Chapter 7
Bulk Free Radical Copolymerization of Allylic Alcohol with Acrylate and Styrene Comonomers
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Shao-Hua Guo A R C O Chemical Company, 3801 West Chester Pike, Newtown Square, P A 19073
The bulk free radical copolymerization of allylic alcohol with acrylate leads to new hydroxyl functional acrylic resins. The copolymerization does not require a process solvent and produces oligomers without need of chain transfer agent. The comonomer feed composition determines not only the copolymer composition but also the molecular weight of the copolymer and the polymerization rate. An excess of allylic alcohol is applied to achieve the high hydroxyl content in the copolymer, and the unreacted allyl monomer is removed and recycled. The copolymers have considerably more uniform hydroxyl group distribution along a polymer chain and among the polymer chains due to the low monomeric reactivity ratio of the allyl monomer.
The
hydroxyl acrylic resins have potential in automotive coatings and many other applications.
Allyl alcohol, a well-known monomer, is commercially available from the isomerization of propylene oxide. Free radical copolymerization of allyl alcohol with other vinyl monomers is a potential route to hydroxyl functional polymers. Such polymers are valuable intermediates for coatings, elastomers, and other thermoset polymers because they can be readily cured with multiple-functional isocyanates, anhydrides, melamine, and many other crosslinking agents. Allyl alcohol readily reacts with an alkylene oxide [1] to form an alkoxylated allyl alcohol such as propoxylated (R: C H ) or ethoxylated (R: H) allyl alcohol (Equation 3
1). Alkoxylated allyl alcohol has lower acute toxicity and lower vapor
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pressure than allyl alcohol, and, therefore, is easier to handle i n the preparation of polymers [2]. The physical properties o f allyl alcohol and allyl propoxylates are listed in Table I. Table I. Physical Properties of Allyl Alcohol and Allyl Propoxylate Monomers Monomer
A l l y l Alcohol
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Structure ^
O
H
Allyl Monopropoxylate
ν\
0 /
A l l y l Propoxylate Α Α Ρ 1.6
γ)Η
Molecular Weight
58
116
150
B o i l i n g Point, °C
97
145
145-245
Flash Point, °C
21
Not Available
63
L D 50, Oral m g / K g
65
Not Available
1,100
Homopolymer Glass
4oC
-15 oC
-33C
Transition Temperature
The allylic alcohols, including allyl alcohol and allyl alkoxylate, readily undergo free radical copolymerization with vinyl comonomers such as styrene, acrylate, and vinyl ether or ester [3-5]. For example, allyl alcohol and styrene free radically copolymerize to form the S A A copolymer (see equation 2) which is a unique class o f resinous polyol for the coating industry [6-8]. Three S A A copolymers are available. They are different i n the composition and the physical properties such as the hydroxyl number, the molecular weight and the glass transition temperature (Table II). Table II.
Compositions and Physical Properties of S A A Copolymers
Product Name
SAA-103
SAA-100
SAA-101
A l l y l Alcohol/Styrene M o l e Ratio
20:80
30:70
40:60
M w (GPC)
8,400
3,000
2,500
M n (GPC)
3,200
1,500
1,200
Hydroxy Number
125
210
255
78
62
57
mgKOH/g Glass Transition Temperature, T , oC g
In Solvent-Free Polymerizations and Processes; Long, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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R E S U L T S A N D DISCUSSION Bulk Free Radical Copolymerization Process The copolymerization of the allylic alcohol with acrylate and styrene was conducted in a semi-batch non-solvent process. All of the allylic alcohol and part of the acrylate and styrene comonomers were initially fed into the reactor, and the remaining acrylate and styrene comonomers were added during the polymerization into the reactor at such a rate as to maintain an essentially constant ratio of the allylic alcohol to the acrylate and styrene comonomers. High polymerization temperature (120C to I6O0C) was applied to achieve commercially acceptable monomer conversion and polymerization rate. The polymerization took 5 to 8 hours to reach 70 to 95% of the total monomer conversion. The unreacted monomers containing mostly the allylic alcohol (larger than 95%) and a small amount of acrylate and styrene comonomers and the initiator were removed after the polymerization. The copolymerization was carried out under the vapor pressure of the allylic alcohol. For example, the vapor pressure of allyl alcohol at 135C is about 2.41 χ 105 Pa. Di-t-butyl peroxide was used as the free