UV-Curable Biobased Polyacrylates Based on a Multifunctional Monomer Derived from Furfural

The controlled polymerization of
a new biobased monomer, 4-oxocyclopent-2-en-1-yl
acrylate (4CPA), was established via reversible addition–fragmentation
chain transfer (RAFT) (co)­polymerization to yield polymers bearing
pendent cyclopentenone units. 4CPA contains two reactive functionalities,
namely, a vinyl group and an internal double bond, and is an unsymmetrical
monomer. Therefore, competition between the internal double bond and
the vinyl group eventually leads to gel formation. With RAFT polymerization,
when aiming for a degree of polymerization (DP) of 100, maximum 4CPA
conversions of the vinyl group between 19.0 and 45.2% were obtained
without gel formation or extensive broadening of the dispersity. When
the same conditions were applied in the copolymerization of 4CPA with
lauryl acrylate (LA), methyl acrylate (MA), and isobornyl acrylate,
4CPA conversions of the vinyl group between 63 and 95% were reached.
The additional functionality of 4CPA in copolymers was demonstrated
by model studies with 4-oxocyclopent-2-en-1-yl acetate (1), which readily dimerized under UV light via [2 + 2] photocyclodimerization.
First-principles quantum mechanical simulations supported the experimental
observations made in NMR. Based on the calculated energetics and chemical
shifts, a mixture of head-to-head and head-to-tail dimers of (1) were identified. Using the dimerization mechanism, solvent-cast
LA and MA copolymers containing 30 mol % 4CPA were cross-linked under
UV light to obtain thin films. The cross-linked films were characterized
by dynamic scanning calorimetry, dynamic mechanical analysis, IR,
and swelling experiments. This is the first case where 4CPA is described
as a monomer for functional biobased polymers that can undergo additional
UV curing via photodimerization.

本研究通过可逆加成-断裂链转移（reversible addition–fragmentation chain transfer, RAFT）（共）聚合策略，成功实现了新型生物基单体丙烯酸4-氧代环戊-2-烯-1-基酯（4-oxocyclopent-2-en-1-yl acrylate, 4CPA）的可控聚合，制备得到带有悬挂环戊烯酮单元的聚合物。
4CPA含有两类反应性官能团——乙烯基与内双键，属于不对称单体。因此，内双键与乙烯基间的竞争反应最终会引发凝胶生成。当采用RAFT聚合且目标聚合度（degree of polymerization, DP）为100时，仅能实现乙烯基19.0%~45.2%的最高转化率，且不会出现凝胶化或分散性显著宽化的问题。
将相同聚合条件应用于4CPA与丙烯酸月桂酯（lauryl acrylate, LA）、丙烯酸甲酯（methyl acrylate, MA）以及丙烯酸异冰片酯的共聚反应时，乙烯基的4CPA转化率可达63%~95%。
通过以乙酸4-氧代环戊-2-烯-1-基酯（1）为模型化合物的研究，证实了共聚物中4CPA所具备的额外官能团活性：该模型化合物可在紫外光照下通过[2 + 2]光环二聚反应顺利发生二聚。
第一性原理量子力学模拟佐证了核磁共振（nuclear magnetic resonance, NMR）实验观测结果。基于计算得到的能量与化学位移，可确定化合物（1）存在头-头与头-尾两种二聚体的混合物。
利用该二聚化机理，将含有30 mol% 4CPA的LA与MA共聚物经溶剂流延成膜后，在紫外光照下发生交联反应得到交联薄膜。通过动态扫描量热法、动态力学分析、红外光谱以及溶胀实验对该交联薄膜进行了系统表征。
本研究首次将4CPA作为可通过光二聚反应实现额外紫外固化的功能性生物基聚合物单体进行报道。