Microwave processing of carbonate monomers

Carbonate monomers are polymerized in the presence or absence of a polymerization initiator by irradiation with electromagnetic radiation in the absence of solvent or diluent.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for curing carbonate monomers, 
e.g., diethylene glycol bis (allyl carbonate) using electromagnetic 
radiation. 
2. Discussion of the Background 
Diethylene glycol bis (allyl carbonate), (DADC), is a monomer which is 
widely used as an optical plastic for production lenses, safety glasses 
and guards, watch crystals, and instrument windows. The most common method 
of preparing DADC is by the reaction of diallyl carbonate and diethylene 
glycol in a relative molar ratio higher than about 10:1, in the presence 
of a basic catalyst, for example, as disclosed in U.S. Pat. No. 4,508,656 
and U.S. Pat No. 4,623,705. This reaction scheme produces at least about 
80-90% by weight pure DADC. If the ratio of diallyl carbonate to 
diethylene glycol is on the order of 2:1, an oligomer or a mixture of 
oligomers of diethylene glycol bis (allyl carbonate) is produced. 
DADC, which has formula (I), polymerizes via a free radical mechanism using 
a suitable polymerization initiator to form a thermosetting polycarbonate. 
One of the most common initiators is benzoyl peroxide. The polymerization 
process involves a long initial curing cycle, typically at least 48 hours, 
at temperatures below 100.degree. C. The initial curing is followed by a 
post-curing cycle at 110.degree.-120.degree. C. for several hours to 
destroy the unreacted catalyst as well release any stresses produced in 
the polymerization step. Above 100.degree. C., benzoyl peroxide (BPO) 
cannot be used as an initiator because it decomposes too rapidly at these 
temperatures. Other initiators are available for polymerization at lower 
temperatures, as described in U.S. Pat. No. 4,607,087. 
##STR1## 
Polymerization of DADC results in a clear, colorless, abrasion-resistant 
polymer casting. The bulk polymerization is slow at first. With increasing 
conversion and upon gelation, polymerization is accelerated. The 
polymerization is highly exothermic. Thus, to prevent a runaway reaction 
with overheating and bubbling from occurring, especially in large pieces, 
a cooling system must be present. After the exothermic reaction, the rate 
of polymerization slows down. 
Due to the slow reaction rate of the monomer, curing cycles on the order of 
48-72 hours are used to obtain complete conversion of the monomer to 
polymer when using benzoyl peroxide as initiator. Even when using more 
reactive initiators, such as peroxydicarbonates, cycle times of 20 hours 
are commonly used. The long curing times which are associated with the 
above-mentioned conventional thermal curing methods for these materials 
limits their applications. 
It is well known that high energy ionizing radiation, such as X-rays, 
electron beams from betatrons, cyclotrons, and other high energy electron 
sources or radiation from radioactive elements, such as cobalt 60, can 
cause chemical reactions. These reactions occur as a result of ionization 
which is induced in the reagents irradiated, and require energy on the 
order of several electron volts. At lower frequencies in the microwave 
range, the energy of the electromagnetic radiation is much too low to 
produce ionization and hence the chemical reactions that are obtained with 
ionizing radiation cannot generally be obtained with electromagnetic 
radiation in the radio-frequency and microwave range. Electromagnetic 
radiation in these frequency ranges, however, has been employed to create 
a heating effect in irradiated dielectric materials, e.g. dielectric 
heating and diathermy. The thermal effect caused by electromagnetic 
radiation in the radio-frequency and microwave range has also been 
utilized in chemical reactions which are activated by heating. In 
particular, the setting of adhesives or the curing of certain resin 
compositions may be accomplished through the dielectric heat caused by 
electromagnetic irradiation in this frequency range. 
U.S. Pat. No. 3,432,413 discloses a method of initiating and conducting 
chemical reactions in a two-component system using non-ionizing radiation. 
Chemical reactions, in particular the polymerization of vinyl monomers, 
are conducted using pulsed non-ionizing electromagnetic radiation in the 
radio-frequency and microwave range to avoid substantial heating and 
undesired thermal effects. 
According to U.S. Pat. No. 3,432,413, electromagnetic radiation in the 
radio-frequency and microwave range can initiate the polymerization of 
vinyl monomers without the use of chemical initiators only when a second 
component is present which is inert with respect to the monomer and to the 
polymerization, and which has a polarity different from that of the vinyl 
monomer. The required use of an inert diluent is expensive and has the 
disadvantage of requiring additional process steps to separate the diluent 
from the desired product. 
