Silicon-containing chain extenders

Details Silicon-containing chain extenders Silicon-containing chain extenders

2000-01-14

2002-07-16

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C528S044000, C528S075000, C528S083000, C528S085000, C525S464000, C525S410000, C525S474000

Reexamination Certificate

active

06420452

ABSTRACT:

The present invention generally relates to silicon-containing chain extenders and their use in the preparation of polyurethane elastomeric compositions having improved properties. These polyurethane compositions are useful for a variety of applications, in particular the manufacture of medical devices, articles or implants which contact living tissues or bodily fluids.
Polyurethane elastomers are amongst the best performing synthetic polymers in medical implant applications. Their excellent mechanical properties coupled with relatively good biostability make them the choice materials for a number of medical implants including cardiac pacemakers, catheters, implantable prostheses, cardiac assist devices, heart valves and vascular grafts. The excellent mechanical properties of polyurethane elastomers are attributed to their two phase morphology resulting from microphase separation of soft and hard segments. In polyurethanes used for medical implants, the soft segment is typically formed from a polyether macrodiol such as poly(tetramethylene oxide) (PTMO) while the hard segment is derived from a diisocyanate such as 4,4′-methylenediphenyl diisocyanate (MDI) and a diol chain extender such as 1,4-butanediol (BDO).
The diol chain extender which is used to link up diisocyanates is a relatively small difunctional molecule of molecular weight between about 60 and 350. The structure of the chain extender makes a significant contribution to the physical properties of the polyurethane elastomers. The most commonly used diol chain extender is 1,4-butanediol.
Despite the long term use of polyurethane elastomers for applications such as cardiac pacemakers, in some cases the polyurethanes biodegrade causing surface or deep cracking, stiffening, erosion or the deterioration of mechanical properties such as flexural strength
1
. Elastomers with high flexibility and low Shore A Durometer hardness in particular degrade faster than the harder and more rigid grades. It is generally hypothesized that the degradation is primarily an in vivo oxidation process involving the polyether soft segment. The currently used medical polyurethanes are polyether-based and the most vulnerable site for degradation is the methylene group alpha to the ether oxygen
2
of the soft segment. Polyurethanes prepared with a lower amount of polyether component generally exhibit improved degradation resistance. However, such materials typically have high elastic modulus and are difficult to process making them less desirable for many implant applications. Pinchuk has recently reviewed the biostability of polyurethanes
3
.
Non-PTMO based polyurethane formulations which show significantly improved in vivo degradation resistance as demonstrated by animal implant experiments have also recently been disclosed in the patent literature. These include polyurethane formulations based on polycarbonate macrodiols disclosed in U.S. Pat. No. 5,133,742 (Pinchuk) and U.S. Pat. No. 5,254,662 (Szycher) and polyether macrodiols with fewer ether linkages in U.S. Pat. No. 4,875,308 (Meijs et al). The aforementioned patents do not disclose polyurethane formulations which provide materials having flexural modulus, hardness and biostability comparable to those of silicon rubber while retaining high tensile strength, abrasion resistance and tear strength of typical polyurethane elastomers. Although the compositions disclosed in U.S. Pat. No. 5,254,662 provide materials with low elastic modulus and high tensile strength, since those compositions are based on polycarbonate macrodiols and aliphatic diisocyanates, their degradation resistance under in vivo conditions is questionable. Hergenrother et al
4
have demonstrated by animal implant experiments that aliphatic diisocyanate based polyurethanes degrade more than the aromatic diisocyanate based polyurethanes. There are also no examples provided in U.S. Pat. No. 5,254,662 to demonstrate the biostability of the disclosed low modulus elastomer compositions.
The conventional method of preparing polyurethane elastomers with low hardness and modulus is by formulation changes so as to have a relatively higher percentage of the soft segment component. However, the materials made this way generally have very poor mechanical properties and biostability. For example, it is reported
2.1
that Pellethane 2363-80A (Registered Trade Mark) which has a higher percentage of soft segment than that in the harder grade Pellethane 2363-55D (Registered Trade Mark), is significantly more prone to stress cracking in the biological environment. However, these reports do not disclose methods for formulating polyurethanes with hardness lower than 80 A while retaining good biostability and mechanical properties. Despite the good stability of silicone rubber in biological environments, its use in the medical implant area is limited by poor properties such as low abrasion resistance and low tensile and tear strengths.
Although the aforementioned non-PTMO based polyurethane elastomers address the issue of biostability, they do not provide methods of formulating polyurethanes having properties such as flexibility and biostability comparable to those of silicone rubber. The formulations disclosed in the above patents (except U.S. Pat. No. 5,254,662) typically have hardness in excess of Shore 80 A.
A requirement accordingly exists to develop polyurethanes having properties such as low durometer hardness, low flexural modulus, good processability and high resistance to degradation, without the disadvantages of silicone rubber such as poor tensile strength, abrasion resistance and tear strength. Such polyurethanes should also preferably have a good biostability for applications such as pacemaker leads, vascular grafts, heart valves and the like.
According to one aspect of the present invention there is provided a chain extender including a silicon-containing diol of the formula (I):
wherein
R
1
, R
2
, R
3
, R
4
, R
5
, and R
6
are the same or different and selected from an optionally substituted straight chain, branched or cyclic, saturated or unsaturated hydrocarbon radical;
R
7
is a divalent linking group or an optionally substituted straight chain, branched or cyclic, saturated or unsaturated hydrocarbon radical; and
n is 0 or greater, preferably 2 or less.
The present invention also provides use of the diol of the formula (I) defined above as a chain extender.
The present invention further provides the diol of the formula (I) as defined above when used as a chain extender.
The hydrocarbon radical for substituents R
1
, R
2
, R
3
and R
4
may include alkyl, alkenyl, alkynyl, aryl or heterocyclyl radicals. It will be appreciated that the equivalent radicals may be used for substituents R
5
, R
6
and R
7
except that the reference to alkyl, alkenyl and alkynyl should be to alkylene, alkenylene and alkynylene, respectively. In order to avoid repetition, only detailed definitions of alkyl, alkenyl and alkynyl are provided hereinafter.
The term “alkyl” denotes straight chain, branched or mono- or poly-cyclic alkyl, preferably C
1-12
alkyl or cycloalkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, amyl, isoamyl, sec-amyl, 1,2-dimethylpropyl, 1,1-dimethylpropyl, pentyl, hexyl, 4-methylpentyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2-trimethylpropyl, 1,1,2-trimethylpropyl, heptyl, 5-methylhexyl, 1-methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1,2-dimethylpentyl, 1,3-dimethylpentyl, 1,4-dimethylpentyl, 1,2,3-trimethylbutyl, 1,1,2-trimethylbutyl, 1,1,3-trimethylbutyl, octyl, 6-methylheptyl, 1-methylheptyl, 1,1,3,3-tetramethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-methyloctyl, 1-, 2-, 3-, 4- or 5-ethylheptyl 1-, 2- or 3-propylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8-methylnonyl, 1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-

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