Polymers used in elastomeric binders for high-energy compositions

Difunctional, hydroxyl-terminated polymers in which the terminal hydroxyl group is non-primary or hindered are end-capped to provide unhindered, primary, terminal hydroxyl groups which promote an improved cure and improved mechanical properties of the cured elastomer. End-capping chemicals have an hydroxyl-reactive group at one end and a group at the other end which is removable to provide a primary, unhindered hydroxyl group. Short-chain, hydroxyl-terminated polymers are chain-extended with a diisocyanate.

The present invention is related to polymers and their use as elastomeric 
binders, particularly those used for elastomeric binders in high-energy 
compositions, such as propellants, explosives, gasifiers or the like. More 
particularly, the invention is directed to end-capping hydroxyl-terminated 
polymers to improve their curability to elastomeric binders and improve 
the mechanical properties of the cured elastomeric binders. The invention 
is also directed to elongating hydroxyl-terminated polymers to enhance the 
mechanical properties of the cured elastomeric binders formed therefrom. 
BACKGROUND OF THE INVENTION 
Solid propellants for rocket motors or the like include a fuel material, a 
high-energy oxidizer and an elastomeric binder matrix in which the fuel 
material and oxidizer are dispersed and immobilized. The binder matrix 
includes the elastomeric binder and, often a plasticizer for the 
elastomeric binder. The elastomeric binder represents an energy limitation 
of a propellant composition in that the binder combination does not 
contribute substantially to the energy of the composition. A good deal of 
attention has therefore been given to developing elastomeric materials, 
useful as binders, having higher energies than the elastomers presently 
used. 
One type of elastomer which has considerable promise as a more energetic 
binder is based upon glycidyl azide polymer (GAP). GAP is a 
hydroxyl-terminated polyether polymer having the general formula: 
##STR1## 
where R is typically a hydrocarbon moiety, such as --CH.sub.2 --CH.sub.2 
--, --CH.sub.2 --1,4--C.sub.6 H.sub.10 --CH.sub.2 --, etc. Having terminal 
hydroxyl groups, GAP can be cured, as is conventional, with polyfunctional 
isocyanates to form elastomers. GAP-based elastomers used as propellant 
binders provide a definite energy advantage relative to polycaprolactone 
(PCP)-based elastomers and polyethylene glycol (PEG)-based elastomers, 
providing a 2-3 second increase in specific impulse (I.sub.sp) relative to 
PEG and an even greater advantage relative to PCP. Unfortunately, the 
mechanical properties of GAP-based elastomers have often been less than 
desired for some applications. To date, GAP, which is derived from 
polyepichlorohydrin (PECH), has been limited in molecular weight to about 
4000, and it is an object of the invention to produce a higher molecular 
weight GAP which would be expected to provide an elastomer with improved 
mechanical properties. Also, because the terminal hydroxyl groups of GAP 
are secondary hydroxyl groups, curing with polyfunctional isocyanates is 
less efficient than is desirable for achieving good mechanical 
characteristics of the cured elastomer. 
Similar curing problems have been encountered with elastomers based upon 
other hydroxyl-terminated polymers where the terminal hydroxyl groups are 
either non-primary hydroxyl groups or the hydroxyl groups are otherwise 
hindered. Of particular interest are polyethers formed from oxetanes and 
combinations of oxetanes and tetrahydrofuran. Such polymers are described, 
for example, in U.S. Pat. No. 4,483,978 to Manser, the teachings of which 
are incorporated herein by reference, and also in U.S. patent application 
Ser. Nos. 925,657-925,660, each filed Oct. 29, 1986, the teachings of 
which are incorporated herein by reference. The oxetane monomers from 
which these polymers are formed generally have the formula 
##STR2## 
and yield mer units having the formula: .paren open-st.CH.sub.2 --CR.sup.1 
R.sup.2 --CH.sub.2 O.paren close-st., where R.sup.1 and R.sup.2 are 
pendant groups. Although the hydroxyl group at the termini of 
oxetane-derived polymers are generally primary alcohols, where R.sup.1 and 
R.sup.2 are bulky groups, e.g., where the terminal mer units have a 
neopentyl type structure, the terminal hydroxyl groups may be sufficiently 
hindered that efficient curing does not take place. 
