Triblock polymers of a monovinyl aromatic compound and myrcene

A tacky thermoplastic elastomeric linear triblock polymer corresponding to the formula A-B-A is made from a monovinyl aromatic hydrocarbon and myrcene. A is a polymer block of a monovinyl aromatic hydrocarbon, e.g. styrene, having an average molecular weight between 2,000 and 100,000 and a glass transition temperature above 25.degree. C. B is a polymeric block of myrcene (7-methyl-3-methylene-1,6-octadiene, C.sub.10 H.sub.16) having an average molecular weight between 10,000 and 1,000,000 and a glass transition temperature below -40.degree. C. B constitutes from 40 to 80 percent of the total. The products are made by sequential polymerization with an organolithium initiator.

FIELD OF THE INVENTION 
Thermoplastic elastomeric tacky linear triblock polymers are made from a 
monovinyl aromatic hydrocarbon and myrcene. 
THE PRIOR ART 
Polymeric products are known which are both thermoplastic and elastomeric 
in the unvulcanized state. In the form here relevant, they are triblock 
polymers of the general structure A-B-A. A is a non-elastomeric block, 
such as linear polystyrene, and B is an elastomeric block of a polymerized 
conjugated diene, e.g. polyisoprene or polybutadiene (U.S. Pat. No. 
3,265,765). These products, though valuable for many purposes, do not have 
the property of tackiness and hence are not useful per se as adhesives. 
It is also known that some trienes, such as myrcene, undergo 
homopolymerization (Marvel et al., J. polym. Sci. 45, 25 (1960)) and 
copolymerization with styrene (U.S. Pat. Nos. 2,383,084 and 2,549,539). 
However, myrcene, and other trienes as well, seem not to have been used 
heretofore in making triblock polymers. 
SUMMARY OF THE INVENTION 
The present invention is based on the discovery that unusual and 
advantageous properties may be imparted to linear triblock polymers by 
making the central block of polymerized myrcene. The polymers thus formed 
are not only thermoplastic and elastomeric but also are solvent soluble, 
clear, and exhibit a tacky consistency and good tensile properties. 
The new materials are triblock polymers of a monovinyl aromatic hydrocarbon 
and myrcene corresponding to the general formula A-B-A. Each A is a 
polymeric block of a monovinyl aromatic hydrocarbon and B is a polymeric 
block of myrcene. To attain the desired properties, the molecular weight 
and glass transition temperature of each of the segments A and B, and the 
relative proportions of A and B, are all controlled within appropriate 
ranges, as will be described. 
The products are best prepared by sequential polymerization of the 
monomeric components with an organolithium initiator. 
The invention has the further advantage that the raw material myrcene is 
prepared from a renewable natural resource, e.g. turpentine, a naval 
store. 
DETAILED DESCRIPTION OF THE INVENTION 
The products of the invention are triblock polymers of a monovinyl aromatic 
hydrocarbon and myrcene corresponding to the general formula A-B-A. Each A 
is independently a block of polymeric monovinyl hydrocarbon having an 
average molecular weight between 2,000 and 100,000 and a glass transition 
temperature above 25.degree. C. B is a polymeric block of myrcene having 
an average molecular weight between 10,000 and 1,000,000 and a glass 
transition temperature below about -40.degree. C. The block B constitutes 
between about 40 and about 80 percent by weight of the total. 
The new products are best made by a sequential polymerization process. In 
the first step, a monomeric polymerizable monovinyl aromatic hydrocarbon 
is contacted with an organolithium initiator until polymerization of the 
monomer is substantially complete. Then, in a second step, myrcene is 
added to the mixture resulting from the first step in a proportion by 
weight from about 1.3 to about 8 times that of the monovinyl compound. 
Polymerization occurs spontaneously and is allowed to proceed until 
substantially all the myrcene has polymerized, forming a second mixture. 
