Branching and pseudo-living technology in the synthesis of high performance fluoroelastomers.

Fluoropolymers are designed for high demanding applications in hostile environments. They represent a high level choice in applications where chemical environment and high temperatures are the dominating factors (ref. 1).

The development of fluorine containing polymers followed the synthesis of low molecular weight polychlorotrifluoroethylene (PCTFE) and the accidental discovery of polytetrafluoroethylene (PTFE) in the late 1930s.

The unique combination of properties of PTFE was immediately apparent, like the difficulties of transforming it into appropriate shapes. In fact, the viscosity of PTFE exceeds 10 billion poise and transformation techniques similar to metal sintering and ceramics have to be used. The need for easier processable polymers led to the development of different highly fluorinated plastics and elastomers.

Although many fluoropolymers have been prepared, major commercial products are homopolymers and copolymers deriving from free radical polymerization of a limited number of fluoromonomers, such as tetrafluoroethylene (TFE), vinyl fluoride (VF), vinylidene fluoride (VF2), chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), perfluoropropylvinylether (PFPVE) and perfluoromethylvinylether (PFMVE).

They mostly find applications in the chemical process industry to manufacture sheets, tubes and fittings working in severe environments, and in the wire and cable industry due to their low dielectric constant.

Vinylidene fluoride (VF2) based fluorocarbon elastomers are the most common fluorinated elastomers with outstanding swelling resistance to aliphatic and aromatic hydrocarbons at high temperatures (ref. 2). For this reason, the main applications are in the automotive sector, for seals, hoses and other engine seals, designed to operate in contact with motor oils and fuels at high temperature. VF2 based fluorocarbon elastomers are mainly copolymers of VF2 with hexafluoropropylene (HFP) and in a smaller volume, terpolymers of VF2, HFP and tetrafluoroethylene (TFE). The presence of TFE has the main purpose of increasing the total fluorine content that gives improved thermal and chemical resistance in many different environments, especially when the elastomeric part is in contact with chemicals having a polar character. Outstanding thermal and chemical resistance in extreme conditions is obtained when the elastomeric structure is completely fluorinated, as in the case of copolymers of TFE and PFMVE.

In recent years, the new performance needs and the increased demand for high quality fluoropolymers are requiring the capability of designing and producing polymer structures using new concepts arising from material sciences and macromolecular chemistry. Precise control of macromolecular structure and architecture is becoming a dominant factor necessary to obtain polymers that are able to satisfy new specifications in advanced applications. The challenge of modern polymer engineering is the capability of producing a tailor-made polymer, that is, a polymer with a complex morphology designed to match the ever-improving properties required by the market.

In this connection, it is worth noting that fluoropolymers can only be produced by free radical polymerization. In this kind of polymerization, the presence of chain breaking events, such as radical coupling and disproportionation, renders the control of the molecular weight and its distribution (MWD), and in the case of multipolymerization, the fine control of microstructure, very difficult. In particular, highly fluorinated fluoropolymers are essentially linear polymers, since the high C-F bond energy prevents chain branching reactions. Although linear polymers offer, in many cases, advantages such as better flow and physical properties, more complex chain architectures, such as star chains and block copolymers, are highly desirable, since they can provide new useful behaviors. Accurate control of the microstructural properties of a polymer during its synthesis is among the main objectives of the current polymer research.

The capability of the currently available polymerization technologies for producing polymers with a complex microstructural architecture will be discussed. Starting from the classical free radical polymerization, we will then highlight the peculiarities of the relatively new pseudo living polymerization process, coming finally to the innovative branching and pseudo living technology that enables the production of fluoropolymers with a tightly controlled molecular architecture.

Free radical polymerization of fluorinated monomers

The general mechanism of the radical polymerization of fluorinated monomers includes six elementary reactions (table 1): (1) the initiation by the homolytic cleavage of a molecule with low thermal stability (usually a peroxide); (2) the chain propagation; (3) the chain transfer to monomer; (4) the chain transfer to polymer; (5) the terminal double bond reaction; and (6) the bimolecular termination (through disproportionation or combination) (ref. 3).