Accordingly, it is a general object of the present invention to improve the 
curability of polymers which do not cure efficiently due to non-primary or 
hindered terminal hydroxyl groups. It is another general object of the 
present invention to elongate polymer chains which are not otherwise 
produceable in chain lengths consistant with good mechanical 
characteristics of the cured elastomer. 
There have been previous attempts to improve the curability of polymers 
which do not cure efficiently due to hindered, terminal hydroxyl groups. 
One example of such a prior attempt is to end-cap GAP with a low molecular 
weight chemical which provides unobstructed primary hydroxyl groups. In 
this prior attempt, GAP is first reacted with phosgene, and the product is 
then reacted either with a straight-chain molecule having primary hydroxyl 
groups at both ends or with a straight-chain molecule having a primary 
hydroxyl group at one end and a primary amine group at the other end 
according to the following scheme: 
EQU A HO--GAP--OH+COCl.sub.2 .fwdarw.Cl--CO--O--GAP--O--CO--Cl (I) 
EQU B (I)+HO--(CH.sub.2).sub.2-4 --OH.fwdarw.HO--(CH.sub.2).sub.2-4 
--O--CO--O--GAP--O--CO--O--(CH.sub.2).sub.2-4 --OH (II) 
EQU B'(I)+H.sub.2 N--CH.sub.2 --CH.sub.2 --OH.fwdarw.HO--CH.sub.2 --CH.sub.2 
--NH--CO--O--GAP--O--CO--NH--CH.sub.2 --CH.sub.2 --OH (III) 
The end product of reactions B and B' each have terminal, primary hydroxyl 
groups which improve the curability of the polymer. However, an important 
disadvantage of this scheme is that reactions A and B are not 
stoichiometric. In reactions A and B, significant side reaction products 
result from chain extension, i.e., through the second Cl of the phosgene 
in reaction A and through the exposed terminal hydroxyl group of product 
(II) with unreacted (I) in reaction B. To minimize chain extension 
reactions in reaction A, the phosgene is provided in substantial excess. 
Likewise, in reaction B, the dihydroxyl compound is provided in 
substantial excess. Even so, unpredictable chain extensions occur. It is 
desirable to avoid use of an excess of reagents because this leads not 
only to polymer purification problems but also to waste and recovery 
problems, particularly with respect to highly toxic phosgene in reaction 
A. 
An improved method for end-capping hydroxyl terminated polymers is 
desirable. 
SUMMARY OF THE INVENTION 
In accordance with the invention, hydroxyl-terminated polymers, in which 
the terminal hydroxyl groups are non-primary or are hindered, are 
stoichiometrically end-capped to provide the polymer with terminal, 
primary, non-hindered hydroxyl groups. The polymers so modified are more 
efficiently cured, e.g., with isocyanates, and the cured elastomers 
exhibit better mechanical properties. Stoichiometric end-capping is 
achieved through the use of a generally linear chemical compound having a 
functionality at one end which reacts stoichiometrically with the terminal 
hydroxyl groups of the polymer and a functionality at the other end which 
is attached to a protected primary hydroxyl group and which is deprotected 
to provide a free primary hydroxyl group under conditions which (a) do not 
break the bond between the functionality which was previously reacted with 
the previously existing terminal free hydroxyl groups of the polymer and 
(b) does not disrupt the chemical integrity of the polymer chain. 
In cases where the polymer has not been synthesized to a molecular weight 
which is most conducive to good mechanical characteristics of a cured 
elastomer, as is often the case with GAP, stoichiometric chain-extension 
is achieved with a diisocyanate. 
