomer 7 mostly at b which is interpreted by preferential formation of more stable silanolate anion, the one with negative charge delocalized to phenyl groups.

Table I. Regioselectivity of monomer opening in various polymerization systems

Monomer

Initiator

7

Me SiOLi/THF 3

Temp

50°

% of the monomer ring opening , , Γ" \ Rf iarkov 1 Markov 1 order order Initiation Initiation statistics (%) experiment e

%

st st

17 35

33

18

49

23

12

48 48

10

73

17

23

35

7

Me SiOK/ 18-crown-6

50°

72 il

13 72

15 15

2

BuLi/THF

-30°

89 8

8 89

3 3

2

BuLi/THF

25°

67 21

21 67

12 12

3

22

78

11

11

22

The copolymerization of a functionalized cyclotrisiloxane with hexamethylcyclotrisiloxane (D ) leads to a copolymer of dimethylsiloxane and siloxane containing functional group which, similarly to the copolymer obtained by polymerization of cyclotrisiloxane with mixed units, contains macromolecules of uniform structures with regard to the size, monomer unit composition and sequencing. However, the distribution of units along the chain is different. The more reactive comonomer enters the chain preferentially, thus the density of the units derivedfromthis monomer in polymer is high at the beginning of the chain formation. It generally decreases during the chain growth as the contribution of the more reactive monomer in the feed decreases. This leads to gradient distribution of the functional groups along the copolymer chain. 3


17


Synthesis of Functionalized All-Siloxane Block Copolymers Block coupling is the method often used for synthesis of siloxane block copolymers (2,3). However, the higher precision of the synthesis may be achieved by building blocks either using macroinitiator or exploiting the sequential copolymerization. The latter method, i.e., sequential copolymerization of cyclotrisiloxanes according to general equation 3, is particularly suitable for the precision synthesis of diblock and triblock all-siloxane functionalized copolymers. This reaction was used as early as in sixties by Bostick in attempts to generate silicone plastic elastomers (P). For this purpose he obtained in a controlled way block copolymers composed of flexible polydimethylsiloxane segments and rigid crystalline polydiphenylsiloxane segments (P). This concept was continued later by Meier et al. (10). Elastomers with good mechanical properties were obtained, but too high melting point of the crystalline domains formed by poly(Ph SiO) segments was the obstacle to their practical use. 2

THF

l/'

Synthesis and Properties of Silicones and Silicone-Modified Materials

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