Table 1

             1   [right arrow]   2R*              (Initiation)

       R* + nM   [right arrow]   [MATHEMATICAL    (Propagation)
                                 EXPRESSION NOT
                                 REPRODUCIBLE
                                 IN ASCII]

[MATHEMATICAL    [right arrow]   [P.sub.n] + R*   (Transfer to monomer)
EXPRESSION NOT
REPRODUCIBLE
IN ASCII]

[MATHEMATICAL    [right arrow]   [MATHEMATICAL    (Transfer to polymer)
EXPRESSION NOT                   EXPRESSION NOT
REPRODUCIBLE                     REPRODUCIBLE
IN ASCII]                        IN ASCII]

[MATHEMATICAL    [right arrow]   [MATHEMATICAL    (Terminal double bond
EXPRESSION NOT                   EXPRESSION NOT   reaction)
REPRODUCIBLE                     REPRODUCIBLE
IN ASCII]                        IN ASCII]

[MATHEMATICAL    [right arrow]   [P.sub.n] +      (Disproportionation)
EXPRESSION NOT                   [P.sub.m]
REPRODUCIBLE
IN ASCII]

[MATHEMATICAL    [right arrow]   [P.sub.n+m]      (Combination)
EXPRESSION NOT
REPRODUCIBLE
IN ASCII]

The molecular weight (MW) and the molecular weight distribution (MWD) of the polymer can be modified through a suitable choice of the operating parameters, that is the initiator and monomer concentration and the polymerization temperature. The molecular weight of the polymer decreases when the initiator concentration is increased or the monomer concentration is reduced. However, in this way just a weak control of the MWD is obtained. In particular, the transfer to polymer mechanism coupled with the terminal double bond reaction and the bimolecular termination through combination may lead, depending on the reaction conditions, to the formation of highly branched macromolecules up the point of gel formation as experimentally observed (ref. 4). A more effective way to control MW and MWD is to use a chain transfer agent (CTA). The CTA causes a decrease of the molecular weight of the polymer by introducing another elementary step in the kinetic scheme:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (transfer to chain transfer agent) Because of the high reactivity of fluorocarbon active chains, almost any type of organic compound can act as a chain transfer agent. The use of a CTA not only reduces the average molecular weight, but also reduces the polydispersity index. As an example, in figure 1, the MWD of a fluoroelastomer copolymer, VF2/HFP produced without a CTA is compared with the MWD of the same copolymer obtained using ethylacetate as CTA. For a better comparison, the MWD curve of the latter copolymer has been shifted along the X-axis towards higher molecular weights so as to super-impose the low molecular weight regions of the two MWDs. In this way, the narrowing of the MWD caused by the use of the CTA is highlighted. The polydispersity index which is equal to five for the polymerization without the CTA is decreased to 2.9 when the ethylacetate is used as CTA. It is worth noting that in the synthesis of these fluoroelastomers, since termination through combination is negligible, the polidispersity index, in accord with the theory, is always greater than two. The index is equal to two only in the case of the absence of chain transfer to polymer and of the terminal double bond reactions.

[GRAPH OMITTED]

The pseudo living polymerization of fluorinated monomers

A living polymerization is defined as a polymerization process where termination and transfer are absent or, in other words, where each growing chain lives all along the reaction. The goal of these peculiar polymerizations is to produce polymers having a very narrow molecular weight distribution and optionally, to produce either fully random or sequential block copolymers.

These processes were first proved possible and effective for anionic or cationic polymerizations of alkenes, ring opening polymerizations and various other ionic polymerizations (refs. 5 and 6). In this kind of polymerization, the termination by coupling is prevented by the polar chain ends, while transfer is the only irreversible chain-stopping event. However, very recently, several works dealing with free radical living polymerizations have appeared in the literature. Although, in this case, irreversible coupling termination cannot be totally avoided (and this is the reason for the definition "pseudo-living"), if this step is sufficiently prevented, living conditions are practically established.

A comprehensive review of the kinetic mechanisms that make living free radical polymerization possible has been given by Greszta et al (ref. 7). Living systems are based on reactions capable of preventing the polymer chains from coupling or terminating by transfer. In order to establish such conditions, the concentration of the growing radicals in the reaction system should be kept very low. This can be done by taking advantage of exchange equilibrium between active chains, which actually polymerize, and dormant chains, unable to add monomer. In fact, in the case where no transfer to monomer is present, if the equilibrium is sufficiently shifted towards the dormant chains, the coupling termination rate, which is proportional to the squared radical concentration, is negligible with respect to the rate of chain propagation, thus resulting in a pseudo-living polymerization. Moreover, if initiation is fast (typically of the order of the propagation rate), i.e., all the polymer chains are produced in a short interval of time at the beginning of the polymerization, all the growing chains should have the possibility of polymerizing in the same conditions (monomer and polymer concentration, temperature, etc.) by means of the continuous activity exchange, and of living approximately for the same time. In other words, the radical functionality on polymerizing chain ends is continuously (and very quickly) shifted from chain to chain, thus allowing the growing species to add approximately the same number of monomer units during each of their many growth periods. This results in a very narrow molecular weight distribution and, in fact, the polydispersity ratio for these processes can be lower than 1.5, the minimum value obtained in ordinary free radical polymerizations.