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
The end-capping method of the present invention is applicable to any 
hydroxyl-terminated polymer. It is most useful for polymers in which the 
terminal hydroxyl groups are secondary or even tertiary. It is also 
applicable to polymers having terminal hydroxyl groups which are primary, 
but nevertheless, hindered. Hindered primary hydroxyl groups are those 
having the general formula: 
##STR3## 
where Bk and Bk' represent bulky chemical moities. How bulky the Bk and 
Bk' groups must be to hinder curing of the polymer cannot be precisely 
determined in advance; however, if an efficient cure is difficult to 
achieve, curability may be enhanced by the end-capping method of this 
invention. Terminal hydroxyl groups bonded to a neopentyl type structure, 
i.e., 
##STR4## 
such as are commonly seen on 3,3-disubstituted oxetane-derived polymers, 
generally exhibit cure properties which may be improved by end-capping 
according to the invention. If there is only one bulky group on the carbon 
that is beta to the terminal, primary hydroxyl group, achieving an 
adequate cure is generally not a problem, nevertheless if the one bulky 
group on the beta carbon is sufficiently hindering to the primary hydroxyl 
group, curability of the polymer may be enhanced by end-capping in 
accordance with the present invention. 
End-capping chemical compounds in accordance with the invention have the 
general formula X--Q--(CH.sub.2).sub.n --O--Z; wherein X is a group which 
reacts readily with the preexisting terminal hydroxyl groups of the 
polymer to attach the compound to the polymer; Z is a blocking group which 
does not react with free hydroxyl groups and which may be removed under 
conditions which do not detach the end-capping compound from the termini 
of the polymer and do not affect the polymer backbone; Q is nothing or is 
any chemical moiety which does not interfere with joining the end-capping 
chemical to the polymer, with detaching the Z moiety of the capping 
compound or with subsequent curing of the end-capped polymer; and n is 1 
or 2. 
Examples of suitable X moieties of the end-capping compound include 
isocyanate (OCN--) isothiocyanate (SCN--); Hal--CO-- where Hal is a 
halogen such as chlorine, bromine, fluorine or iodine, activated esters, 
such as NO.sub.2 --C.sub.6 H.sub.4 --O--CO--; and mixed anhydrides. X may 
also be an alkyl halide, particularly an activated alkyl halide, such as a 
benzyl halide. Isocyanate and isothiocyanate groups are preferred moieties 
as they do not produce a byproduct which might present purification 
problems. 
The Z moiety may complete (with the oxygen) a silyoxy moiety, e.g., 
trimethyl silyoxy or triethyl silyoxy, or may complete an ester moiety, 
e.g., --O--Z being --O--CO--CH.sub.3, --O--CO--C.sub.2 H.sub.5, 
--O--CO--CF.sub.3, --O--CO--C.sub.6 H.sub.5, etc. 
The (CH.sub.2).sub.1 or 2 group of the end-capping chemical provides that 
the hydroxyl group which is residual upon removal of Z is primary and that 
it is non-hindered. To ensure that the hydroxyl group is sufficiently 
unhindered for good cure properties and yet does not add unnecessary 
non-functional weight, it is preferred that n is 2. 
If a Q moiety is present, it is advantageous if it spaces the --O--Z group 
further from the terminus of the polymer, e.g., Q may be additional 
--CH.sub.2 -- groups. The Q moiety may also be selected to enhance 
activity of the X moiety with hydroxyl groups; thus, if X is OCN--, Q may 
be an aromatic ring. Although there is relatively little restriction to Q 
moieties which might be included in an end-capping compound, it is 
preferred not to include a Q moiety without purpose, as non-functional Q 
moieties add to the weight of the polymer and generally do not enhance 
energy of the propellant composition. 