A need continues to exist, therefore, for improved methods of producing 
polymers of carbonate monomers which do not have the limitations of 
existing methods. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a method for 
polymerizing carbonate monomers efficiently using non-ionizing radiation. 
This and other objects which will become apparent from the following 
specification have been achieved by the present process. In the process of 
the present invention, a carbonate monomer is polymerized, with or without 
an initiator using non-ionizing electromagnetic radiation in the microwave 
or radio-frequency range. This method allows for the polymerization of 
carbonate monomers in the absence of significant thermal effects. The 
monomer is used without solvent or diluent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It has now been found that carbonate monomers polymerize very efficiently 
using microwave or radio-frequency radiation and that the radiation heats 
a monomer sample to the desired reaction temperatures in a matter of 
seconds or minutes, depending on input power level, without the need of a 
second component. Further, the use of microwaves as the heating source 
allows reaction at an isothermal temperature to proceed at a faster rate 
than is possible using conventional thermal methods. 
By "carbonate monomers" is meant monomers having two polymerizable terminal 
carbon-carbon double bonds available for polymerization and a carbonate 
group, preferably compounds having the general formula shown below. 
##STR2## 
In this formula, A is a divalent (preferably hydrocarbon) organic group 
such as an alkylene, cycloalkylene, alkylene ether, arylene, aralkylene or 
arylene group. Preferably, A is C.sub.2-10 alkylene or cycloalkylene (more 
preferably C.sub.2-6 alkylene or cycloalkylene), C.sub.2-8 alkylene ether 
(more preferably C.sub.2-5 alkylene ether), C.sub.6-10 arylene, C.sub.7-12 
aralkylene or arylene-B-arylene, where the arylene groups are as defined 
above and B is a straight-chain or branched C.sub.1-6 alkylene group (more 
preferably C.sub.1-4 alkylene group). Group A may be substituted with one 
or more (preferably 1-3) C.sub.1-4 alkyl groups. In formula (II), n and m 
are independently integers of 1-5. Preferably, n=m=1-3. 
The carbonate monomers of formula (II) are prepared by heating diallyl 
carbonate, CH.sub.2 .dbd.CH(CH.sub.2).sub.n --O--C(O)--O--(CH.sub.2).sub.m 
CH.dbd.CH.sub.2 where n=m=1 or a homologue thereof where one of n and m is 
2-5, with an appropriate diol or glycol having the formula HO--A--OH, 
where A is as defined above. The carbonate monomers may be prepared in the 
presence of a base catalyst according to the method described in U.S. Pat. 
No. 4,508,656 and U.S. Pat. No. 4,623,705, incorporated herein by 
reference. 
Specific examples of alkylene or cycloalkylene diols which can be used to 
prepare the carbonate monomers include ethylene glycol, 1,3-propanediol, 
1,4-butanediol, 1,6-hexanediol and cyclohexane dimethanol. Suitable 
alkylene ether diols include diethylene glycol, triethylene glycol, 
tetraethylene glycol, dipropylene glycol, etc. Specific arylene glycols 
include hydroquinone, resorcinol, 2,5-naphthalenediol and 
2,6-naphthalenediol. Specific arylene-B-arylene diols include 
2,2-bis(p-hydroxyphenyl)propane (bisphenol A). A particularly preferred 
monomer is DADC. For simplicity, the carbonate monomer will be referred to 
below as simply the "monomer". 
The present invention is applicable to carbonate monomers, oligomers of the 
same, or any mixture which contains the same. 
In accordance with the present invention, the carbonate monomer is 
polymerized, in the presence or absence of suitable polymerization 
initiators, by subjecting it to pulsed or continuous electromagnetic 
radiation. The electromagnetic radiation causes dielectric heating of the 
monomer. Accordingly, the pulse duration and frequency are adjusted to 
avoid significant thermal effects. 
The frequency of the electromagnetic radiation used in the present 
invention is in the microwave and radio frequency range and varies from 
10.sup.8 to 10.sup.11 Hertz (Hz). The input power necessary to heat and 
react the monomer is dependent on the final desired result. The initial 
heating rate can be controlled by monitoring and varying the input power 
into the sample. In general, the power is such that under pulsed or 
continuous radiation, dielectric heating will result. The preferred output 
power is up to about 1-2 kilowatts. 