radical initiator due to its high decomposition temperature. The initiator was gradually fed into the reactor mixture to maintain a steady ratio of initiator to the monomer and to achieve an increased monomer conversion [ 9 ]. A typical laboratory procedure for conducting the copolymerization of the allylic alcohol with the acrylate and styrene comonomers is described in the Experimental section. In order to design the semi-batch copolymerization process, we have investigated the bulk copolymerizations of the allylic alcohol, including allyl alcohol and allyl propoxylate, with methyl methacrylate and styrene, respectively. We measured the monomelic reactivity ratios and the copolymerization rates. Monomeric Reactivity Ratios and Copolymer Compositions. The initial comonomer feed composition and the addition rate of the acrylate and styrene comonomers are determined by the desired hydroxyl group content (or hydroxyl number) of the copolymer, and by the monomeric reactivity ratios of the allylic alcohol and the acrylate and styrene comonomers. Most studies on allyl monomer polymerization or copolymerization in the literature were conducted at low polymerization temperature [10], and, therefore, they are of limited value for the
In Solvent-Free Polymerizations and Processes; Long, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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commercial process. W e experimentally measured the monomeric reactivity ratios o f a l l y l alcohol and a l l y l propoxylate i n bulk copolymerization
w i t h methyl
methacrylate and styrene, respectively, at 135C. The method known i n the literature was applied for measuring the monomeric reactivity ratios [11]. For example, by determining the copolymer compositions of allyl alcohol with methyl methacrylate for several different comonomer feed compositions (Table III) and plotting the left side o f the copolymer composition equation (Equation 3) against the coefficient o f r\
9
we obtained slope π (monomeric reactivity ratio o f methyl methacrylate)
and
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intercept Γ2 (monomeric reactivity ratio o f allyl alcohol). The monomeric reactivity ratios of three monomer pairs are listed in Table I V . Table Ι Π . Experimental Determination of the Monomeric Reactivity Ratios of Allyl Alcohol and Methyl Methacrylate @ 135«C fi χ 100
4.94
7.24
9.42
17.3
F i x 100
59.2
66.6
73.6
81.9
-1.62x10-2
-3.89x10-2
-6.67 χ 10-2
-16.3 χ 10-2
-1.86x10-3
-3.06 χ 10-3
-3.88 χ 10-3
-9.67 χ 10-3
2
fi (F l) r
2
d-fi) ^
fid-2Fi) (1-f^Fx
a
fi (F l)
=
r
2
2
(l-fO ^
where fi is the molar fraction of methyl methacrylate in the total monomers, and F i the molar fraction of methyl methacrylate monomeric unit i n the copolymer being formed at any instant. [MMA] fi =
d[MMA] Fj =
[ M M A ] + [AA]
(4 ) d[MMA] + d[AA]
A s the monomeric reactivity ratio o f allylic alcohol is dramatically lower than that o f the acrylate or styrene, a large ratio o f the allylic alcohol to the acrylate or styrene comonomer is always needed to produce a resin o f high hydroxyl group content. The excess amount of the allylic alcohol is removed after the polymerization by the vacuum distillation. The amount of allylic alcohol recycled depends on the
In Solvent-Free Polymerizations and Processes; Long, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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desired hydroxyl content o f the copolymer product, and it can be predicted by the copolymer composition equation. It is usually i n a range o f 5-30% of the total reactor charge for preparing a copolymer containing 10-50 molar % of the allylic alcohol. Table IV. Monomeric Reactivity Ratios of Allyl Alcohol and Allyl Propoxylate in Bulk Radical Copolymerization with
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Methyl Methacrylate and Styrene, respectively, at 135oÇ Mi
M
M e t h y l Methacrylate
Π
Γ2
A l l y l Alcohol
19.0
0.02
M e t h y l Methacrylate
A l l y l Propoxylate
11.7
0.05
Styrene
A l l y l Alcohol
69.0
0.05
2
Copolymerization Kinetics. is rather complex
The rate expression of free radical copolymerization
and, therefore, few rate constants o f copolymerizations are
measurable. The copolymerization o f the allylic alcohol with acrylate or styrene provides a simpler system for evaluating the copolymerization kinetics since the allylic alcohol hardly undergoes homopolymerization ( r close to zero, see Table I V ) . 2
For example, the rate expression o f styrene and allyl alcohol copolymerization can be approximated by (internal technical report of A R C O Chemical Company)
Rp=([st] ^-)Km" +
2