A specific example of an end-capping compound useful in the practice of the 
present invention is 2-trimethylsilyoxyethylisocyanate (TMSI), 
OCN--CH.sub.2 --CH.sub.2 --O--Si(CH.sub.3).sub.3, H. R. Kricheldorf, 
Liebigs Ann. Chem. pp. 772-792 (1973). Other useful end-capping compounds 
include 4-(2-trimethysilyoxyethyl) phenylisocyanate OCN--pC.sub.6 H.sub.4 
--CH.sub.2 --CH.sub.2 --O--Si(CH.sub.3).sub.3 and 
6-trimethylsilyoxyhexylisocyanate OCN--(CH.sub.2).sub.6 
--O--Si(CH.sub.3).sub.3. 
In accordance with a preferred aspect of the invention, where the X moiety 
is an isocyanate group, the reaction with the hydroxyl-terminated polymer 
is facilitated by being carried out in the presence of a catalyst. Tin 
catalysts, such as dibutyltin diacetate, promote the reaction of an 
isocyanate with terminal hydroxyl groups, and such catalysts were used in 
initial end-capping experiments. However, tin catalysts are somewhat 
undesirable from the standpoint that they are difficult to remove from the 
end-capped polymer. Unfortunately, residual tin catalysts also promote 
more rapid curing of the end-capped polymer with multifunctional 
isocyanate curatives. It is a general problem with propellant formulations 
to provide a sufficiently long pot life (the time after mixing when the 
propellant formulation is castable, e.g., before viscosity becomes 
excessive due to the amount of cure), and residual tin catalysts reduce 
pot life. Accordingly, it is preferred to use tertiary amine catalysts, 
such as triethyl amine, which are more readily removable from the 
end-capped polymer and therefore do not reduce pot life of the uncured 
propellant formulation. 
The addition of the end-capping compound to the polymer is generally 
carried out in an appropriate non-reactive organic solvent, such as 
methylene chloride chloroform, trichloroethane, dichloroethane, benzene, 
toluene, acetonitrile and tetrahydrofuran. Time and temperature conditions 
will vary depending upon the nature of the X moiety, the presence or 
absence of a catalyst and the degree to which the terminal hydroxyl groups 
of the polymer are hindered. If X is an isocyanate moiety, end-capping may 
proceed at room temperature, but the rate is improved with elevated 
temperatures. 
End-capping in accordance with the invention is substantially 
stoichiometric, that is, if two moles of end-capping compound are used 
with each mole of di-functional polymer molecule, at least about 90% of 
the terminal hydroxyl groups will be capped, and end-capping of close to 
100% of the terminal hydroxyl groups may be expected with many X moieties, 
e.g., isocyanate. Generally, the end-capping compound is added at about a 
2 molar ratio relative to difunctional polymer molecules. Those skilled in 
the art will recognize that determinations of polymer equivalent weight 
are not always accurate and that the end-capping compound may interact 
with impurities; thus, to achieve full end-capping, some excess of 
end-capping chemical may be used. Large excesses, however, are wasteful 
and could result in polymer purification difficulties from excess reagent 
or reagent-derived impurities. Less than a 2 molar ratio of end-capping 
chemical to polymer may be used, where less than complete end-capping is 
consistant with effecting a sufficient cure and achieving required 
elastomer mechanical properties. Generally, a molar ratio of end-capping 
chemical to polymer of at least about 1 is used to enhance curability of 
the polymer and enhance mechanical characteristics of the elastomer which 
is formed. 
Subsequent to end-capping the polymer, the blocking Z group is removed. 
This is generally carried out in an organic solvent using a trace amount 
of an organic acid, such as trifluroacetic acid. The solvent may enter 
into the reaction, attaching to the Z moiety. For example, when TMSI (or 
any other chemical containing a trimethylsilyoxy blocking group) is the 
end-capping compound and methanol is the solvent, CH.sub.3 
--O--Si(CH.sub.3).sub.3 is formed. This compound is volatile and easily 
removed. Advantageously, CH.sub.3 --O--Si(CH.sub.3).sub.3 is readily 
converted to ClSi(CH.sub.3).sub.3, which in turn may be reconverted to 
TMSI, whereby the --Si(CH.sub.3).sub.3 group may be recycled. 