When using pulsed radiation, any arrangement of pulse duration and pulse 
repetition frequency which allows for the dissipation of adverse heat 
build-up in the reaction mixture may be used in the present invention. The 
pulse duration may be varied from 1 to 100 microseconds and the pulse 
repetition frequency from 2 to 1,000 pulses per second. The mixture of 
monomer and initiator may be irradiated for any period of time sufficient 
to achieve polymerization. Generally, a sample will be irradiated for at 
least about 5 minutes, commonly for a period of time ranging from about 10 
minutes to about 150 minutes to achieve the desired extent of 
polymerization. Obviously, the time required to achieve polymerization 
will be shorter for higher power settings. The amount of time, frequency 
and power ranges can be readily adjusted by one having ordinary skill in 
the art to achieve the desired extent of polymerization based on simple 
calibration experiments. 
When continuous radiation is utilized, the sample is also heated for a time 
sufficient to achieve the desired polymerization, generally at least about 
5 minutes, preferably about 10-150 minutes. As with pulsed radiation, the 
time, frequency and power input can be routinely adjusted to achieve the 
desired extent of polymerization. 
Irradiation of the monomer sample may be conducted in any microwave and/or 
radio frequency heating device which is capable of continuous or pulse 
radiation and has the power requirements necessary to polymerize the 
sample. Suitable heating devices include microwave ovens, waveguides, 
resonant cavities, etc. Suitable heating devices are well known in the art 
and commercially available. 
The preferred device for performance of the present invention is a 
single-mode resonant cavity. Any available mode for heating in this device 
can be used in the present invention. However, the present invention is 
not be limited to use of this device but can be performed in any microwave 
or radio-frequency heating equipment. 
In general, the process of the present invention is carried out by placing 
the monomer/initiator inside of a microwave or radio frequency device and 
applying the appropriate input power. The present invention is 
particularly suitable for batch processing but is not be limited in this 
regard. The microwave reactions are generally carried out at elevated 
temperatures, as required by the decomposition half-lives of the 
polymerization initiators. 
The polymerization reaction is preferably carried out in the presence of a 
polymerization initiator. Suitable initiators are known in the art and any 
initiator which is capable of initiating polymerization of the monomer may 
be used. Preferably, the initiator is chosen from the following types of 
compounds: 
(a) Organic peroxyesters, preferably C.sub.4-30 alkyl or C.sub.6-30 aryl 
peroxyesters such as tert-butyl peroxymaleate, tert-butyl peroxylaurate, 
tert-butyl peroxy 3,5,5-trimethylhexanoate, 2,5-dimethyl-2,5-di(benzoyl 
peroxy)hexane, tert-butyl peroxyacetate, tert-butyl peroxybenzoate, 
tert-butyl peroxyisophthalate, cumyl peroxyneodecanoate, tert-butyl 
peroxyneodecanoate, tert-butyl peroxypivalate, tert-butyl 
peroxy(2-ethylhexanoate), and tert-butyl peroxyisobutyrate. 
(b) Organic peroxydicarbonates, preferably C.sub.6-30 peroxydicarbonates 
such as di-isopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, 
di-myristyl peroxydicarbonate, di-(2-ethoxyethyl) peroxydicarbonate, 
di(methoxyisopropyl)peroxydicarbonate, di-(2-ethylhexyl) 
peroxydicarbonate, and di-(3-methyl-3-methoxybutyl) peroxydicarbonate. 
(c) Diacyl peroxides, preferably C.sub.4-30 alkyl or C.sub.6-30 aryl diacyl 
peroxides such as 3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, 
decanoyl peroxide, lauroyl peroxide, acetyl peroxide, meta-toluyl peroxide 
and benzoyl peroxide. 
(d) Peroxycarbonates, preferably C.sub.4-30 peroxycarbonates such as 
tert-butyl peroxyisopropylcarbonate. 
(e) Peroxyketals, preferably C.sub.6-30 alkyl or cycloalkyl peroxyketals 
such as 1,1-bis(tert-butyl peroxy)-3,3,5-trimethylcyclohexane, 
1-1-bis(tert-butyl peroxy)cyclohexane, and 
2,2-bis(tert-butyl-4,4-bis(tert-butyl peroxy)valerate. 