Thereafter, the same or another monomeric polymerizable monovinyl aromatic 
hydrocarbon is added to the second mixture, preferably in a proportion 
approximately equal to that in the first step. Again polymerization occurs 
spontaneously and is allowed to continue until substantially all the 
monomer has polymerized, forming a third mixture which contains the 
desired triblock polymer. The latter may then be recovered from the third 
mixture. 
The polymerization process is preferably carried out by procedural 
techniques known and generally used for organolithium-initiated 
polymerizations. Reaction ordinarily takes place in the presence of an 
inert liquid hydrocarbon diluent or solvent for the monomer. Benzene is 
preferred. The proportion of diluent is not critical but it is usually in 
great excess, several times by volume, relative to the total of the 
monomers. The temperature of the reaction is also not critical but is 
usually in the range -20.degree. C. to 100.degree. C. While reaction at 
the higher temperatures in this range is more rapid, operation is somewhat 
simpler, and excellent results are obtained, at room temperature. 
The reaction initiator may be any organolithium compound commonly used in 
the linear polymerization of unsaturated hydrocarbons. A lower alkyl 
lithium is convenient, with n-butyl lithium and especially sec-butyl 
lithium being preferred. A conventional proportion of initiator suffices, 
as little as will insure adequate reaction, seldom more than about 0.1 mol 
percent of total monomers. When polymerization to form a triblock polymer 
is essentially complete, it may be ended by injecting a known terminator, 
such as a lower alcohol or water. The triblock polymer is then recovered 
from the reaction mixture by standard procedures. Thus, it may be 
coagulated from solution in the diluent by adding a precipitant such as a 
lower alcohol, steam, or water. The resulting crumbs of polymer may be 
separated, washed, and dried. 
The A component in the new tripolymers may preferably be a non-elastomeric 
linear homopolymeric block made from any polymerizable monomeric monovinyl 
aromatic hydrocarbon, preferably one of the benzene series. Typical are 
styrene, vinyl toluene, vinyl xylene, ethyl styrene, tert. butyl styrene, 
etc. Styrene itself gives excellent results and is usually chosen for 
commercial reasons. 
The B or central component of the triblock polymer is polymerized myrcene, 
having a glass transition temperature below about -40.degree. C., 
preferably below about -60.degree. C. Myrcene is an acyclic terpene triene 
C.sub.10 H.sub.16, specifically 7-methyl-3-methylene-1,6-octadiene. It may 
be considered a conjugated diene with an unsaturated sidechain, viz. the 
4-methyl-3-pentenyl radical. In polymerized myrcene a side chain of this 
unit structure is believed to be attached to each repeating unit of the 
elastomeric polymer block. 
Myrcene, when highly pure, i.e. with far less than one percent of 
impurities, is satisfactory for use in the invention, though expensive. 
Commercial myrcene, of 90 to 95 percent purity, sometimes contains 
gel-forming impurities which interfere with formation of a clear, linear 
triblock polymer. If such are present, they may be removed by brief 
pre-treatment of the impure myrcene with sodium to destroy gel-formers, 
followed by simple distillation. 
In the polymerization to form the thermoplastic non-elastomeric monovinyl 
aromatic hydrocarbon block segments A of the triblock polymer, the 
reaction is controlled by known procedures so that each homopolymeric 
block has an average molecular weight between 2,000 and 100,000 and a 
glass transition temperature above 25.degree. C. Preferred ranges are 
10,000 to 60,000 and a glass transition temperature above 70.degree. C. 
With styrene as the monomer, a transition temperature above 85.degree. C. 
affords optimum thermoplastic behavior and strength. It is preferred that 
the two monovinyl aromatic hydrocarbon polymer blocks in the triblock 
polymer be of the same monomer and as nearly equal as possible in 
molecular weight and glass transition temperature. 
For polymerization of the elastomeric myrcene block, the reaction is 
controlled so that the block molecular weight is between 10,000 and 
1,000,000 and the glass transition temperature is below -40.degree. C. 
Most suitable are molecular weights from 50,000, better 100,000, to 
200,000 and a glass transition temperature below about -60.degree. C., in 
order to impart optimum elastomeric behavior, tack, and tensile strength. 