In accordance with another aspect of the invention, where the 
hydroxyl-terminated polymer, as polymerized by conventional methods, is of 
insufficient chain length to impart good mechanical properties to a cured 
elastomer, the hydroxyl-terminated polymer may be chain-extended by 
reaction with a diisocyanate. The chain-extending reaction is 
substantially stoichiometric, the relative proportion of the 
chain-extending isocyanate to polymer molecules determining the average 
number of polymer molecules joined together and thereby the average 
molecular weight or length of the joined polymer. Thus, if the average 
molecular weight is to be approximately doubled, one mole of diisocyanate 
will be used per two moles of polymer molecules; if the average molecular 
weight is to be approximately tripled, two moles of diisocyanate will be 
used per three moles of polymer molecules; etc. 
As in the end-capping reaction, the chain elongation reaction is generally 
carried out in organic solvent and in the presence of a catalyst, such as 
a tin catalyst, but preferably a more readily removable amine catalyst. 
Suitable diisocyanates include, but are not limited to toluene 2,4 
diisocyanate (TDI), hexamethylenediisocyanate (HDI) and isophorone 
diisocyanate (IPDI). 
It is noted above that chain extension of hydroxyl-terminated polymers has 
been achieved, as an undesirable side reaction, for example, in attempts 
to end-cap hydroxyl-terminated polymers with phosgene. It is to be noted 
that although hydroxyl-terminated polymers might be chain-extended with 
phosgene, if end-capping, as taught by this invention, is further 
contemplated, phosgene chain extension is undesirable. The C--O--CO--O--C 
chain extension bond formed by phosgene may be significantly cleaved under 
the conditions which remove the end-capping Z moiety. On the other hand, 
the urethane chain extension bond formed using a diisocyanate is stable 
under the mild conditions used to deblock the end-capped polymer. 
Modified polymers may be (and in producing elastomeric binders generally 
are) cured with polyisocyanate curatives. Typically the amount of 
isocyanate used to cure propellants containing modified polymers is 
determined by the desired mechanical properties. This determination is 
arrived at empirically by making the same propellant formulation with 
varying concentrations of isocyanate curative. The ratio of polymer to 
curative is referred to as the NCO/OH ratio. After the propellants have 
cured, the mechanical properties of the propellants made with varying 
NCO/OH ratios are determined. The propellant with the NCO/OH ratio that 
gives the desired mechanical properties is used for all future mixes of 
that propellant formulation. This method applies to high and lower energy 
propellant formulations. 
The curative must provide sufficient functionality to both chain-extend and 
cross-link the polymer. For hydroxyl-terminated polymers having 
functionalities of 2, an isocyanate curative having a functionality of 
greater than 2 and typically 3 or more may be used. One commonly used 
curative is a mixed isocyanate sold under the tradename Desmodur 
N-100.RTM. having a functionality of about 3.6. Alternatively, a 
diisocyanate may be used in conjunction with an additional cross-linking 
agent having higher functionality, such as a low-molecular weight 
trihydroxyl compound, e.g., trimethylol propane or 1,2,6 hexanetriol. 
Propellant compositions have NCO/OH equivalencies in the range of about 
0.5-2.0 and more generally in the range of about 0.8-1.5. The isocyanate 
curative generally comprises between about 0.5 and about 1.5 weight 
percent of the cured elastomer components, i.e., polymer plus curative. 
Curing is effected at elevated temperatures to promote relatively rapid 
curing. Typically, hydroxyl-terminated polymers are cured with isocyanates 
at temperatures of 120.degree.-130.degree. F. (49.degree.-54.degree. C.) 
for a period of several days. 