There are various methods to achieve a pseudo-living free radical polymerization. One is based on the principle of the degenerative exchange, where the radical concentration is kept low using a very small initial amount of initiator. The degenerative exchange reaction, then, shifts radical functionality from chain to chain:

(1) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is a radical of length n and [P.sub.n]X is the reversibly terminated chain. This is the case, for instance, of fluorinated polymers and will be dealt with here.

It is well known that perfluoroalkyl iodides easily generate perfluoroalkyl radicals, by cleavage of the C-1 bond, initiated by heating, UV or peroxides. In order to establish pseudo-living conditions, a chain transfer agent, containing the C-1 bond, can be used for the degenerative exchange reaction (ref. 8). The homolytic dissociation energy of the carboniodine bond is low enough (55Kcal/mole) to allow the shift of iodine from chain to chain, according to the scheme shown in equation 1. In particular, iodo-substituted fluorinated molecules of the general formula, Rf-I, where Rf is a perfluoroalkyl group, turned out to efficiently support pseudo-living polymerization conditions. Moreover, in order to reduce as much as possible the irreversible termination mechanisms, the polymerization reaction must be performed with an initial amount of initiator sensibly lower than that typical of ordinary processes (up to two orders of magnitude). Since this would cause the polymerization rate to be very low if carried out in a traditional emulsion system, to obtain acceptable polymerization rates, the polymerization reaction can be performed in a microemulsion environment (ref. 9), so as to increase the number of polymer particles per unit volume of water.

Under these conditions, the molecular weight of the macromolecules increases linearly with conversion following a stepwise chain growth mechanism. When an active chain reacts with [R.sub.f]I it undergoes a reversible termination.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The polymer chain, [P.sub.n]I is only reversibly terminated (dormant reactive species) because the carbon-iodine bond can be easily broken by a radical and the chain can restart growing:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The reactivity of the iodine chain-end leads to a continuous chain extension through an activation-deactivation mechanism of the polymeric chain. The polymerization is not fully living because irreversible termination events such as the chain transfer to monomer and the bimolecular termination can not be completely avoided. However, the MWD of the polymer produced with this technology is very narrow, the polydispersity index being less than two, thus confirming the pseudo living evolution of the polymerization. A comparison between the MWD obtained using a classical CTA and a iodo-substituted fluorocarbon is reported in figure 2. It should be noted that the polydispersity index of the MWD obtained using the [R.sub.f]I is 1.7 [is less than] 2. As previously stated, the termination through combination being negligible, the polydispersity index must be greater than two. The experimental evidence of polydispersity index lower than two can be explained only assuming a living behavior of the polymerization process.

[GRAPH OMITTED]

The branching and pseudo-living technology

Besides the interest that by itself the pseudo-living free radical polymerization can offer for the synthesis of polymers having narrow MWD, this methodology is also important because it is the basis for the implementation of a more advanced technology. This technology can be applied to the production of polymers having more complex molecular architecture. This goal is obtained by adopting the concept of controlled branching of the polymeric chains.

In order to realize the above concept, it is necessary to find a polyfunctional molecule able to react with the growing polymer chains and to link them in a strictly controlled way, i.e., only the desired links must be done, avoiding all other reactions.

We have found that this can be achieved using a highly reactive fluorinated diene, and introducing it in the reactive medium in a starved way. Because the fluorinated diene is so reactive, the entire amount introduced is immediately incorporated into the growing chain and then immediately produces a link with another growing chain. As a consequence, the amount of crosslinks produced is exactly determined by the amount of fluorinated diene introduced in the reaction.

Fluorinated dienes of the general formula [CH.sub.2] = CH - [([CF.sub.2]).sub.n] - CH = [CH.sub.2] have proved able to satisfy the above conditions. Their hydrogenated electron rich double bonds guarantee a very fast reaction with the electron poor fluorinated growing polymer radicals. In particular, the 1,6 divinylperfluoroexane is very efficient and an easily controlled crosslinker during polymerization of fluoromonomers. Moreover, the formation of dormant species following the reaction of the growing polymer radicals with the fluorinated diene, due to the pseudo-living polymerization environment, allows the formation of branches of similar length. This results in the formation of "star"-like polymers.