(f) Ketone peroxides, preferably C.sub.4-20 ketone peroxides such as 
cyclohexanone peroxide and methyl ethyl ketone peroxide. 
A particularly preferred initiator is benzoyl peroxide. 
Such initiators are employed at a concentration, with respect to the 
monomer, which generally varies from 0.01 to about 10% by weight, 
preferably about 1-5%. The monomer and initiator are mixed together in the 
appropriate proportions by weight in the same vessel and the entire 
mixture is placed into the microwave device. 
Other features of the invention will become apparent in the course of the 
following descriptions of exemplary embodiments which are given for 
illustration of the invention and are not intended to be limiting thereof. 
EXAMPLES 
Example 1 
Samples of DADC containing 3% benzoyl peroxide were placed in TEFLON 
containers and heated to 90.degree. C. using both a single-mode resonant 
cavity and a conventional thermal oven. Input power for the microwave 
experiments was 5 watts of power. Heating time for the microwave 
experiments was 90 seconds while for the thermal experiments was 400 
seconds. 
DADC alone was placed in a TEFLON container and heated to 90.degree. C. in 
a single-mode resonant cavity using an input power of 20 watts. Total 
heating time was 30 seconds. 
Samples containing 2%, 3%, 4% and 5% benzoyl peroxide by weight were made 
with DADC monomer at room temperature. This temperature was used to ensure 
that the sample did not begin to react upon mixing. The compositions thus 
obtained were poured into small TEFLON cylinders which were used to cure 
the samples. 
Microwave curing of DADC was conducted in a six inch diameter single-mode 
resonant cavity. The microwave source had an operating frequency of 2.45 
GHz which was produced by an OPTHOS microwave generator that had a maximum 
power output of 40 watts. 
Samples were placed in the cavity so that the TM.sub.012 mode was utilized. 
The TM.sub.012 mode produces an axial electromagnetic field. The sample 
temperature was monitored by a LUXTRON fluoroptic probe. The probe was 
placed in the cylinder with the sample and protected by a glass probe 
cover. Isothermal microwave curing was performed in the temperature range 
of 85.degree. to 100.degree. C. for times up to 90 minutes. This 
temperature range corresponds to the usual use temperature range for 
benzoyl peroxide. The same time and temperature ranges were used for 
thermal curing experiments which were done using a circulating air oven. 
After both the microwave and thermal cure, the reacted samples were 
quenched with dry ice to prevent further reaction. Differential Scanning 
Calorimetry (DSC) was used to measure the extent of cure of the samples. 
All measurements were run in a nitrogen atmosphere using a heating rate of 
10.degree. C./min. 
The extent of cure, x, is calculated according to the equation: 
##EQU1## 
where Hr is the residual heat of the partially cured sample and Hp is the 
heat of polymerization of the uncured sample. Hp was determined to be 525 
J/g by DSC examination of an uncured sample. 
FIGS. 1-4 compare the extent of cure versus time profiles for DADC 
solutions containing 2, 3, 4 and 5 wt. % benzoyl peroxide. These figures 
show that using microwaves as the heating source allows the same extent of 
reaction to be obtained in a shorter time, compared to conventional 
thermal heating. As an example, for the 3% benzoyl peroxide solution, at 
90.degree. C. it takes about 15 minutes to reach an extent of cure of 0.2 
while at the same temperature under thermal conditions it takes about 30 
minutes to reach this level of cure. In general, the reduction in reaction 
time corresponds to about a 2-3 fold time reduction. This effect is most 
pronounced for the solutions which contain 4-5 weight benzoyl peroxide but 
there is still an effect for the lower benzoyl peroxide levels. 
Example 2 
Samples of the monomer having the structure shown below 
##STR3## 
and containing 2%, 3%, 4% and 5% by weight benzoyl peroxide are placed 
into small TEFLON cylinders and cured in a six-inch diameter single-mode 
resonant cavity in a manner described in Example 1. The cured samples are 
quenched to provide the cured polymer. 
Example 3 
Samples of the monomer having the structure shown below 
##STR4## 
and containing 2%, 3%, 4% and 5% by weight benzoyl peroxide are placed 
into small TEFLON cylinders and cured in a six-inch diameter single-mode 
resonant cavity in a manner described in Example 1. The cured samples are 
quenched to provide the cured polymer. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.