In high-energy compositions, the polymer is mixed with solids, including 
fuel material particulates, e.g., aluminum, and oxidizer particulates, 
e.g., ammonium perchlorate (AP), cyclotetramethylene tetranitramine (HMX) 
and/or cyclotrimethylene trinitramine (RDX). Then, the isocyanate curative 
is added and the uncured formulation is cast, e.g., into a rocket motor 
casing, and curing is effected at appropriate temperatures and for 
appropriate time periods. 
High-energy compositions typically contain between about 70 to 90% solids, 
including oxidizer particulates and/or fuel material particulates. The 
balance comprises the components of the elastomeric matrix, including the 
binder polymer and the curative. The elastomeric matrix components may 
also contain a plasticizer. If the polymer is miscible with a high-energy 
plasticizer, the binder components may advantageously include such a 
plasticizer. In propellants where specific impulse is of primary 
importance and a class 1.1 propellant is acceptable from a hazards 
sensitively standpoint, a high level of nitrate ester plasticizer is 
preferably included in the binder matrix. In such propellant binders, 
plasticizer-to-polymer ratios of at least about 2.0:1 and preferably at 
least about 2.5:1 are used. Nitrate ester plasticizers include, but are 
not limited to, nitroglycerine, butanetriol trinitrate and 
trimethylolethane trinitate. High-energy formulations may also contain a 
variety of minor components, such as processing aids, additional 
cross-linking agents, flow control agents, etc. 
The invention will now be described in greater detail by way of specific 
examples:

EXAMPLE 1 
Hydroxyl-terminated GAP (A) was used to produce (B) end-capped GAP, (C) 
chain-extended GAP and (D) chain-extended and end-capped GAP as follows: 
(B) Endcapping GAP with TMSI 
To a solution of 20.17 gms (0.0173 eq) of GAP (A) (1166 gms/eq., M.sub.w 
=2530, M.sub.w /M.sub.n =1.26) in 40 ml of dry methylene chloride under a 
dry nitrogen atmosphere was added 2.8 gms (0.0173 eq) of 
trimethylsilyoxyethylisocyanate. To this stirred solution was added 0.06 
ml of a 2.5% solution of dibutytin diacetate in methylene chloride. The 
reaction was stirred and refluxed under a nitrogen atmosphere for 18 hrs. 
The reaction was cooled to room temperature. Methanol (20 ml) was added, 
followed by 0.66 ml of trifluoroacetic acid. The mixture was stirred at 
room temperature for 2 hours. Saturated aqueous sodium bicarbonate 
solution (35 ml) was then added with vigorous stirring. The aqueous layer 
was separated, and the methylene chloride solution was washed with water 
and dried over anhydrous magnesium sulfate. After removal of the drying 
agent by filtration, the methylene chloride solvent was removed from the 
modified polymer under reduced pressure. The final traces of solvent were 
removed at 50.degree. C. under high vacuum. The H-1 NMR spectrum of this 
material showed about 90% primary and 10% secondary hydroxyl 
functionality. Eq. wt.=1366 gms/eg., M.sub.w =2460, M.sub.w /M.sub.n 
=1.24. 
(C) HDI Chain Extension of GAP 
To a solution of 94.8 grams (0.0813 eq.) of GAP (A) (1166 gms/eq., M.sub.w 
=2530, M.sub.w /M.sub.n =1.26) in 200 ml of dry methylene chloride under a 
nitrogen atmosphere was added 3.42 gms (0.04065 eq.) of hexamethylene 
diisocyanate (HDI). To this stirred solution was added 0.32 ml of a 2.5% 
solution of dibutyltin diacetate in methylene chloride. The reaction was 
stirred and refluxed under nitrogen for 18 hours. After this time, the 
methylene chloride solvent was removed under reduced pressure. The final 
traces of solvent were removed at 50.degree. C. under high vacuum. Eq. 
wt.=2532 gms/eq., M.sub.w =5640, M.sub.w /M.sub.n =1.72. 