Both pseudo-living and the controlled branching concepts have been used to design the polymer chain morphology that matches the desired applicative properties. As an example, in figure 3, the MWD of a copolymer produced with the branching and pseudo-living technology is compared with the MWD of the same polymer obtained in absence of the fluorinated diolefin. It appears that while the high molecular weight region is deeply modified by the diene, the low molecular weight region is mainly determined by the pseudo-living mechanism. In conclusion, the experimental results show that the two technologies make possible a close control of the shape of the MWD. The pseudo-living concept allows reducing the polydispersity index and the molecular weight, while the controlled branching concept allows increasing the molecular weight, the polydispersity index and the number of iodine-chain ends per macromolecule. This last property is particularly important because the iodine functionality can be used for further reactions such as adding other polymer blocks with different monomer composition and/or for crosslinking reactions, in order to obtain more complex macromolecular architectures.

[GRAPH OMITTED]

Branching and pseudo-living technology can be exploited for the production of a series of different materials having structural features designed to fulfill desired application needs. Examples are the synthesis of peroxide curable fluoroelastomers and of fluorinated thermoplastic elastomers. In all cases, maximizing the number of iodine chain ends per macromolecule is desirable and, as it has been shown before, branching and pseudo-living technology allows a precise tuning of it (ref. 10).

Synthesis of peroxide curable fluoroelastomers

Early peroxide curable fluoroelastomers have been obtained by incorporating into the macromolecular chain a "cure site" monomer susceptible to radical attack. Along this line, iodine terminated chain ends can be used to cure the elastomer, taking advantage of the low energy of the carbon-iodine chemical bond. In this connection, it is worth noting that the mechanical stability of the network is strictly related to the number of iodine chain end groups per macromolecule: The higher this number, the better the network. A very important point is that by using the pseudo-living process, the polymer chains have less than two iodine chain end groups per macromolecule (ref. 11), and thus, the curing process leads to poor results in terms of mechanical properties and compression set. By using the branching and pseudo-living approach, however, the number of iodine chain end groups per macromolecule can be increased until values much higher than two are achieved, thus improving the mechanical properties and compression set of the final elastomeric network.

From the scheme in figure 4, it appears that the pendant double bond reaction, by linking two macromolecules into one, provides a step growth of the molecular weight, number of long chain branches and number of iodine chain end groups of the resulting macromolecule. The polymer chain structure is thus deeply modified, and in particular, the number of iodine chain end groups per macromolecule can be much higher than two.

[ILLUSTRATION OMITTED]

This feature enables us to obtain polymer networks with improved properties. For example, in table 2, the properties of a fluoroelastomer synthesized with the pseudo-living technology (with 1.8 iodine chain ends per macromolecule) are compared to those of a fluoroelastomer produced with the branching and pseudo-living process (with 3.3 iodine chain ends per macromolecule). It clearly appears that the second one has better properties.

Table 2 - mechanical and sealing properties of
block copolymers after press molding (180 [degrees] C,
5 minutes) and annealing (150 [degrees] C, 4  hours.)

                          Polymerization technology

Properties                 Pseudo-   Branching and
                           living    pseudo-living

Iodine atoms per             1.8          3.2
  macromolecule
Modulus 100% (MPa)           4.8          5.8
Tensile strength (MPa)        21           21
Elongation at break (%)      270          242
Hardness (durometer A)        71           73
Compression set (200          32           19
  [degrees] C/70 h.)

Starting from these results, Ausimont has developed and commercialized a new class of peroxide curable fluoroelastomers synthesized with the branching and pseudo-living technology: Tecnoflon P/PL fluoroelastomers. Currently, more than 10 grades of different composition, ranging from 67 up to 72 wt% fluorine content are commercially available. It is important to note that these polymers demonstrate better mechanical and sealing properties after very short press cure and post curing cycles. In addition, these polymers show a marked improvement in processability. Thanks to their precrosslinked structure, the viscosity in the melted state of this new class of fluoroelastomer is much lower than that of linear polymers of the same molecular weight. Accordingly, the fluoroelastomer easily fills even elaborate molds and thus, complex items can be easily molded.

Synthesis of fluorinated thermoplastic elastomers

At present, block copolymers are widely produced using the anionic polymerization process, as well as the cationic and the Ziegler Natta polymerization approaches. In the past years, the attempts to produce block copolymers using the free radical polymerization process were unsuccessful because of the short lifetime (generally less than one second) of the highly reactive propagating radicals.

More recently, block copolymerization has been performed using the pseudo-living approach. The weakness of the iodine-carbon bond is used to produce block copolymers. For this purpose, the reaction is carried out sequentially, producing in a first step the elastomeric segment A and in the second step the crystalline segment B (figure 5).