In addition to controlling the molecular structure of each polymeric block, 
the properties of the final triblock polymer can also be varied as desired 
by regulating the relative proportions of the blocks A to the myrcene 
block B. A myrcene content of at least 65 percent of the total is 
preferred within the range previously stated. The effect of varying all 
these parameters and proportions will be evident from the examples 
hereinafter. 
The basic conditions which may be controlled during the sequential 
reactions to achieve these ranges of molecular weight and glass transition 
temperature are the quantities of each monomer added, the identity and 
proportion of initiator, the operating temperature, and the identity of 
the inert diluent. Each reaction stage is adjusted so that essentially no 
monomer is left unreacted in the mixture when the polymeric block being 
formed is within the desired range of molecular weight and transition 
temperature. Procedures for this type of control are known in the art. 
The linear triblock polymers made according to the invention are both 
thermoplastic and elastomeric. They may be molded under heat and pressure 
and yet are energy absorptive or rubbery without requiring vulcanization 
or compounding. They exhibit good tensile properties of elongation on 
stretching and high strength. These qualities alone equal or in some 
instances exceed those of the thermoplastic elastomeric triblock polymers 
of styrene and diolefins heretofore known. In addition, uniquely and 
surprisingly, the triblock polymers of the invention also exhibit a tacky 
consistency. This property, together with moldability, elasticity, and 
strength, renders them useful as hot-melt adhesives. The new tripolymers 
are also soluble in hydrocarbon solvents, such as benzene, a quality which 
makes them useful in formulating flowable adhesive cements. The new 
tripolymers are also clear, virtually transparent in the molded state. 
These important properties are thought directly attributable to the 
polymerized myrcene forming the central block in the tripolymers. This 
behavior of myrcene seems quite unexpected, since on polymerization, a 
triene monomer such as myrcene, especially one with a long side chain 
containing a reactive double bond, might have been expected to undergo 
extensive cross-linking, forming insoluble opaque products. Instead, the 
polymyrcene block is linear and adds tackiness to the block polymer, as 
well as elastomeric behavior and strength.

EXAMPLES 
A series of sequential polymerizations was carried out on a laboratory 
scale. In all runs, the initiator was sec-butyl lithium, the diluent 
benzene, and the temperature 30.degree. C. The monovinyl aromatic 
hydrocarbon in both the first and third stages of each triblock 
polymerization was styrene of polymerization grade. The myrcene was 
commercially available material of 90 percent purity. It was dried by 
stirring with freshly crushed calcium hydride and distilling. The 
distillate was then mixed with a dispersion of sodium in wax and 
redistilled into ampoules at reduced pressure. These were used in the 
triblock polymerization. 
EXAMPLE 1 
Polymerization was conducted in an all-glass vessel adapted to be evacuated 
and heat-sealed and fitted with three ampoules, two containing styrene 
(7.2 g. each) and the other myrcene (54.4 g.). To start, the initiator 
solution (0.5256 millimol (g.)) of sec-butyl lithium in cyclohexane (a 
commercially available mixture) was introduced into the vessel under argon 
pressure through a sidearm, which was then sealed off. Benzene (550 
milliliters) was distilled into the reactor, which was then sealed off 
from the distillation line under vacuum. The first polymerization step was 
then carried out by introducing the styrene from one ampoule into the 
vessel, allowing 12 hours for completion. In the second step, the myrcene 
was introduced and polymerization continued 10 hours. Then in the third 
step styrene from the other ampoule was admitted to the reactor, for an 
additional 12 hours of polymerization. Thereafter the process, which 
appeared substantially complete, was terminated by adding degassed 
methanol (3 ml.). The triblock polymer was then coagulated by pouring the 
reacted mixture into methanol, after which the polymer was separated and 
dried. The dry product was a tacky linear tripolymer which was both 
thermoplastic and elastic. It was visually clear and soluble in benzene, 
tetrahydrofuran, and other common solvents. On testing, it exhibited 
physical properties closely similar to those of the product of Example 3 
in the Tables. 