(D) Endcapping of HDI Chain Extended GAP with TMSI 
The above chain-extended polymer (C) (0.05 eq.) was dissolved in 125 ml 
CH.sub.2 Cl.sub.2 and treated with TMSI (0.05 eq.) followed by 
trifluoroacetic acid/methanol exactly as described above for the 
end-capping of unmodified GAP (A). The isolated polymer showed: eq. 
wt.=2597 gms/eq., M.sub.w =5610, M.sub.w /M.sub.n =1.67. NMR analysis 
showed about 90% primary hydroxyl terminal functionality. 
The following properties of GAP's A, B, C and D are as follows: 
______________________________________ 
Equivalent wt. 
Equivalent 
M.sub.N M.sub.W by titration 
wt. by NMR 
______________________________________ 
A 2008 2530 1174 1166 
B 1984 2460 1366 1415 
C 3270 5640 2532 2344 
D 3350 5410 2597 2459 
______________________________________ 
Gumstocks from the GAP's were prepared by mixing eight grams of polymer 
with the required amount (based on polymer equivalent weight) of 
isocyanate curative (Desmodur N-100.RTM.) for the desired NCO/OH ratio. 
The fluid gumstock material was cast into uniaxial tensile specimen molds 
and allowed to cure for several days at about 120.degree.-130.degree. F. 
Mechanical properties of the gumstocks are compared in the following table: 
______________________________________ 
Gumstock 
______________________________________ 
CAPPED 
GAP (A) GAP (B) 
______________________________________ 
NCO/OH 0.8 0.9 1.0 1.1 0.8 0.9 1.0 1.1 
Mechanical 
Properties 
E.sup.1.0 (PSI) 
122 165 241 300 95 175 215 275 
.epsilon..sub.m .sup.t (%) 
41 32 30 21 78 51 53 36 
.sigma..sub.m (PSI) 
36 40 46 58 43 56 70 82 
Shore A 34 35 46 51 29 39 48 50 
______________________________________ 
CHAIN EXTENDED 
CAPPED, CHAIN 
GAP (C) EXTENDED GAP (D) 
______________________________________ 
NCO/OH 0.8 0.9 1.0 1.1 0.8 0.9 1.0 1.1 
Mechanical 
Properties 
E.sup.1.0 (PSI) 
48 59 81 134 54 95 138 165 
.epsilon..sub.m .sup.t (%) 
141 99 95 51 104 102 78 61 
.sigma..sub.m (PSI) 
30 33 41 51 30 55 59 65 
Shore A 15 15 28 35 13 25 35 40 
______________________________________ 
E.sup.1.0 = modulus at a gauge length of 1.0 
.epsilon..sub.m .sup.t = strain (elongation) 
.sigma..sub.m = stress 
In conclusion, all modified GAP polymers show cured mechanical property 
improvements. 
Propellants were prepared from unmodified GAP and from end-capped GAP (B). 
All propellants had 77% total solids, 76% ammonium perchlorate, 4% 
trimethylolethane trinitrate/triethyleneglycoldinitrate 1/3. Mechanical 
properties are as follows: 
______________________________________ 
GAP 
Modification 
NCO/OH E.sup.2.7 (PSI) 
.epsilon..sub.m.sup.t,c (%) 
.sigma..sub.m.sup.c 
______________________________________ 
(PSI) 
None 0.7 394 23 101 
Capped (B) 
0.8 1718 14 276 
Capped (B) 
0.7 926 21 189 
______________________________________ 
E.sup.2.7 = modulus at a guage length of 2.7 
.epsilon..sub.m.sup.t,c = true, corrected strain 
.sigma..sub.m.sup.c = corrected stress 
EXAMPLE 2 
Endcapping Poly BAMO/NMMO with TMSI. 