[ILLUSTRATION OMITTED]

It is worth noting that this technology requires a number of iodine chain ends per macromolecule at the end of the first step at least equal to two, in order to obtain the B-A-B block copolymer. Actually, the elastomeric segment A, in the presence of a di-iodo derivative [Rfl.sub.2], has an experimental iodine functionality always less than two because there are chain ends from the initiator and from the inevitable termination mechanisms such as transfer to monomer and bimolecular terminations (ref. 11). As a consequence, only a fraction of the block polymer would have the A-B-A structure, while other macromolecules would have the A-B or A morphology.

Very recently, a triblock fluorinated copolymer FTPE, has been synthesized using the branching and pseudo-living technology (ref. 13). The small amount of the fluorinated diene, added to the pseudo-living reacting system, gives the important consequence of increasing the number of iodine-chain-ends per molecule (figure 6).

[ILLUSTRATION OMITTED]

Using a suitable amount of the fluorinated diolefin, the iodine functionality of the elastomeric segment A can be increased to two or more than two. In this way, in the second step of the FTPE synthesis, a real B-A-B block copolymer is obtained (figure 7).

[ILLUSTRATION OMITTED]

It is worth pointing out that with the branching and pseudo-living technology, the structure of the FTPE block copolymer can be modified in detail so as to produce a tailormade polymer that matches the required end-use properties. The morphology of the macromolecules is deeply modified by the diene and the produced polymer shows improved mechanical and sealing properties.

A very significant improvement is being achieved moving from the past to the present technology also from the point of view of the crosslinking properties. As a matter of fact, the pseudo-living technology leads to only physically crosslinked structure as indicated in figure 8. The new branching and pseudo-living technology, on the contrary, gives a final item having both physically and chemically crosslinked structure (figure 9).

[ILLUSTRATIONS OMITTED]

The differences in the crosslinked structure of items lead to a different mechanical and sealing behavior. As an example, in table 2 the results are shown for a FTPE in which the elastomeric segment is a VF2/PMVE/TFE terpolymer and the plastomeric segment is a VF2 homopolymer with a melting point of 160-170 [degrees] C. The new technology developed by Ausimont has allowed manufacturing polymers with markedly improved sealing properties at high temperature.

Table 3 - mechanical and sealing properties of
block copolymers after press molding (180 [degrees] C,
5 minutes) and annealing (150 [degrees] C, 4  hours)

                              Polymerization technology

Properties                     Pseudo-   Branching and
                               living    pseudo-living

Modulus 100% (MPa)                 5             6
Tensile strength (MPa)           9.9          11.8
Elongation at break (%)          410           230
Hardness (durometer A)            80            72
Compression set (ASTM):
  100 [degrees] C 24 hr (%)       60            23
  150 [degrees] C 24 hr (%)    Broken           29

This new family of fluorinated polymers can be advantageously used in industrial, automotive, aerospace and chemical/petroleum applications for the manufacture of hoses, tubes, gaskets, o-rings and other molded parts for high temperature service and in contact with very aggressive fluids and chemicals (ref. 14.

References

(1.) "Modern fluoropolymers," J. Scheirs Ed., J. Wiley & Sons, Chicester, 1997.

(2.) Arcella, V. and Ferro, R., in "Modern fluoropolymers," J. Scheirs Ed., J. Wiley & Sons, Chicester, Ch. 2, 1997.

(3.) Apostolo, M., Arcella, V., Storti, G. and Morbidelli, M., Macromolecules, 1999, 32, 989.

(4.) Logothetis, A.L., Prog. Polym. Sci., Vol 14, 251-296, 1989.

(5.) Szwarc, M., Carbanions, Living Polymers and Electron Transfer Processes; Wiley; New York, 1968.

(6.) Flory, P.J., Principles of Polymer Chemistry, Cornell University Press: Ithaca, NY, 1953.

(7.) Greszta, D., Mardare, D. and Matyjaszewski, K., Macromolecules, 1994, 27, 638.

(8.) Tatemoto, M., "Proceedings of the IX International Symposium on Fluorine Chemistry," Daikin Koygo Co., Ltd., Osaka, Japan, 1979.

(9.) Giannetti, E. and Visca, M., U.S. Pat. 4,486,006.

(10.) Arcella, V., Brinati, G., Albano, M. and Tortelli, V., U.S. Pat. 5,585,449.

(11.) Apostolo, M., Arcella, V., Storti G. and Morbidelli M., Macromolecules, submitted (2000).

(12.) Tecnoflon P/PL, Technical Information, Ausimont S.p.A., Viale Lombardia 20, 20021 Bollate (MI), Italy.

(13.) Arcella, V., Brinati, G., Albano, M. and Tortelli, V., U.S. Pat. 5,612,419.

(14.) Tecnoflon FTPE XPL, Technical Information, Ausimont S.p.A., Viale Lombardia 20, 20021 Bollate (MI), Italy.