EXAMPLES 2 TO 7 
In a series of runs, triblock polymers according to the invention were made 
by the procedure of Example 1 except that various other proportions of 
styrene relative to myrcene were used. The proportions for each run are 
given in Table A. For convenience the proportions are stated in terms 
first of the molecular weight M.sub.c (g./mol.) of the entire triblock 
polymer and then of the block molecular weights styrene-myrcene-styrene, 
all figures being calculated from the weights of the monomers used. In all 
cases, linear triblock polymers were obtained having thermoplasticity, 
elasticity, solubility and appearance similar to that of the product of 
Example 1. 
For additional characterization, the triblock polymer of each such example 
was subjected to physical testing by known standard procedures. Total 
molecular weights were measured two ways for comparison with the 
calculated value M.sub.c. The value M.sub.n was determined by osmometry, 
while the value M.sub.w was derived by gel phase chromatography. All three 
molecular weight figures for each example are shown in Table A. Also shown 
is the value M.sub.B /M.sub.A derived as the quotient of M.sub.B (the 
block molecular weight of the myrcene block) divided by M.sub.A (the total 
molecular weight of the two styrene blocks), all such weights being 
calculated from the known quantities of the monomers used. The ratio 
M.sub.B /M.sub.A indicates the relative proportions of myrcene and 
styrene. 
Further given in Table A is the yield of polymer (calculated from the 
weight of triblock polymer obtained relative to the weight of the monomers 
used). In addition, there is shown the weight percent of styrene in the 
final polymer, both as calculated from the starting materials and as 
measured experimentally by known NMR spectroscopic techniques. 
Other tests on the same examples are reported in Table B. The glass 
transition temperatures T.sub.g of the blocks of myrcene and of styrene in 
the triblock polymer were measured by differential scanning calorimetry 
(DSC). The intrinsic viscosity [.eta.] was determined by conventional 
method. Tensile tests were performed on a portion of each triblock polymer 
molded into a sheet and then die-cut into a standard test "dumbbell" bar 
which was stretched to the breakpoint on an Instron tester. Reported in 
Table B are the elongation at break (as a percent of original length) and 
tensile strength (MP.sub.a, megapascals). 
From Table A it will be observed that the measured molecular weights 
M.sub.n and M.sub.w are nearly equal in all samples. This close 
correspondence is indicative of low polydispersity. The triblock polymers 
are thus essentially linear, termination and transfer free. 
While tackiness was not measured numerically, it was noted that the degree 
of tackiness increased significantly as the proportion of myrcene in the 
triblock polymer increased. 
TABLE A 
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--M.sub.c .times. 10.sup.-3 
--M.sub.n .times. 10.sup.-3 
--M.sub.w .times. 10.sup.-3 
Polymer 
Styrene - wt. % 
Example 
(g/mol) 
(g/mol) 
(g/mol) 
--M.sub.B /--M.sub.A 
Yield % 
calcd. 
exptl. 
__________________________________________________________________________ 
2 190 187 194 0.73 91 57.9 
58.1 
(55-80-55) 
3 180 190 210 3.50 88 22.2 
23.1 
(20-140-20) 
4 170 194 186 1.83 90 35.3 
36.0 
(30-110-30) 
5 150 165 157 1.50 89 40.0 
41.2 
(30-90-30) 
6 110 130 125 2.24 88 31.2 
32.6 
(17-76-17) 
7 80 92 85 1.86 89 35.0 
35.5 
(14-52-14) 
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TABLE B 
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T.sub.g.sbsb.(DSC) (.degree.C.) 
[.eta.] Elongation 
Tensile Strength 
Example 
B A (dl/g) 
% (MPa) 
______________________________________ 
2 -63 103 0.815 670 12.8 
3 -60 87 1.065 1200 4.3 
4 -61 95 0.853 1290 7.0 
5 -62 95 0.714 1000 9.2 
6 -63 85 0.683 580 3.5 
7 -63 85 0.495 340 3.6 
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