To a solution of 25.11 g (0.005073 eq) of poly (3,3-bis(azidomethyl) 
oxetane-copoly-3-nitratomethyl, 3-methyl oxetane) (4950 gms/eq) in 50 ml 
of dry methylene chloride under a dry nitrogen atmosphere was added 0.808 
gms (0.005073 eq) of trimethylsiloxyethyl isocyanate. To this stirred 
solution was added 0.007 ml of a 1.34% solution of dibutyltin diacetate in 
methylene chloride. The reaction was stirred and refluxed under a nitrogen 
atmosphere for 24 hours. The reaction was cooled to room temperature. 
Methanol (5 ml) was added followed by 0.20 ml of trifluoroacetic acid. The 
mixture was stirred at room temperature for 2 hours. Aqueous sodium 
bicarbonate solution (25 ml) containing 0.25 grams of sodium bicarbonate 
was then added with vigorous stirring. The aqueous layer was separated and 
the methylene chloride solution was washed with water, separated and dried 
over anhydrous magnesium sulfate. After removal of the drying agent by 
filtration, the methylene chloride solvent was removed under reduced 
pressure leaving the modified polymer. The final traces of solvent were 
removed at 50.degree. C. under high vacuum. 
Gumstocks were prepared by mixing 4 gm of polymer with the required amount 
of curative, cast into uniaxial tensil specimen molds and cured for 
several days at 120.degree. F. 
______________________________________ 
Mechanical Properties of Poly BAMO/NMMO Gumstock 
Poly BAMO/NMMO 
NCO/OH E.sup.1.0 (psi) 
.epsilon..sub.m.sup.t (%) 
.sigma..sub.m psi 
______________________________________ 
Unmodified 0.9 19 390 36 
Endcapped 0.9 58 208 61 
______________________________________ 
The end-capped poly BAMO/NMMO shows a significant increase in stress and 
modulus due to higher cross-link density resulting from the end-capping. 
The invention is addressed to providing an improved cure for 
hydroxyl-terminated polymers, which cure is effected, at least in part, 
with isocyanate curatives. At the same time, preferred end-capping 
chemicals contain isocyanate or isothiocyanate moieties for reaction with 
the pre-existing terminal hydroxyl groups. Although this may appear to be 
an inconsistancy, it is not. In propellant compositions, in addition to 
the polymer, there are several additional ingredients. Also, impurities 
are present and moisture may enter the reaction. Any additional 
ingredients, impurities and moisture may react with the isocyanate 
curative. For example, MNA used in some of the above formulations is 
reactive with isocyanate curative. Thus, if the polymer itself reacts too 
slowly with isocyanate because its terminal hydroxyl groups are 
non-primary or otherwise hindered, sufficient levels of side reactions may 
occur such that the propellant composition which is formed upon curing 
lacks the requisite mechanical properties. On the other hand, the 
end-capping reaction can be much more carefully controlled. Additional 
isocyanate-reactive ingredients are eliminated, impurities are minimized, 
moisture can be more easily kept out and the reaction is carried out in a 
non-reactive organic solvent. Thus, even if the reaction of an isocyanate 
or isothiocyanate group with terminal hydroxyl groups in an end-capping 
reaction is relatively slow, it may be substantially stoichiometric. 
Several advantages of the methods of the present invention may now be more 
fully appreciated. Reagents are used in stoichiometric amounts, avoiding 
the use of large excesses of materials with resulting purification 
problems. End-capping reagents are monofunctional in alcohol reactivity 
which eliminates the potential for undesired dimerization of polymer 
chains often observed when using difunctional polymer modifying chemicals, 
such as phosgene. The end-capping and extension reactions are "one-pot" 
reactions, eliminating the need for several polymer isolation/purification 
steps which are often necessary, especially when large excesses of 
reagents are used, in conventional polymer capping chemistry. 
While the invention has been described with respect to certain preferred 
embodiments, modifications obvious to one with ordinary skill in the art 
may be made without departing from the scope of the invention. 
Various features of the invention are set forth in the following claims.