Silane terminated polymers

The following results give an overview of the possibilities of new silane-terminated polymers (STP) using isocyanatomethylalkoxysilanes (a-NCO-silanes) for endcapping. These STP systems show dramatically enhanced reactivities and curing rates. Furthermore two possible applications in new fast rtv-silicone copolymer systems were examined. 

Reactive adhesives for elastic bonding are available as one-part and two-part systems, based on polyurethane (PUR), polysulphide, silicone or silane-terminated polymers, such as PUR-Hybrids and Modified Silicones (MS). These adhesive systems can he applied cold or warm or mixed with a water-based Booster component. 

Silane-terminated polymers do not cause bubble formation either, see below. 

Apart from the development of polyurethane adhesives and sealants with isocyanate reactive groups, a new type of silane-terminated polymer adhesive has been developed called Polyurethane (PUR)-Hybrids. They and modified silicones (MS) and provide the benefits of ... 

Note of the editor For more information on Silane terminated Polymers the readers should refer to the chapter selyl terminated polymer adhesives and sealants in volume 6 of this Handbook. 

Such silane-terminated polymers cross-link through reaction with water. A small amount of an alcohol is released. 

At present, these silane-terminated polymers have mechanical properties suitable for general use in elastic bonding applications, but usually do not achieve the same high mechanical strengths as pure polyurethane adhesives. 

Pohl, E. and Osterholz, F. D., Novel Vulcanizable Silane-Terminated Polyurethane Polymers, U.S. Patent 4,645,816, 1987. 

The thiocarbonylthio and trithiocarbonate end groups which result from RAFT polymerization can also be converted to hydrogen-terminated polymer in the presence of a free-radical reducing agent, composed of a free-radical source and a hydrogen atom donor. Examples of free-radical reducing agents include tributylstannane, tris(trimethylsilyl)silane, hypophosphite salts, and isopropyl alcohol. ... 

Figure 2 shows the reactivity, measured as tack-free time in STP-S polymer systems (see below) of systems terminated with different NCO-silanes. The new a-silanes show dramatically enhanced curing rates. Additionally, the curing rates of compounds made from these polymers can be varied over a wide rage, using different amounts of catalyst (usually tin) and water scavengers (usually alkoxysilanes). Comparable results were achieved in silane-terminated polyethers and polyurethanes. 

Silane-terminated poiysiioxanes are a new class of high reactive rtv-1 silicone systems. The synthesis of these polymers is shown in Eq. 1. In a first step a hydroxy-terminated polydimethylsiloxane is converted to the corresponding aminopropyl-polydimethylsiloxane by reaction with stoichiometric amounts of 3-[(2,2-dimethyl-l,2-a2asiiolidin-l-yl)dimethylsilyl]-l -propylamine at room temperature. The silane termination reaction in a second step is carried out under the same conditions without adding further catalysts, due to the fast reaction of NCO groups with the primary amino groups. 

The addition of reagents containing X-H bonds in which X is more electronegative than H typically lead to addition across the M-C bond in the direction opposite to the addition of silane or borane to the early metal catalysts. Polymerization of etiiylene with lanthanide catalysts in the presence of phosphines generates phosphine-terminated polymers (Scheme 22.12) - by a mechanism in which the alkyl chain is protonated, and a metal-phosphido complex is generated. This phosphido complex then inserts olefin to start the growth of a phosphine-functionalized polyolefin. Marks subsequently showed that a similar process can be conducted witii amines. In this case, the bulky dicyclohexylamine was needed to sufficiently retard the rate of protonation to allow chain growth. The steric bulk also makes the olefin insertion more favorable thermodynamically. 

In the absence of hydrogen, metallocene-based catalyst systems produce well-defined polymers which are olefin- or aluminum-terminated. Miilhaupt has polymerized propylene with a chiral metallocene and MAO under conditions where P-hydrogen elimination was the predominant chain transfer process. In a post-polymerization functionalization, the olefin endgroups of the highly isotactic polypropylene chains were converted to bromo-, epoxy-, anhydride-, ester-, amine-, carboxylic acid-, silane-, borane-, hydroxy-, thiol-terminated polymers as intermediates for the preparation of block copolymers. Using olefin-terminated atactic and isotactic polypropylene formed with MAO-activated Cp2ZrCl2 and (EBTHI)ZrCl2 Shiono has synthesized amine- and aluminum-terminated polymers." ... 

The above procedure with l-naphthyldimethyl(dimethylamino)silane was applied to four polymers, two of them hydroxyl terminated, the other two with methoxy end groups of unknown, but very low, hydroxyl content. Two nominal molecular weights were chosen for both types of polymers, 600 and 20,000 representing extreme types regarding the difficulties involved in the purification of the silylated polymers by precipitation and filtration. For this operation, the best pair of solvents proved to be 1,2-dimethyloxyethane and low boiling petroleum ether. The samples were redissolved and precipitated repeatedly and each time a portion of the precipitate was subjected to photometric analysis. Apparent hydroxyl contents expressed as a function of the number of precipitations showed that in the case of the hydroxyl terminated polymers, a constant silyl concentration is attained after the first precipitation. Three precipitations were necessary, however, for the methylated polymers in order to obtain consistent results. 

FIGURE 26.22 Phenyl acetylene-terminated poly(carborane-silane) (PACS) polymer. 

Figure 10 Formation of linear and crosslinked polydimethylsiloxane networks. Room temperature vulcanization (RTV) is performed by reacting methoxysilyl-terminated polymer 75 and methyltrimethoxysilane 76. Hydrolysis produces transient silanols 77 and 78 whose dehydration yields silicone precursor 79. Two-part systems are based on the platinum-catalyzed addition reaction between vinyl-terminated polysiloxane 80 and polydimethylsiloxane carrying either only terminal 81 or multiple silane (Si-H) pendent groups. Silicone main chains of the resulting polymer 82 are linked through silicon-...

Another type of sealant uses a combination of chemistry to yield a product useful as a sealant. This chemistry is a marriage of silicone and organic chemistry known as a polyether silicone. The basic polymer for this type of sealant is the reaction product of a polyether diol with a diisocyanate followed by further reaction with an amino-functional alkoxy silane. Thus, the central portion of this polymer is a polyether that is terminated with a urethane linkage that leads to the terminal group which is an alkoxy silane. This polymer can cure by the methods described above for a standard silicone but it contains the properties of a polyether. As with the normal silicones described above, these materials also have to be reinforced in order to obtain the tensile properties desired for a sealant. The same list of fillers as described above can be used in polyether silicones. Adhesion promoters such as silane coupling agents, diluents of various sorts, plasticizers, and catalysts are also added to these materials. 

In choosing a SAM system for surface engineering, there are several options. Silane monolayers on hydroxylated surfaces are an option where transparent or nonconductive systems are needed. However, trichlorosilane compounds are moisture-sensitive and polymeri2e in solution. The resulting polymers contaminate the monolayer surface, which occasionally has to be cleaned mechanically. CarboxyUc acids adsorb on metal oxide, eg, AI2O2, AgO through acid—base interactions. These are not specific therefore, it would be impossible to adsorb a carboxyUc acid selectively in the presence of, for example, a terminal phosphonic acid group. In many studies SAMs of thiolates on Au(lll) are the system of choice. 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

It is difficult to find crosslinking systems that are ideal in that all functional groups are of equal reactivity and intramolecular cyclization is negligible. The crosslinking of vinyl terminated poly(dimethylsiloxane) polymers with tri- and tetrafunctional silanes appears to be an exception. Thus the calculated and experimental pc values were 0.578 and 0.583, respectively, for the tetrafunctional silane and 0.708 and 0.703, respectively, for the trifunctional silane (with r — 0.999) [Valles and Macosko, 1979]. 

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